Modeling of Voltage Source Converter Based HVDC

Modeling of Voltage Source Converter

Based HVDC Transmission

System in EMTP-RV

by

Hiteshkumar Patel

A Thesis Submitted in Partial Fulfillment

of the Requirements for the Degree of

Master of Applied Science

in

The Faculty of Engineering and Applied Science

Electrical and Computer Engineering

University of Ontario Institute of Technology

August, 2010

© Hiteshkumar Patel, 2010

ii

CERTIFICATE OF APPROVAL

Submitted by [Hiteshkumar, Patel]

In partial fulfillment of the requirements for the degree of

[Master of Applied Science] in [Electrical and Computer Engineering]

Date of Defense: [2010/08/17]

Thesis title: Modeling of Voltage Source Converter Based HVDC Transmission System in EMTP-RV

The undersigned certify that the student has presented [his] thesis, that the thesis is acceptable in form and content and that a satisfactory knowledge of the field covered by the thesis was demonstrated by the candidate through an oral examination. They recommend this thesis to the Office of Graduate Studies for acceptance.

Examining Committee

_________________________________________ [first name, last name] Chair of Examining Committee _________________________________________ [first name, last name] External Examiner

_________________________________________ [Vijay, Sood]

Research Supervisor

_________________________________________ [first name, last name]

Examining Committee Member

_________________________________________ [first name, last name]

Examining Committee Member

As research supervisor for the above student, I certify that I have read and approved changes required by the final examiners and recommend the thesis for acceptance:

_________________________________________ [Vijay, Sood] Research Supervisor

iii

AUTHOR'S DECLARATION

I hereby declare that I am the sole author of this thesis. This is a true copy of the thesis,

including any required final revisions, as accepted by my examiners.

I understand that my thesis may be made electronically available to the public.

Hiteshkumar Patel

iv

Abstract

Voltage Source Converter (VSC) applications include but are not limited to HVDC,

Flexible AC Transmission System (FACTS) devices such as STATCOM, SSSC, UPFC and

Wind generators and active filters. The VSC based HVDC system is a feasible option for

bulk power transmission over long or short distances and the grid integration of renewable

energy sources in existing transmission and distribution systems.

The main requirement in a power transmission system is the precise control of active and

reactive power flow to maintain the system voltage stability. The VSC operating with the

specified vector control strategy can perform independent control of active/reactive power at

both ends. This ability of VSC makes it suitable for connection to weak AC networks or even

dead networks i.e. without local voltage sources. For power reversal, the DC voltage polarity

remains the same for VSC based transmission system and the power transfer depends only on

the direction of the DC current. This is advantageous when compared to the conventional

Current Source Converter (CSC) based HVDC system. Furthermore, in case of VSC, the

reactive power flow can be bi-directional depending on the AC network operating conditions.

In this thesis, a 3-phase, 2-level, 6-switch VSC connected to an active but weak AC

system at both ends of the HVDC link is developed using EMTP-RV. The VSC-HVDC

transmission system model is developed using both direct control and vector control

techniques. The direct control method is an approximate method in which the active power,

AC voltages at both ends of HVDC link and DC link voltage are controlled directly by using

PI-controllers. In vector control method, closed loop feed-forward control system is used to

control the active power, reactive power at both ends and DC voltage.

v

By comparing the simulation results, it is concluded that the vector control method is

superior to the direct control because of the removal of the coupling between control

variables to achieve the independent control of active and reactive powers at both ends of the

HVDC link.

vi

Acknowledgements

I would like to express my deep appreciation for my supervisor, Dr. Vijay Sood, who is

leading the electrical power engineering program at UOIT. Without his supervision and

consistent guidance, completing this thesis would not have been possible.

I am also thankful to my colleagues who have inspired me and make me laugh at critical

phases during the entire study period. Thanks to the library staff for providing support

throughout the whole program.

A special thanks to my wife and daughter for their support and encouragement. Without

their unconditional, pure love, I would not achieve such a challenging target. Their powerful

dreams are my fuel to make everything happen.

At last, I cannot forget to mention the inspiration I got from God and my final

appreciation is to those who I may have failed to mention.

vii

Dedication

This thesis is dedicated to my parents (Baldev Patel and Savita Patel), without their

continuous love and inspiration it would not have been possible to complete this work.

viii

Table of Contents

Author’s Declaration ................................................................................................................ iii

Abstract .................................................................................................................................... iv

Acknowledgements .................................................................................................................. vi

Dedication. .............................................................................................................................. vii

Table of Contents ................................................................................................................... viii

List of Figures ........................................................................................................................ xiii

List of Tables ........................................................................................................................ xvii

Chapter 1 Introduction .............................................................................................................. 1

1.1 Background ..................................................................................................................... 1

1.2 Related literature review ................................................................................................. 3

1.3 Scope of the project ......................................................................................................... 9

1.4 Objective of the thesis ................................................................................................... 10

1.5 Outline of the chapters .................................................................................................. 11

1.6 Summary ....................................................................................................................... 12

Chapter 2 VSC-HVDC System ............................................................................................... 13

2.1 Introduction ................................................................................................................... 13

2.2 VSC-HVDC system ...................................................................................................... 14

2.2.1 AC shunt RLC-filters.............................................................................................. 15

2.2.2 Converter Transformer ........................................................................................... 15

ix

2.2.3 Voltage Source Converter ...................................................................................... 15

2.2.4 DC transmission ..................................................................................................... 16

2.2.5 DC capacitors ......................................................................................................... 17

2.3 Topologies of the VSC .................................................................................................. 18

2.3.1 Two-level voltage source converter ....................................................................... 18

2.3.2 Multi-level voltage source converter ...................................................................... 19

2.3.3 Multi-pulse topologies ............................................................................................ 23

2.4 PWM for VSC ............................................................................................................... 24

2.4.1 Sinusoidal PWM ..................................................................................................... 24

2.5 VSC-HVDC Operation ................................................................................................. 26

2.5.1 Operating principle of VSC-HVDC ....................................................................... 26

2.5.2 Four quadrant operation of VSC ............................................................................ 28

2.5.3 Rectifier- Inverter operation of VSC ...................................................................... 30

2.6 Summary ....................................................................................................................... 32

Chapter 3 Control of VSC-HVDC System ............................................................................. 33

3.1 Introduction ................................................................................................................... 33

3.2 Direct control method.................................................................................................... 34

3.3 Vector control method ................................................................................................... 35

3.4 Limitations .................................................................................................................... 39

3.5 Outer controller ............................................................................................................. 40

3.5.1 DC voltage controller ............................................................................................. 40

x

3.5.2 Active power controller .......................................................................................... 42

3.5.3 Reactive power controller ....................................................................................... 42

3.6 Summary ....................................................................................................................... 45

Chapter 4 Methodology .......................................................................................................... 46

4.1 Introduction ................................................................................................................... 46

4.2 Power system modeling ................................................................................................ 46

4.2.1 Modeling of weak AC networks ............................................................................. 47

4.2.2 Modeling of converter transformer ......................................................................... 47

4.2.3 Modeling of AC shunt RLC-Filters ........................................................................ 48

4.3 Converter Modeling (Rectifier and Inverter) ................................................................ 51

4.3.1 Converter topology modeling ................................................................................. 51

4.3.2 IGBT-Diode module modeling ............................................................................... 52

4.4 DC capacitors and transmission system modeling ........................................................ 54

4.5 Control system modeling .............................................................................................. 56

4.5.1 Carrier frequency generation .................................................................................. 56

4.5.2 Sinusoidal pulse width modulation (SPWM) ......................................................... 57

4.5.3 Direct Control system modeling ............................................................................. 58

4.5.4 Vector control system modeling ............................................................................. 63

4.5.5 PI-Controller Gains................................................................................................. 70

4.6 Summary ....................................................................................................................... 74

Chapter 5 Simulation Results and Analysis ............................................................................ 75

xi

5.1 Introduction ................................................................................................................... 75

5.2 Steady state operation-Direct control method ............................................................... 76

5.2.1 Case study-1 ........................................................................................................... 76

5.2.2 Case-study-2 ........................................................................................................... 77

5.3 Step changes-Direct control method ............................................................................. 78

5.4 Active power flow reversal-Direct control method....................................................... 80

5.5 Steady state operation -vector control method .............................................................. 82

5.5.1 Case study-1 ........................................................................................................... 82

5.5.2 Case study-2 ........................................................................................................... 83

5.5.3 Case study-3 ........................................................................................................... 84

5.5.4 Case study-4 ........................................................................................................... 85

5.6 Step changes -vector control method ............................................................................ 86

5.7 Active and reactive power reversal -vector control method ......................................... 89

5.8 Comparison between direct and vector control technique ............................................ 91

5.9 Effect of DC capacitance on VSC-HVDC performance ............................................... 92

5.10 Summary ..................................................................................................................... 95

Chapter 6 Fault Analysis ......................................................................................................... 96

6.1 Introduction ................................................................................................................... 96

6.2 Fault analysis at VSC1 ................................................................................................... 96

6.2.1 Single line to ground (SLG) fault at VSC1 ............................................................. 97

6.2.2 Asymmetrical double line to ground (DLG) fault on AC filter bus at VSC1 ......... 98

xii

6.2.3 Symmetrical three lines to ground (TLG) fault on AC filter bus at VSC1 ............. 99

6.3 Fault analysis at VSC2 ................................................................................................. 101

6.3.1 Single line to ground fault at VSC2 ...................................................................... 101

6.3.2 Asymmetrical double line to ground fault on AC filter bus at VSC2 ................... 102

6.3.3 Symmetrical three lines to ground fault on AC filter bus at VSC2....................... 103

6.4 Fault at DC transmission system ................................................................................. 105

6.5 Summary ..................................................................................................................... 105

Chapter 7 Conclusions and Future work ............................................................................... 106

7.1 Conclusions ................................................................................................................. 106

7.2 Future work ................................................................................................................. 109

Appendix A: Per Unit System Representation................................................................... 115

Appendix B: Filter Design Calculations…...…...………………………………………117

Appendix C: ABC to dq Conversions….……………………………………………….119

Appendix D: EMTP-An Overview…….………………………………………………123

Appendix E: List of Publications………………………………………………………124

xiii

List of Figures

Figure 2.1: Single Line Diagram of VSC-HVDC System .................................................... 14

Figure 2.2: 2-Level, 6-Switch, 3-phase Converter Topology ............................................... 18

Figure 2.3: 3-Level, NPC-Converter Topology .................................................................... 20

Figure 2.4: 3-Level, Flying Capacitor Converter Topology ................................................. 21

Figure 2.5: 3-Level, Actively Neutral Point Clamped Converter Topology ........................ 22

Figure 2.6: 5-Level, Cascaded H-bridge Converter Topology ............................................. 22

Figure 2.7: Comparison between Sinusoidal and Triangular waveform and PWM Signals 25

Figure 2.8: Single Line Diagram for VSC-HVDC Operating Principle ............................... 26

Figure 2.9: P-Q Circle Diagram for VSC Operation ............................................................ 29

Figure 2.10: VSC Operation as Rectifier or Inverter ............................................................ 30

Figure 2.11: VSC Operation as a Rectifier ........................................................................... 31

Figure 2.12: VSC Operation as an Inverter .......................................................................... 32

Figure 3.1: VSC-HVDC Control System.............................................................................. 34

Figure 3.2: Vector Control System of VSC-HVDC System ................................................. 36

Figure 3.3: Active and Reactive Power Control System ...................................................... 44

Figure 4.1: Converter Transformer Model............................................................................ 48

Figure 4.2: EMTP-RV Filter Modeling and Frequency Scan graph ..................................... 49

Figure 4.3: Converter Model in EMTP-RV .......................................................................... 52

Figure 4.4: IGBT Valve Modeling........................................................................................ 53

xiv

Figure 4.5: DC Transmission System Modeling................................................................... 55

Figure 4.6: Carrier Frequency Generation Block ................................................................. 56

Figure 4.7: SPWM Block ...................................................................................................... 58

Figure 4.8: Angle delta block................................................................................................ 59

Figure 4.9: Active power control block for direct control method ....................................... 59

Figure 4.10: AC voltage control block at VSC1 for direct control ....................................... 60

Figure 4.11: DC voltage control block at VSC2 for direct control ....................................... 61

Figure 4.12: AC voltage control at VSC2 for direct control ................................................. 61

Figure 4.13: Direct control system for VSC-HVDC ............................................................ 62

Figure 4.14: Modulation Index and Angle Delta Block ....................................................... 64

Figure 4.15: Outer current controller loop for active power control block .......................... 64

Figure 4.16: Inner current controller loop for active power control block ........................... 65

Figure 4.17: Outer current controller loop for reactive power control block ....................... 66

Figure 4.18: Inner current controller loop for reactive power control block ........................ 67

Figure 4.19: Outer current controller for DC voltage control system ................................... 67

Figure 4.20: Inner current controller for DC voltage control system ................................... 68

Figure 4.21: Inner current controller loop for reactive power control block ........................ 69

Figure 4.22: Outer current controller loop for reactive power control block ....................... 69

Figure 4.23: VSC-HVDC model created using EMTP-RV software package ..................... 71

Figure 4.24: Rectifier Control Block of Proposed VSC-HVDC model ................................ 72

Figure 4.25: Inverter control block for proposed VSC-HVDC model ................................. 73

xv

Figure 5.1: Steady-state operation of VSC-HVDC with direct control method (a) 1 pu active

power flow from VSC1 to VSC2 (b) 1 pu AC bus voltage at VSC1 filter bus (c) 1 pu AC bus

voltage at VSC2 filter bus (d) 1 pu DC link voltage ............................................................... 76

Figure 5.2: Steady-state operation of VSC-HVDC with direct control method (a) 1 pu active

power flow from VSC2 to VSC1 (b) 1 pu AC bus voltage at VSC1 filter bus (c) 1 pu AC bus

voltage at VSC2 filter bus (d) 1 pu DC link voltage ............................................................... 77

Figure 5.3: Dynamic-state operation of VSC-HVDC with direct control method (a) 0.5 pu

active power flow from VSC2 to VSC1 (b) 1 pu AC bus voltage at VSC1 filter bus (c) 1 pu

AC bus voltage at VSC2 filter bus (d) 1 pu DC link voltage .................................................. 79

Figure 5.4: Steady-state operation of VSC-HVDC with direct control method (a) Initially

0.5 pu active power flow from VSC2 to VSC1 (b) Initially 1 pu AC bus voltage at VSC1 filter

bus (c) Initially 1 pu AC bus voltage at VSC2 filter bus (d) DC link voltage is maintained

constant at 1 pu ....................................................................................................................... 81

Figure 5.5: Steady-state operation of VSC-HVDC (a) 0.45 pu active power flow from VSC2

to VSC1 (b) Reactive power of 0 pu at VSC1 (c) DC voltage of 1 pu in DC link (d) Reactive

power of 0 pu at VSC2 ............................................................................................................ 83

Figure 5.6: Steady-state operation of VSC-HVDC (a) 0.45 pu active power flow from VSC1

to VSC2 (b) Reactive power of 0 pu at VSC1 (c) DC voltage of 1 pu in DC link (d) Reactive

power of 0 pu at VSC2 ............................................................................................................ 84

Figure 5.7: Full load active power flow (a) 1 pu active power flow from VSC1 to VSC2 (b)

0.65 pu reactive power flow from VSC1 to AC system (c) DC voltage of 1 pu in DC link (d)

0.45 pu reactive power flow from AC system to VSC2 .......................................................... 85

Figure 5.8: Full load active power flow (a) 1 pu active power flow from VSC2 to VSC1 (b)

0.25 pu reactive power flow from VSC1 to AC system (c) DC voltage of 1 pu in DC link (d)

0.5 pu reactive power flow from VSC2 to AC system ............................................................ 86

Figure 5.9: Step changes in the active power, reactive power and DC voltage (a) 0.5 pu

active power flow from VSC2 to VSC1 (b) 0.5 pu reactive power flow from AC system to

xvi

VSC1 (c) DC voltage of 1 pu in DC link (d) 0.5 pu reactive power flow from VSC2 to AC

system ..................................................................................................................................... 88

Figure 5.10: Active and reactive power reversal (a) Initial active power flow of 0.5 pu from

VSC2 to VSC1 (b) Initial reactive power of 0.15 pu flow from AC system to VSC1 (c) Initial

reactive power of -0.5 pu flow from VSC2 to AC system (d) DC voltage of 1 pu in DC link 90

Figure 5.11: Step changes with the DC capacitance of 100µF at each terminal .................. 93




Figure 6.1: SLG fault at VSC1 converter transformer .......................................................... 97

Figure 6.2: DLG fault at VSC1 converter transformer .......................................................... 99

Figure 6.3: TLG fault at VSC1 converter transformer ........................................................ 100

Figure 6.4: SLG fault at VSC2 converter transformer ........................................................ 102

Figure 6.5: DLG fault at VSC2 converter transformer ........................................................ 103

Figure 6.6: TLG fault at VSC2 converter transformer ........................................................ 104

Figure 61: ABC to dq conversion block modeling in EMTP-RV ........................................ 119

Figure 62: Phasor diagram for relationship between ABC and αβ ....................................... 120

Figure 63: Phasor diagram for relationship between ABC, dq and αβ quantities ................ 122

xvii

List of Tables

Table 4-1 Filter Ratings .......................................................................................................... 50

Table 4-2 DC Cable RLC-Parameters .................................................................................... 55

Table 4-3 VSC-HVDC PI-controller gains ............................................................................. 70

Table 5-1: Parameters of VSC-HVDC Simulation Model ................................................... 75

Table B-7-1: Filter ratings at VSC1 ...................................................................................... 117

Table B-7-2: Filter ratings at VSC2 ...................................................................................... 117

xviii

List of Acronyms

AC Alternating Current

ANPC Actively Neutral Point Clamped

B-B Back to Back

CC Current Controller

CSC Current Source Converter

DC Direct Current

DLG Double Line to Ground

EMT Electromagnetic Transient

EMTP-RV Electromagnetic Transient Program Restructured Version

FACTS Flexible AC Transmission System

FC Flying Capacitor

GTO Gate Turn-off Thyristor

HV High Voltage

HVDC High Voltage Direct Current

IGBT Insulated Gate Bi-polar Transistor

LCC Line Commutated Converter

MOSFET Metal Oxide Field Effect Transistor

NPC Neutral Point Clamped

PI Proportional-Integral

PSCAD Power System Computer Aided Design

PWM Pulse Width Modulation

R & D Research and Development

SCR Short Circuit Ratio

xix

SLD Single Line Diagram

SLG Single Line to Ground

SPWM Sinusoidal Pulse Width Modulation

STATCOM Static Synchronous Compensator

SSSC Static Synchronous Series Compensator

SVC Static Var Compensator

TLG Triple Line to Ground

UPFC Unified Power Flow Controller

VSC Voltage Source Converter

1

Chapter 1

Introduction

1.1 Background

The continuous increase in global power demand has fuelled the R & D of new

technologies for bulk power transmission with improved reliability and controllability [1-

11]. High Voltage Direct Current (HVDC) technology has been in operation for more

than 50 years and has been gaining in importance within the existing power grids due to

its advantages compared to conventional AC transmission [1-5]. The Line Commutated

Converter (LCC) based HVDC system is a mature technology which is primarily used for

bulk power transmission over long distances but it has certain constraints such as the

requirement of fairly strong AC networks to which it is connected and reactive power

demand requirement by the converters.

The Voltage Source Converter (VSC) based HVDC system is a relatively new

technology for low and medium power transmission which can be connected to weak or

even passive AC networks and is capable of supplying or absorbing reactive power

according to the system operating conditions [6-11]. The converter which uses the

voltage parameters as an input of it is called VSC. The main switching component of the

VSC is a high-power high-frequency static switch (GTO, IGBT or MOSFET). The high-

power high-frequency static switches, which were previously used for motor drives and

other similar applications, found applications in the HVDC system due to advances in

semi-conductor manufacturing technology, DC transmission and converter control

Chapter 1 Introduction

2

strategies [1, 8]. Capability of these switches to turn ON and turn OFF at any desired

instance makes it a very attractive technology for various power system applications. A

constant DC voltage is required for operation of VSC which generates an AC output

voltage of controlled magnitude, phase angle and frequency without presence of any

rotating machines. Consequently VSC is also known as a synchronous machine without

any inertia. VSC-HVDC may be used to supply passive loads without local generation

and can connect islanded loads or even passive AC systems [13].

It is also possible to use VSC-HVDC to construct a DC grid with multiple converters.

The concept of multi terminal DC grid can be used for power supply to city centers [9].

With further developments for increasing the current and voltage capability of the

converter switches and DC cables, the possibility of augmented use of VSC based HVDC

system is very high. At present VSC-HVDC is used up to the 400 MW power level, but

there are strong possibilities for raising this limit [9, 36]. The limitations associated with

thyristor valve technology regarding the active and reactive power control are easily

overcome by VSC-HVDC technology. Possible applications of VSC-HVDC exist feeding

of weak AC systems, feeding to a passive network, or the grid integration of renewable

energy sources like wind farms and solar farms. Remote power transmission makes VSC-

HVDC technology superior than its counterpart Current Source Converter (CSC) based

HVDC technology [1, 2, 3, 4, 5, 9, 10, 11].


3

1.2 Related literature review

Detailed information about the development of HVDC transmission system

technologies is presented in [1, 2, 7, 8]. Economical, technical and environmental

considerations of the AC and DC power flow are detailed in [7, 8]. A comparison

between VSC-HVDC and CSC-HVDC systems regarding the capital cost, capitalized

losses, DC capacitor size, commutation inductances and the footprint of the converter

station has been discussed in [3]. The analysis of the power increase that can be achieved

on an existing distribution network when a 3-phase link is substituted by a DC link is

provided in [7].

At present, the total installed capacity of HVDC systems around the world is

approximately 60 GW and it is increasing at a very high rate [1]. Reference [5] explained

the experiences starting from research stage to the commercial implementation of VSC-

HVDC technology pioneered by a leading electrical manufacturing company. Theoretical

derivations and simulation results have been given to prove the advantages of VSC-

HVDC for power transmission and operation with neighboring AC networks. Detailed

explanations regarding independent control of reactive power as an important feature for

voltage control and network operation in connection with system recovery after

disturbances is also provided.

Different topologies of VSC have been studied in [2, 3]. Two-level is the simplest

converter configuration and has been used in many practical applications. Many multi-

level converter topologies have been developed and studied regarding operational

improvement of VSC performance [2, 3, 6]. The operating principle of diode clamped,


4

flying capacitor and cascaded-inverters with separate DC source converter are explained

in [6].

Reference [4] focused on the advantages related to VSC in FACTS (Flexible AC

Transmission Systems) and HVDC applications. VSC applications like STATCOM

(Static Synchronous Compensator), SSSC (Static Synchronous Series Compensator),

UPFC (Unified Power Flow Controller) and Back-to-Back DC links have been provided

in [4]. Reference [8] has focused on power system reliability issues by adding new

HVDC transmission system in to existing synchronized AC network. Information about

the viability and advantages of implementing VSC-HVDC for city center load feeding

has been provided in [9, 10]. The discussion regarding the technical viability of VSC

HVDC for bulk transmission of power in a meshed grid is provided in [10].

Detailed information about operation and performance of VSC-HVDC system

requires knowledge of the control system design and mathematical modeling which has

been provided in [13-34]. Reference [13] has approximated the VSC-HVDC transmission

system with an equivalent model where the AC side is represented as a controlled voltage

source and DC side is represented as a controlled current source. Steady-state model for

the VSC-HVDC system and design of its controllers has been discussed in [14]. The VSC

model is represented as a 2-input and 2-output non linear control device and used voltage

control strategy. The dynamic and transient model of VSC-HVDC system has been

provided in [15]. The control techniques of VSC-HVDC can implement either direct or

indirect current control strategy. The indirect current control has a simple structure and

gives better static performance but its dynamic performance and system transition

characteristics are not ideal. Reference [15] has implemented the direct current control


5

strategy and dual closed loop structure consisting of current feedback and voltage feed

forward in the inner current control loop which can track the reference current quickly.

Outer loop controller is a power regulator which combines the inverse steady state model

with PI controller which can control active and reactive power separately. Reference [16]

has used state-space average method to derive the non-linear VSC-HVDC system. By

using non-linear transformation and power balance, linear model of back-to-back VSC-

HVDC system is developed. The VSC-HVDC model in an equivalent continuous-time

state-space model in synchronous dq reference frame is presented in [17] & [18]. Active

power controller is designed based on the multi-input multi-output control scheme. To

reduce the response time of the PI controller, a derivative term is introduced which can

add an early correction signal to the system. Reference [19] used a PSCAD/EMTDC

simulation package based electromagnetic transient simulation of Back-to-Back VSC

based transmission scheme. Two-level, 6-switch converter topology is used at both the

ends and vector control strategy is applied for convertor controllers. DC link voltage and

the reactive power are controlled at the sending end side while active and reactive power

is controlled at the receiving end side in this model. The coupling between the d and q

current components is studied and removed using the decoupled controller configuration

with the vector control. The simulation results of a 2-level 6-switch VSC based

transmission system which is connected to a passive load at the receiving end has been

provided in [20]. The simplex optimization algorithm was applied to optimize the gains

of the different controllers at both the ends. The benefits of vector control strategy are

established which provides a better performance compared to direct control. Reference

[21] has presented the detailed control model of VSC-HVDC where AC voltage and


6

reactive power control strategy with active power and DC voltage control is explained in

detail. Active power reversal is also explained with the effect on other parameters like

DC voltage and reactive power at both ends. A mathematical model for the VSC in

synchronous reference frame for investigating VSC-HVDC for transferring wind

generated power through a long distance line has been given in [22]. Tuning of PI

controllers is critical for system operation and performance so three different tuning

techniques are discussed and analytical expressions are derived for calculating the

parameters of the current and voltage controllers in [22].

Line commutated converter based HVDC system needs a reversal of voltage polarity

during the reversal of power flow [23]. The conventional system cannot operate at a fixed

DC voltage level for both rectifier and inverter modes whereas a multi terminal system

requires the unipolar DC voltage during all the conditions. VSC-HVDC does not require

reversal of voltage polarity for changing the direction of power flow and is also capable

of independent active and reactive power control. Reference [23] has discussed a multi-

terminal VSC-HVDC system proposed for integration of deep sea wind farms and

offshore oil and gas platforms. Proposed system was able to secure power to passive

loads during loss of DC voltage regulating VSC-HVDC terminal without the use of

communications between terminals.

Reference [24] has focused on the dynamic mathematical model for rectifier and

inverter side controllers of VSC-HVDC. Reference [25] has provided the model for the

PWM-VSC controlled power transfer. The control loop of DC voltage controller and

current controller is developed which has the second order characteristic equations.


7

A model of VSC based transmission system that can be used for a wide range of

stability studies and controller design with MATLAB is presented in [26]. The converter

model is non linear in nature, but it is linearized by assuming very small deviations

around the steady-state values. Detailed study of Phase Locked Loop (PLL) gains has

been provided to investigate the influence on the system stability which shows that

increased gains of the PLL reduce the system stability.

Modeling and control design of VSC-HVDC using 12-pulse 3-level converter is

presented in [27] where active power controller is designed using Proportional-Integral-

Derivative (PID) controller. Eigenvalue analysis is done to study the damping of rectifier

and inverter operation of the converter. In reference [32], the VSC-HVDC system is

realized using 12- pulse converter with fundamental frequency switching for the static

switches.

The idea of the voltage and current control techniques in PWM based rectifiers and

detailed analysis of different current control techniques has been provided in [35].

Synchronous Proportional-Integral (PI) current controller (CC), resonant current

controller and artificial neutral network based current controller are presented and

comparison is done regarding the requirements. At present a synchronous frame PI

current controller has become the standard solution for current regulation of AC

machines and PWM rectifiers due to its various advantages. PI-current controller can be

equipped with a decoupling network which eliminates the negative impact and improves

the PI controller overall performance.

Embedding the VSC-HVDC in meshed AC networks will open up the possibilities to

improve the power grid reliability and efficiency [29]. The multilevel VSCs provide


8

noteworthy advantages with respect to the system performance and harmonic impacts.

Application of VSC-HVDC using modular multilevel converter technology has been

studied in [38]. Reference [39] has focused on the asynchronous back-to-back connection

of VSC based system in which the converters can operate as a STATCOM during zero

active power transfer between two connecting AC networks. References [36, 39, 40, 41]

have discussed the VSC-HVDC technology for various power system applications. In

[36], the possibility for 1000 MW power transfer with 300 kV DC voltage is explained

using VSC-HVDC simulation model and results. References [41, 43] have discussed the

benefits of power system stability improvement which can be achieved by VSC-HVDC

transmission system. A mathematical model of an HVDC-VSC transmission system is

developed and its harmonic performance is investigated for steady-state operating

conditions in [42].


9

1.3 Scope of the project

The main scope of this project is to build a VSC-HVDC transmission system model

and study its operational performance using the EMTP-RV software package (Appendix

D). The control system for both rectifier and inverter stations is to be designed using both

direct and vector control methods. Design of controllers and optimization of gains at both

converter stations according to transmission of active and reactive power flow in either

direction is also part of the scope. Detailed studies will be done regarding independent

control of active/reactive power at sending and receiving ends which can be achieved

using vector control strategy.

The operational performance of the complete system is to be studied with regard to

the active and reactive power variation on sending and receiving ends, DC voltage

control, harmonics reduction and short time overload capacity. The performance under

symmetrical and asymmetrical faults is also to be studied.

By using the high frequency switching of the converter valves, the low order

harmonics are reduced and the harmonic filters’ size can be reduced. The size of the filter

and the effect of it on the system are to be studied. The relationship of control variables

(modulation index and angle delta) with controlled variables (active power, reactive

power/voltage and DC voltage) is also to be studied.


10

1.4 Objective of the thesis

The main objective of this thesis is to evaluate the potential use of VSC-HVDC

transmission system connecting relatively weak AC networks. The assessment of

operational performance of VSC-HVDC with respect to independent control and bi-

directional flow of active and reactive power will also be done.

The major objectives of the work reported in this thesis are:

1. To study the VSC, and its operating principle.

2. To study the HVDC transmission system based on VSC.

3. To evaluate two different control methods (i.e. direct and vector control method)

and provide a detailed comparison and study the effect of controller gains on

system performance.

4. To develop an EMTP-RV simulation model for the VSC-HVDC transmission

system (with its rectifier and inverter controllers) and ensure the desired active

and reactive power flow on both sides of the HVDC link.

5. To study the operational performance of proposed VSC-HVDC system regarding

the steady-state and dynamic operation, the converter losses, harmonic levels in

the AC and DC systems.

6. To study the performance under different fault scenarios at various locations in

VSC-HVDC system and to study the system stability in case of small and large

signal system disturbances on both AC and DC sides.


11

1.5 Outline of the chapters

The thesis is laid out as follows:

The introduction to the project is presented in chapter 1. The scope and the main

objectives of this project are also presented in chapter 1.

Chapter 2 provides an introduction to the operating principles and topologies of VSC.

A single line diagram of the proposed VSC-HVDC system is provided with detailed

explanation of each element associated with the system.

Chapter 3 discusses the control strategies for VSC-HVDC. Direct and vector control

methods are discussed and a detailed mathematical presentation is given for the vector

control method.

Chapter 4 presents the methodology for the VSC-HVDC system modeled in EMTP-

RV. A system description and different parameters of converters, harmonic filters,

transformers and DC network are presented. The direct and vector control systems

explained in chapter 3 are modeled in this chapter.

Chapter 5 provides the simulation results of the proposed system. The steady-state

and dynamic operating conditions (i.e. step changes in active power, reactive power/AC

voltages at both ends and DC voltage; active and reactive power reversal) are established

and operational performance and stability of the system is studied.

The response of the VSC-HVDC system to faults is presented in chapter 6.

Symmetrical and asymmetrical faults at various locations in the system are created and

the operational performance of the system is studied.


12

Chapter 7 presents the conclusions and closing remarks. The recommendations for the

future work to extend this research are suggested.

1.6 Summary

This chapter provides the back ground of this thesis and related literature review. The

scope and objectives of this thesis are provided here. The outline of the thesis is also

provided in this chapter.

13

Chapter 2

VSC-HVDC System

2.1 Introduction

The existing power transmission networks in most developed countries are heavily

loaded which has created the risk of system instability and power quality deterioration.

Furthermore, these countries have the problem of obtaining right of ways and face

environmental issues for the construction of new transmission lines. The necessity for a

new technology by which increased power transmission can be achieved using the

existing network infrastructure is clear. As explained in chapter 1, the VSC-HVDC

transmission system can take the place of this technology to solve the problem to some

extent. The VSC-HVDC system is a relatively new technology which comprises the high-

power high-frequency semi-conductor switches (i.e. IGBTs and GTOs). These devices

incorporated with anti-parallel diodes makes one complete unit. Several units are

connected in series to build a valve to get the required voltage level and to increase the

redundancy in the converter. The control of these switches is done by using PWM (Pulse

Width Modulation) techniques by which it is possible to control the magnitude and phase

angle of the generated fundamental (60 Hz) voltage component. This high controllability

of VSC results in a wide range of power system applications like VSC-HVDC, Static

Synchronous Compensator (STATCOM), Static Synchronous Series Compensator

(SSSC), Unified Power Flow Controller (UPFC), Interline Power Flow Controller (IPFC)

and active filters. In addition to bulk power transmission, strategically located VSC-

Chapter 2 VSC-HVDC System

14

HVDC system in an existing AC power system network augments the network stability

and reliability during disturbances and contingencies.

2.2 VSC-HVDC system

RL

C

AC filters AC filters

Transformer

DC cable

Converter-1

RL

C

Converter-2

Transformer

Active Power

Reactive

Power

Reactive

Power

DC cable

AC system AC systemZsource Zsource

Figure 2.1: Single Line Diagram of VSC-HVDC System

Figure 2.1 shows the single-line diagram of a VSC-HVDC system which

interconnects two AC networks of same operating frequency for power transmission and

stability improvement during contingencies. The system consists of different equipments,

such as shunt connected RLC-filters, interfacing transformers, VSCs and their control

systems, DC overhead or underground cables and DC capacitors. The following sub-

sections provide explanations regarding the function of each apparatus comprising the

VSC-HVDC.


15

2.2.1 AC shunt RLC-filters

Switching of high-power high-frequency switches generates harmonics in the

converter output voltages and currents. The order of harmonics generated is directly

related to the PWM switching frequency of the semi-conductor switches.

Single- or double-tuned Low-pass and High-pass damped RLC filters are connected

to suppress these harmonics. The frequency modulation index is defined as mf which is

defined as the ratio of the switching frequency of the converter to the fundamental

frequency of the AC voltage. In case of 3-phase VSC, the harmonics in the line to line

voltages are present. The odd harmonics exist as sidebands, centered around the mf and

its multiples where mf is odd. The value of mf is chosen such that it should be an odd

integer to eliminate the even harmonics; also, if it is a multiple of 3, then most of the

triplen harmonics are cancelled out in the line-to-line voltage.

2.2.2 Converter Transformer

A converter transformer interfaces the VSC AC terminals to AC network with

suitable voltage level at both sides. It provides electrical isolation and operates as a filter

between converter and AC networks and provides the appropriate AC voltage level to

optimize the available converter valve voltage rating. Configuration and rating of the

transformer can be decided from the total system power rating, converter topology and

specified requirements.

2.2.3 Voltage Source Converter

The VSC is the main building block of the VSC-HVDC system. The number of

switches employed depends on the operating characteristic and voltage level required.


16

The series connected IGBT switches, connected with anti parallel diodes, make up a

valve. Two valves are connected in series to make up the converter leg for one phase.

VSC can operate as a rectifier or an inverter at either end depending on the current flow

direction. Two converters can be connected as a back-to-back system to connect two

different AC grids, DC transmission line or overhead cables for bulk power transmission

or underground cables for connection of offshore oil rigs, islands or off-shore wind

farms. The 6-switch converter comprises a group of six valves. Different types of PWM

techniques are used to operate and control the converter switches. Typical IGBT has

rating of 6.5 kV and 2.4 kA. The voltage across the IGBT switch in the ON state is

typically in the range of 1 to 3 V which creates the ON state losses. The switching losses

are the second major loss mechanism and are due to fact that, during the turn ON and turn

OFF transition, current is still flowing while voltage is present across the device. In order

to minimize the switching losses, the maximum switching frequency needs to be

carefully considered. VSC is equipped with the RC snubbers and series R-L elements to

reduce the dv/dt & di/dt stresses on the switches during ON/OFF transitions.

2.2.4 DC transmission

The DC system can be in the form of a transmission line, with underground or

overhead cables. The overhead cables can be used with a moderate size of towers. The

extruded polymer cable is a newly developed technique in which the insulation is highly

resistant to the DC voltage.

The DC transmission system adopted here is a bi-polar transmission system in which

the power can flow in both the overhead poles and ground is used as a return path in case

of monopolar operation. In case of failure or preplanned maintenance of one pole,


17

reduced power can still be transmitted using the other pole. Two smoothing inductors are

connected on DC transmission system to reduce the ripples in the DC current.

2.2.5 DC capacitors

As shown in Fig.2.1, two series connected DC capacitors of same size are employed

across the DC terminals of converter with grounded midpoint. The size of the capacitor

depends on the required DC voltage level and the acceptable DC ripple.

The DC voltage should be adequate for the generation of sinusoidal voltages on AC

side of the converter. For the system to operate in the linear range of the PWM, the DC

voltage should be 1.634 times the value of root mean square (RMS) AC voltage.

The PWM switching of high-power high-frequency switches generates the harmonics

in the DC current flowing in the transmission line. This generates the harmonics in the

DC voltage which is strongly related to the converter AC voltage. By evaluation of the

magnitude of ripple and the switching frequency of the semiconductor switches, the DC

side capacitor ratings can be finalized.

The value of DC capacitor CDC used here can be small which may result in a faster

converter response. The energy stored in the capacitor is released during transients which

will help to stabilize the power flow on the DC transmission system. The DC capacitor

size is characterized with a time constant τ, which can be defined as the ratio between the

energy stored in the capacitor at the rated DC voltage VDC and the apparent power rating

of the converter SC:

(2.1)


18

The time constant is equal to the time needed to charge the capacitor from zero to

rated voltage VDC if the converter is supplied with a constant active power equal to SC.

The time constant τ can be selected to be small to satisfy ripple and transient over

voltages requirement on the DC voltage. A relatively small time constant allows fast

control of active power and reactive power.

2.3 Topologies of the VSC

VSC can be divided into topologies according to the number of levels for converter

output AC voltage; either two-level or multi-level. These are explained in following sub-

sections.

2.3.1 Two-level voltage source converter

Vdc/2

Vdc/2

VCa

VCb

VCc

2Cdc

2Cdc

S1 S3 S5

S4 S6 S2

D1 D3 D5

D4 D6 D2

Figure 2.2: 2-Level, 6-Switch, 3-phase Converter Topology

A two-level bridge is the simplest circuit configuration that can be used to construct a

3-phase VSC. It has been widely used in many applications for a range of power levels.


19

As shown in Fig. 2.2, the converter can generate DC voltage waveform of only two

values, ±Vdc/2, where Vdc is the total DC voltage. Relatively simple power and control

circuitry, small DC capacitor size, small foot print environmentally and same semi

conductor duty ratio, are some of the advantages related to a two-level VSC.

2.3.2 Multi-level voltage source converter

Converters topologies with more than 2-level voltages are known as multilevel

converters. In this type of VSC, the output AC voltage waveform has different level

according to the construction of the VSC. The main requirement for multilevel converter

is to synthesize a sinusoidal voltage from several voltage levels which are obtained from

the DC side capacitor voltages. Multilevel VSCs are divided into different types

according to their application. The multilevel VSC found various applications in power

systems due to its high power density, excellent performance and high reliability. Various

multi-level topologies are explained next.

2.3.2.1 Neutral point diode clamped VSC

Neutral point diode clamped VSC topology is shown in Fig. 2.3. As shown, this

topology uses clamping diodes to limit the dynamic and static over voltages of switching

devices. The clamping diodes are connected to the centre point of the DC bus capacitor.

The advantages of this topology include reasonable small size of the DC capacitors,

small foot print environmentally and good quality AC output waveform. An n-level

diode-clamp converter typically consists of (n-1) capacitors on the DC bus and produces

n-levels of the converter output AC voltage. With a higher number of levels, harmonics

generation is very low which can even avoid the need for filters in some cases. The


20

disadvantages are related to voltage balancing across the DC capacitors, uneven loss

distribution between devices and excessive clamping diodes necessity in higher levels.

N

Vdc/2

Vdc/2

Vdc A

++

+

__

_

O

Figure 2.3: 3-Level, NPC-Converter Topology

2.3.2.2 Flying capacitor VSC

Figure 2.4 shows the 3-level flying-capacitor type VSC topology. This topology uses

capacitors instead of diodes in the diode-clamped topology. For a n-level converter, (n-1)

numbers of DC capacitors at DC bus and (n-1) x (n-2)/2 number of auxiliary capacitors

per phase leg are required. These capacitors are known as floating capacitors.

The AC waveform synthesized by this topology is better than the diode-clamped

topology. The advantages of this topology are the extra ride through capabilities during

power outage and severe disturbances due to large number of storage capacitors, switch

combination redundancy for balancing different voltage levels and avoidance of

harmonic filters in higher level applications. The requirement of an excessive number of


21

storage capacitors on DC side and across the semi-conductor switches in higher number

of levels makes this topology very expensive. The converter control system is complex

and switching losses are higher compared to diode clamped topology.

N

Vdc/2

Vdc/2

Vdc A

+

+

_

_

CfVdc/2

+

_

O

Figure 2.4: 3-Level, Flying Capacitor Converter Topology

2.3.2.3 ANPC (Actively Neutral Point Clamped) converter

Figure 2.5 shows the 3-level ANPC converter topology in which the clamping diodes

are replaced by semi-conductor switches. The disadvantage of uneven loss distribution

across various devices in the neutral point diode clamped converter topology is mitigated

by this newly developed topology.


22

N

Vdc/2

Vdc/2

VdcA

+

+

+

_

_

_

O

Figure 2.5: 3-Level, Actively Neutral Point Clamped Converter Topology

2.3.2.4 Cascaded H-bridge converter

This VSC topology (Fig. 2.6) is a cascaded structure consists of full bridge inverter

units connected in series.

Vdc/4

Vdc/4N

Figure 2.6: 5-Level, Cascaded H-bridge Converter Topology


23

Each unit is fed by a separate DC capacitor and no other circuit to balance the voltage

matching of the switching devices is necessary. There is no need to add the clamping

diodes or voltage balancing capacitors in this topology which results in a relatively

simple construction. Main drawback of this topology is the requirement of several

independent DC power supplies.

2.3.3 Multi-pulse topologies

In a 6-switch converter, the commutation of current takes place six times per cycle. In

case of 12-switch converter, two six-switch converters are connected in series to form a

twelve-switch converter topology. By using the 6-switch converter, the additional multi-

switch topologies in multiples of six can be created to form the required pulse-level. The

24-switch and 48-pulse converters are implemented in some research papers for

application as a STATCOM in power system. The main purpose of using these topologies

is to reduce the voltage and current harmonics in the system. As the number of switches

increases the harmonic distortion in the system decreases proportionally.


24

2.4 PWM for VSC

PWM is the most accepted switching technique in which the comparison between a

fundamental frequency modulating waveform and a fixed carrier frequency triangular

waveform generates the firing pulses for VSC. The magnitude and phase angle of the

generated fundamental frequency component of the converter output AC voltage can be

controlled by varying the magnitude and phase angle of the modulating waveform. In this

technique, the converter output AC voltage contains harmonic frequency components in

addition to fundamental frequency component located around the multiples of the

triangular waveform carrier frequency.

2.4.1 Sinusoidal PWM

The PWM technique used for switching of the semiconductor devices of converter

decides the frequency and nature of the converter output AC voltage so to get the

sinusoidal waveform as an output, sinusoidal PWM technique is used. The sinusoidal

modulating waveform of 60 Hz system frequency is compared with a high frequency

triangular waveform to generate the pulse width modulated firing pulses for converter.

Two different SPWM methods can be implemented to get the required output.

(a) Bi-polar SPWM

In case of bi-polar sinusoidal PWM method, a single fundamental frequency

modulating waveform per phase is compared with the high frequency triangular

waveform. The output voltage switches between –Vdc/2 and +Vdc/2 voltage levels where

Vdc is the total DC voltage. It is the reason why this type of switching is called a bi-polar

PWM switching. As shown in Fig. 2.7, during comparison, when the sinusoidal


25

waveform is greater than the triangular waveform, the PWM trigger signal is high;

otherwise at lower level.

Figure 2.7: Comparison between Sinusoidal and Triangular waveform and PWM Signals

(b) Uni-polar SPWM

In case of uni-polar sinusoidal PWM technique, two same phase fundamental

frequency modulating waveforms with opposite polarity are compared with the high

switching frequency triangular waveforms. The output voltage switches between +Vdc/2

and zero or between zero and –Vdc/2. For this reason, this type of PWM scheme is called

a PWM with a uni-polar voltage switching.


26

2.5 VSC-HVDC Operation

The VSC-HVDC operation can be explained by considering its operating principle,

four-quadrant operation in PQ-diagram and capability to operate in either rectifier or

inverter modes.

2.5.1 Operating principle of VSC-HVDC

The VSC-HVDC operating principle can be explained from the SLD shown in Fig.

2.8, where two AC networks are connected by a VSC-HVDC transmission system.

AC

ACDC

DC

Sending end

Converter-1

Receiving end

Converter-2

Vs1 /_0 Vc1 /_δ1 Vs2 /_0Vc2 /_δ2

VDC

AC system AC systemTransformer Transformer

XT1 XT2

Figure 2.8: Single Line Diagram for VSC-HVDC Operating Principle

The converter transformer connecting AC network and converter can be modeled as

an equivalent series resistance and reactance. The transformer resistance is very small

compared to the transformer reactance so it can be neglected in power calculations. The

phase shift between both sides’ voltages due to transformer reactance is responsible for

active power flow. At sending end, the voltage VS1 at AC bus is considered as a reference

so the converter output AC voltage VC1 has phase shift δ1 with respect to the bus voltage

VS1. The magnitude and phase angle of converter output AC voltage VC1 is controlled by


27

the VSC control system. The active and reactive power flow between the AC system and

the converter depends on the magnitude of the voltages at both sides of the transformer,

the transformer reactance and the phase angle δ1 between them. The active and

reactive power flow between AC bus and converter AC terminals can be expressed as

follows:

The active power at sending end and receiving end is given by,

(2.1)

The reactive power at sending end and receiving end is given by,

(2.2)

The amplitude, phase angle and frequency of fundamental component of converter

output AC voltage VC1 and VC2 can be controlled using the SPWM technique. If the

voltage at the DC side of the converter is VDC which is assumed to be constant then the

fundamental frequency component of converter output AC voltage can be derived from

the following equation:

(2.3)

Where VDC is the voltage on DC side, ω is the angular frequency, Mi is modulation

index and δ is the phase angle between the converter output AC voltage and the AC bus

voltage.

Mi can be defined as the ratio of peak value of voltage of modulating waveform

(fundamental frequency sinusoidal waveform) to the peak value of voltage of carrier

waveform (switching frequency triangular waveform). From Eqn. 2.3, it is seen that the


28

fundamental component of converter voltage depends on the variables Mi, angle δ and

frequency which are independently controlled by VSC controllers. The active and

reactive power can be controlled by controlling the phase angle δ and the amplitude of

the fundamental frequency component of converter AC output voltage VC respectively.

In a VSC-HVDC, the polarity of DC voltage is constant so the power reversal is done

by current reversal in the DC link. The current in DC link flows from higher to lower DC

voltage level. For stable VSC operation and flow of active power, the DC link voltage is

maintained at a desired reference value by using a feedback control loop. The power flow

to and from the AC source can be controlled according to the DC link voltage

requirements. The voltage VDC is measured and controlled in such manner that the DC

current flow can be in either direction depending on the requirement of active power flow

direction.

2.5.2 Four quadrant operation of VSC

Active power flow direction for rectifier or inverter operation and reactive power

flow direction for inductive or capacitive operating mode of VSC can be explained by a

PQ-circle diagram shown in the Fig. 2.9. The diagram explains the ability of VSC to

operate in all four quadrants of PQ-circle and to achieve the independent control of active

and reactive power. The direction of current flow and hence active power flow decides

the rectifier or inverter operation of converter.

In the first quadrant both the active and reactive power are positive which means that

the converter injects both powers to AC system which shows the capacitive mode of

inverter operation. The converter output AC voltage magnitude is higher than AC bus

voltage and leads AC bus voltage by an angle δ.


29

In the second quadrant, the active power is negative and the reactive power is positive

which explains the capacitive mode of rectifier operation. In this case, the converter

output AC voltage amplitude is higher than AC bus voltage but it lags the AC bus voltage

by an angle δ.

Inverter

OperationRectifier

Operation

Capacitive

Mode

Inductive

Mode

Injects P

Injects QAbsorbs P

Injects Q

Absorbs P

Absorbs Q

Injects P

Absorbs Q

δ

δVs

Vc Vs

Vc

δVs

Vc δVs

Vc

P-P

Q

-Q

1st Quadrant2

nd Quadrant

3rd

Quadrant 4th Quadrant

Figure 2.9: P-Q Circle Diagram for VSC Operation

Both the powers in the third quadrant are negative which means converter absorbs

both powers from the AC system which explains the inductive mode of rectifier

operation. In this case, the AC bus voltage magnitude is higher than the converter output

AC voltage and it leads by an angle δ.

In the fourth quadrant, the active power is positive and the reactive power is negative

which explains the inductive mode of inverter operation. Here the converter output AC

voltage leads the AC bus voltage but its magnitude is less than the AC bus voltage.


30

According to the converter MVA capacity and system requirements, converter can

operate in any mode (i.e. capacitive or inductive) of rectifier or inverter operation.

2.5.3 Rectifier- Inverter operation of VSC

Vdc

Vs /_0 Vc /_δ

RT XT

2Cdc

2Cdc

i

IT Idc

VdcT

Figure 2.10: VSC Operation as Rectifier or Inverter

The rectifier or inverter operation of VSC can be explained by considering Fig. 2.10.

AC bus voltage VS can be taken as a reference and VC is the converter AC output voltage

with phase shift δ. The converter transformer can be represented by an equivalent

resistance RT and reactance XT. Two capacitors of same rating 2Cdc are connected in

series across the DC terminals of the converter. The direction of current flow decides the

operation of VSC as a rectifier or inverter.

As shown in Fig 2.11, the AC system voltage VS leads the converter AC output

voltage VC by an angle δ. The active power flows from the AC system to the converter so

the converter operates as a rectifier. As shown in figure, ΔV is the voltage drop across the

transformer impedance which is controlled in order to control the angle δ. In the rectifier

mode of operation, the current Idc is considered positive and the capacitor CDC is


31

discharged through the DC transmission system. The error signal demands the control

circuit for more power from the AC supply. The control circuit thereby generating the

appropriate PWM signals for the switching devices and accordingly, more current flows

from the AC to DC side and the capacitor voltage recovers its predefined value.

Vs

Vc

δ

I

ΔV

Re

Im

Figure 2.11: VSC Operation as a Rectifier

As shown in Fig 2.12, the AC system voltage VS lags the converter AC output

voltage by an angle δ. In this case, the active power flows from the converter to AC

system so the converter operates as an inverter. In the inverter mode of operation, Idc

becomes negative and the capacitor CDC is overcharged. The error signal demands the

controls to discharge the capacitor CDC and return power to the AC system. The PWM

can control both the active power and reactive power independently. Thus, this type of

converter can be used for power factor correction also in addition to power transmission.


32

Vs

Vc

δ

I

ΔV

Re

Im

Figure 2.12: VSC Operation as an Inverter

Depending on the control strategy, VSC can be operated as either inverter or rectifier;

therefore it is often referred to as a converter. Two such converters are often cascaded to

control the power flow between two AC networks. The first converter converts AC

voltage to variable DC link voltage and the second converter converts the DC voltage to

variable AC voltage with fixed or variable frequency.

2.6 Summary

In this chapter, the VSC-HVDC system construction, its principle of operation and

operational function of each component is explained in detail. The different topologies of

VSC and PWM techniques are explained. The principle of operation for the VSC-HVDC

is explained regarding the active and reactive power flow. PQ-circle diagrams are

explained regarding the rectifier/inverter operation and capacitive or inductive mode of

operation.

33

Chapter 3

Control of VSC-HVDC System

3.1 Introduction

VSC-HVDC transmission system has flexibility in terms of controlled quantities

offering two degrees of freedom at each end to provide stable and robust control. As

described in section 2.5, VSC-HVDC system has advantage of independent control of

active and reactive power. The direction of active power flow can be changed without

changing the DC voltage polarity. The reactive power can be controlled independently at

either converter station by controlling converter AC output voltage.

Although both current and voltage control schemes are possible, current control is

generally preferred for its excellent dynamic characteristics and inherent over-current

limitation capabilities. When current control is used, the rectifier input current and

inverter output currents are measured and compared with reference signals; the current

errors are used as inputs to the PWM modulator, which then provides the required

switching signals. The requirements for the control system to operate the VSC-HVDC

system are:

It must be capable of controlling the desired active power in either direction.

The reactive power of system must be controlled independently at both ends.

The DC voltage must be maintained at desired level for power transfer.

Chapter 3 Control of VSC-HVDC System

34

The control modes are chosen according to the application and requirement of the

given power system. The rectifier and inverter controllers take the control of any two of

above mentioned four control strategies according to the system requirements and

operating conditions. VSC1 controllers operate on active power and reactive power

control whereas VSC2 controllers deal with DC voltage and reactive power control (Fig.

3.1).

AC

ACDC

DC

AC Voltage /

Reactive power

Control

AC Voltage /

Reactive power

Control

Active Power

Control

DC Voltage

Control

Converter-1 Converter-2

AC System

Reactance Reactance

AC System

Figure 3.1: VSC-HVDC Control System

3.2 Direct control method

Direct control method is a simple method for the control of VSC-HVDC. The angle δ

and modulation index Mi are controlled directly by using PI controllers. The measured

values are compared with the reference values and the resultant errors are fed to the PI-

controllers. The output of which is used as either modulation index or angle delta. The

modeling of direct control system is provided in section 4.5.3. Though this method is not

used in practical applications, it is useful in comparative studies of operation and

performance of VSC-HVDC.


35

3.3 Vector control method

The P and Q controlled quantities of VSC-HVDC are coupled to each other such that

any change in one quantity strongly affects the other. The vector control method removes

the coupling between these quantities to ensure the independent control of each quantity.

The vector control strategy consists of a cascade control system with faster inner

controllers. The vector controller is accomplished by additional outer current controller

which provides the reference values for inner controller. The outer controllers include

active power controller, reactive power controller, AC voltage controller, DC voltage

controller or frequency controller where the implementation of particular control strategy

depends on the application and operating conditions of VSC-HVDC system. It is not

possible to implement all the control strategies at the same time and requires an extensive

system operation and performance studies for implementation of particular control

method.

For feeding the passive system at receiving end without any local voltage source, it is

necessary to control the frequency and AC voltage whereas the active power and reactive

power control is required in active AC system. However, since the active power flow into

the DC link must be balanced, the DC voltage controller is necessary in any case to

achieve active power balance. On the DC side, the active power input to the DC link must

equal the active power output from the DC link plus the losses in the DC link. Any

difference would result in a change in the DC voltage.

In order to understand the vector control technique of the VSC-HVDC transmission

system, the mathematical model for VSC-HVDC system needs to be derived. The vector


36

control strategy can be derived from the mathematical modeling of the system shown in

Fig. 3.2.

Vdc

Vs Vc

RT LT

Vac

Ls

VSC

2CDC

2CDC

AC Filters

Rf Lf

Cf

It

Rs

Is

IDCt IDC

TransformerSource

Impedance

Figure 3.2: Vector Control System of VSC-HVDC System

The AC filters connected to the VSC-HVDC system behave as pure capacitors at

fundamental frequencies. Hence in the mathematical model presented here, the filter

resistances and inductances can be neglected. From the single line diagram (Fig. 3.2), the

voltage across the transformer, the current to the filter and the voltage across the source

impedance can be obtained in three phase instantaneous form as follows:

The voltage across the source impedance is

(3.1)

The current through the filters is

(3.2)

The voltage across the transformer is


37

(3.3)

From (3.1), (3.2) and (3.3), the following differential equations can be derived:

(3.4)

(3.5)

(3.6)

By using Clark’s transformation matrix (Appendix C), quantities from ABC-frame can be

converted to αβ-reference frame as follows,

(3.7)

Where γ=2π/3.

Equations (3.4), (3.5) and (3.6) can be converted in to αβ frame as follows:

(3.8)

(3.9)

(3.10)

By using the transformation angle Ө derived from a phase-locked loop (PLL), the above

equations are further transferred to the synchronously rotating dq-reference frame, using

Park’s transformation, as follows:


38

(3.11)

(3.12)

(3.13)

(3.14)

The AC filter voltage, the current through the transformer and the AC side voltage of

VSC can be expressed as:

(3.15)

(3.16)

(3.17)

From equation (3.17), the dq-current component through the transformer can be given as,

(3.18)

(3.19)

Equations (3.18) and (3.19) present the relationship between input reference current and

the converter output AC voltage in dq-reference frame.


39

3.4 Limitations

The VSC-HVDC link with its semi-conductor switches has a limited overload

capacity. Large transient currents due to system disturbances can stress or damage the

converter valves. Therefore, a current limit must be implemented in the control system.

Moreover, since the maximum voltage amplitude out of the VSC is limited by the VSC

voltage capability, the produced reference voltage from the vector controller must be

appropriately limited. The current limit Ilim is compared with the current magnitude

computed from the active and reactive reference currents. When the current limit is

exceeded, both the active and reactive reference currents have to be limited. The choice

of how to limit both reference currents will depend on the application. For instance, if the

converter is connected to a strong system, the active component of the reference current

will be given priority, when the current limit is exceeded, to produce more active power.

The produced active power is estimated from the equation

(3.20)

If the converter is connected to a weak system or used to supply a bulk industrial user

(or island), the VSC will give priority to the reactive component of the reference current

to maintain the AC voltage when the current limit is exceeded. The remaining capacity is

then available for active power production. The produced reactive and active power may

be estimated from the following equations:

, (3.21)

Where, is the pre-set maximum reactive reference current.


40

3.5 Outer controller

The outer controller consists of the DC voltage controller, active power controller and

reactive power controller. The proposed simulation model consists of active power and

reactive power controllers at VSC1 and DC voltage and reactive power controllers at

VSC2. The outer controller creates the reference values of the dq-current components for

inner current controllers. The outer controller gains are smaller when compared to the

inner controller to ensure the stability of the complete system.

3.5.1 DC voltage controller

As shown in the Fig. 3.2, the instantaneous active and reactive powers in dq-reference

frame at the AC side of the VSC and the power PDC transmitted on the DC side of the

VSC are expressed as:

(3.22)

(3.23)

(3.24)

Where, is the current flowing in the DC terminals of the VSC.

During steady state operation, the dq-components of voltages are constant at the rated

values. Therefore, the voltage is rated voltage VS and is zero. By considering the

above assumptions, the active and reactive powers are expressed as follows:

(3.25)


41

If the losses in the converter trasformer and converter are neglected, the power at the AC

and DC sides can be considered equal, i.e.

(3.26)

Hence,

(3.27)

The current in the DC line of the VSC is given as:

(3.28)

Any imbalance between the AC and DC powers leads to a change in voltage at the DC

link capacitors, which is given as:

(3.29)

Where, is the current in the DC terminals of the VSC, is the total DC side

capacitance, VDC is the DC voltage across the capacitor and IDC is the DC current flowing

through the transmission line. The value of DC current from equation (3.28) can be used

in equation (3.26), i.e.

(3.30)

(3.31)

From equation (3.30), the value of the reference current id *

is derived as follows:


42

(3.32)

The d-component of the current derived in the equation (3.32) gives the reference current

for the inner current controller for DC voltage control.

3.5.2 Active power controller

The active current reference is obtained from equation (3.25)

(3.33)

Where, Pref is the reference active power. For accurate control of the active power, a

combination of a feedback loop and an open loop is used.

(3.34)

Where, Kp1 and Ki1 are the proportional and integral gains respectively of the active

power controller.

3.5.3 Reactive power controller

A reactive power controller similar to the active power controller is obtained from

equation (3.25) as:

(3.35)

Where, Qref is reference reactive power. Again, same method is used to combine a

feedback loop with an open loop,

(3.36)


43

Where, Kp2 and Ki2 are the proportional and integral gains respectively of the reactive

power controller.

Fig. 3.3 shows active and reactive power control system for VSC1. The control block of

PLL is shown from which the synchronization angle Ө is derived and fed to the ABC to

dq block.


44

PI1 PI2Pref

Pmeas Id meas

ωLIq

Vsd

Vcd+++ __

PI3 PI4Qref

Qmeas Iq meas

ωLId

Vsq

_+++ __

dc

cqcd

V

VVM

222

cd

cq

V

V1tan

Sinusoidal

PWM

Vdc

Firing

Pulses

Outer Current Control Loop Inner Current Control Loop

Inner Current Control Loop

PLL θVsaVsbVsc

abc

dq

VsaVsbVsc

Vsd

Vsq

θSynchronization Block

Vcq

Outer Current Control Loop

+

_

+

Figure 3.3: Active and Reactive Power Control System


45

3.6 Summary

This chapter deals with the control system of VSC-HVDC transmission system. Two

different control systems, direct control and vector control are explained. The VSC-

HVDC mathematical model is provided in this chapter. Block diagram of active power

and reactive power control systems in vector control technique is also provided here.

46

Chapter 4

Methodology

4.1 Introduction

For simulation of the provided VSC-HVDC system, it is necessary to model each and

every component in a realistic manner which can then provide a clearer understanding of

the actual system behaviour. The different components of the system are modeled in

EMTP-RV package. The parameters of voltage sources, filters, transformers, DC

capacitors and overhead DC cables are provided. The complete model of the VSC-HVDC

transmission system can be divided in the following different blocks which are connected

to each other.

4.2 Power system modeling

The power system components such as transformers, filters, VSC, DC capacitors and

DC overhead cables are integrated in VSC-HVDC model (Fig. 2.1). The necessary

calculations to convert the system data and equipment ratings to per unit (pu) values are

provided in Appendix A. The simulated measuring instruments of the software package

are calibrated using these per unit calculations to measure system values.

Chapter 4 Methodology

47

4.2.1 Modeling of weak AC networks

Two, 230 kV (RMS L-L), 100 MVA weak AC networks with Short circuit ratio

(SCR) of 3 are connected through a VSC-HVDC link. A source impedance of 176.05 Ω

is connected at both ends to present the SCR of 3. The weak AC systems presented here

are balanced 3-phase sources at a fixed frequency of 60 Hz.

4.2.2 Modeling of converter transformer

Converter transformer reactance works as a necessary element for power transfer

between AC and DC systems. The transformer configuration used here is a star ground-

star (Yg-Y) which avoids the phase shifting between primary and secondary voltages.

The rating of the converter transformer is 115 MVA, 230 kV primary voltage and 49 kV

secondary voltage (230/49 kV), 60 Hz. The rated DC voltage of ± 40 kV is chosen for

VSC-HVDC which results in the 49 kV (RMS L-L) as a converter transformer secondary

voltage. The rated per unit resistance is Rpu= 0.00375 pu and the reactance is Xpu= 0.12

pu. The resistance of transformer is very small compared to reactance so it is neglected in

power calculations.

The transformer is operating in the linear range and operation under saturation is not

considered. The aforementioned configuration is shown in Fig. 4.1 where three single

phase units are connected in star configuration at primary and secondary sides.


48

Figure 4.1: Converter Transformer Model

4.2.3 Modeling of AC shunt RLC-Filters

The order of harmonics generated by the switching action of the semiconductor

devices of converter depends on the switching frequency and configuration of the

converter. The single-tuned low-pass and high-pass damped RLC filters are connected in

parallel at appropriate location(s) in AC system to mitigate the harmonics. The low-pass

filters are modeled as a series connected RLC branch and high-pass damped filters are

modeled by parallel combination of resistance and inductance connected in series with

the capacitor.

3 identical non-ideal units

YY transformer

inY outY

NeutralW1 NeutralW2

+

xfmr_YY_unitO

xfmr_A

+

xfmr_YY_unitO

xfmr_C

+

xfmr_YY_unitO

xfmr_B

Y

b

a

c

NW2NW1

Yout

b

a

c


49

Figure 4.2: EMTP-RV Filter Modeling and Frequency Scan graph

4th, 5 MVA

5th, 5 MVA

7th, 5MVA

8th, 5 MVA

High pass filtertuned to 6 th25 MVA

High pass filtertuned to 17 th 25 MVA

FILTERS

+

5.51,0.4385,0.25072uF

+

14.11,1.1226,0.251uF

+

29.385

+

1.5uF

+

22.05,1.754,0.25072uF

+

2.91,0.24,0.25072uF

+

0.1871

+

3

+

1.5uF

+

0.0223117


50

The 4th

, 5th

, 7th

and 8th

harmonic frequency low-pass tuned filters and 6th

and 17th

harmonic frequency high-pass damped filters are connected in parallel to make a single

filter unit (Fig. 4.2). The rating of any particular harmonic frequency filter is decided

using the value of that particular (voltage and current) harmonic content. The frequency

scan graph for the filter is also provided in Fig. 4.2, which shows the least impedance for

the particular harmonic frequency component. In case of VSC-HVDC system, the

reactive power compensation is less important due to the capability of VSC to operate as

a reactive power compensator which results in smaller size of the capacitor required for

the filters. As explained earlier, the harmonics in VSC-HVDC are multiples of the

switching frequency of the semiconductor valves so the high-pass damped filters are

required to eliminate these harmonics. The necessary calculations for the filter design are

provided in APPENDIX-B. The filters of 70 MVAR in total are connected at high

voltage side of the transformer and the individual filter ratings are provided in Table 4.1.

The MATLAB code for filter RLC-parameters calculations and values of RLC-

parameters are provided in Appendix B.

Table 4-1 Filter Ratings

Filters at VSC1 Filters at VSC2

Harmonic Order Rating (MVAR) Harmonic Order Rating (MVAR)

4th

5 4th

5

5th

5 5th

5

7th

5 7th

5

8th

5 8th

5

6th

50 6th

25

17th

0 17th

25


51

4.3 Converter Modeling (Rectifier and Inverter)

The converters are most important components of VSC-HVDC system since they

convert AC voltage into DC voltage and vice versa. The converters are composed of

number of devices such as semi conductor switches, diodes, RC snubbers and resistances

and inductances.

4.3.1 Converter topology modeling

The 3-phase, 2-level, and 6-switch VSC topology is implemented for this model. The

output voltage of the converter can have only two levels; –VDC/2 and + VDC /2 where VDC

is the total DC voltage between two DC poles. The modeling of high-power high-

frequency switching device is done by using an ideal controlled switch. This semi-

conductor switch connected with anti-parallel diode make one unit. In actual installation,

there are several units connected in series to get the required voltage level. In this

simulation model, only one unit is used to study the operational performance of the

converter. The voltage pulses generated by SPWM technique in control system are the

firing signals for these switches.

The units are connected in the order of predetermined firing sequence. The units 1

and 4 are connected to phase A, units 3 and 6 are connected to phase B and units 2 and 5

are connected to phase C. As shown in Fig. 4.3, the firing of the switches in the same

phase is complementary to prevent the risk of the short circuit across the DC terminals of

the converter. Therefore, a finite blanking time of a few nano-seconds is added to each

firing signal to achieve some degree of protection. In ideal case, it is considered that all

the switches are ideal without any kind of time delay so no blanking time is added to the


52

firing signals generated in this simulation. RL-snubbers are connected in DC terminals to

provide di/dt limitation for the converters.

Figure 4.3: Converter Model in EMTP-RV

4.3.2 IGBT-Diode module modeling

The IGBT-Diode unit is modeled as shown in Fig.4.4 where the controlled switch is

represented by an ideal switch. The diode is also considered as an ideal element so on-

state losses and switching losses are neglected. The anti-parallel diode operates in the

complementary manner with the IGBT which makes the converter a bi-directional

operating device. As explained earlier in section 2.5.1, the converter can operate in all

four quadrants of V-I plane so the direction of current flow must be bi-directional. The

diode connected in series with IGBT switch serves the purpose of protection due to IGBT

switch being a unidirectional device where the current cannot flow in opposite direction.

firing

Pc

Pb

Pa

DC2

+

1k

P

P2

P1

1

P

P2

P1

3

P

P2

P1

5

P

P2

P1

4

P

P2

P1

6

P

P2

P1

2

+

1mH

+

1mH

+

1k

f1f2f3f4f5f6

DC1

firing


53

Each unit is equipped with a RC snubber connected across the switch and the diode in

order to reduce the rate of change of voltage (dv/dt) during the switching of the semi

conductor devices. The switching frequency of converter is 1440 Hz which results in 24

ON-OFF operations per cycle.

Figure 4.4: IGBT Valve Modeling

In actual installations, during each process of switching from one state to another

state, there is a power loss in the switching device which is directly related to the

DC TERMINAL

AC TERMINALP1

P2

+

1.5uF

0.7

0

+

16k

+

0.0

1m

H

+

0.0

1m

H

+

0.0

0001

P

+

1.5uF

0.7

0

+

+

1.5uF

+

16k+

16k

+

0.0

000000001

+

0.0

000000001


54

switching frequency. During the switching action, the rate of change of voltage and

current is very high which can expose the device to failure so the RC snubber circuit in

parallel with semiconductor device is employed for protection. The optimized RC

snubbers reduce the stress on the switching devices by limiting the voltage stresses at turn

ON and turn OFF. The series resistance and inductors also reduce the rate of change of

current stresses (di/dt) in the semiconductor devices. The rating of the RC snubber is

estimated from the system parameters like voltage and current rating of the devices and

the total amount of power loss through the valve.

4.4 DC capacitors and transmission system modeling

DC capacitors are connected across the DC terminals of the VSC to maintain the DC

voltage required for the dynamics of the system. The PWM switching of converter

creates the harmonics in the current flowing in DC transmission system. This current

generates ripple in the DC voltage; magnitude of which depends on the DC capacitor size

and converter switching frequency. The rating of capacitor is decided according to the

DC voltage level and the rating of VSC. Two capacitors of 400 µF are connected in

cascade form with the common terminal grounded. An identical DC capacitor is used at

both ends of the DC system.

The DC system can be in the form of overhead transmission line, underground DC

cables or overhead DC cables. DC transmission line is modeled in the same way as an

AC transmission line. The bi-polar transmission system is implemented here with a

grounded midpoint providing the return path during monopolar operation. The power can


55

still be transmitted by one pole in case of failure or scheduled maintenance of other pole

in actual systems. The model presented here is based on the overhead DC cables of 75

km length; divided into three T-sections of 25 km on each pole (Fig 4.5).

Figure 4.5: DC Transmission System Modeling

The resistance, inductance and capacitance of the DC cable are calculated from the

data collected for the per unit length of cable. The DC system used here is a bi-polar

transmission system so two identical cables are considered for positive and negative

poles. The RLC-parameters per unit length are provided in Table 4-2.

Table 4-2 DC Cable RLC-Parameters

Parameters Values per km Total Value

Resistance RDC 0.013 Ω/ km 1 Ω

Inductance LDC 0.16 mH/ km 12 mH

Capacitance CDC -- 100 µF

p1

p2

p3

p4

+

0.25

+

3mH

+

0.25

+

0.25

+

3mH

+

0.25

+

33uF

+

33uF

+

0.25

+

3mH

+

0.25

+

3mH

+

33uF

+

33uF

+

0.25

+

3mH

+

0.25

+

3mH

+

33uF

+

33uF

+

3mH

+

3mH


56

4.5 Control system modeling

The controller comprises of measurement devices by which various parameters are

measured including voltage, current, active power, reactive power, voltage phase shift,

power factor angle, DC voltage and DC current at both ends of the DC system. The

filtering devices are implemented to reduce the noise and small disturbances in the

signals coming from the power system to alleviate the risk of malfunctioning of the

control system in the event of faults, disturbances or transients. As mentioned in chapter

3, four different controllers, two at each end of VSC-HVDC link are implemented. The

various control system blocks are explained in following sub-sections.

4.5.1 Carrier frequency generation

Figure 4.6: Carrier Frequency Generation Block

Fig. 4.6 explains the generation of the triangular waveform. The square wave

representing the converter switching frequency of 1440 Hz is integrated to get a

triangular waveform. The resultant triangular waveform is then biased to get the

symmetric positive and negative values of ± 1 per unit.

Carrier Frequency (1440 Hz) triangular wavefroms

step

rc rv

++

-

c1.035

c

0

1

2

1440


57

4.5.2 Sinusoidal pulse width modulation (SPWM)

As explained in section 2.4, bi-polar SPWM method is implemented in this model.

The modulation index Mi is defined as,

(4.2)

Where, is peak amplitude of the fundamental frequency modulating waveform

and is the peak amplitude of the switching frequency triangular waveform.

The VSC output AC voltage should be sinusoidal with controlled magnitude, phase

angle and frequency. The sinusoidal modulating waveform at 60 Hz frequency is

multiplied with the modulation index Mi which results in changed amplitude according to

the value of modulation index. This resultant waveform is compared with the 1440 Hz

frequency triangular waveform to generate the firing pulses for the converter (Fig. 4.7).

The frequency of triangular waveform establishes the converter switching frequency

which is kept constant. Three, 120 degree phase shifted waveforms (a, b and c in Fig 4.7)

are compared with triangular waveform to get three 120 degree phase shifted firing

pulses for 3-phase legs of the converter. Each firing pulse is then extrapolated into two

complementary firing pulses to operate two switches in the same phase leg.


58

Figure 4.7: SPWM Block

4.5.3 Direct Control system modeling

As explained in section 3.2, the direct control system modeling is provided in this

sub-section. On the sending end, active power and AC filter bus voltage at VSC1 are

controlled whereas at receiving end side, DC voltage and AC filter bus voltage at VSC2

are controlled.

4.5.3.1 Angle delta block

The delta block comprises of 3-phase sinusoidal waveform generator which generates

three 120 degree phase shifted unitized sine waves (sin0, sin1, sin2 in Fig. 4.8). The angle

delta derived from the active power or DC voltage controller is added to the phase angle

Carrier Frequency(1440 Hz) triangular wavefroms

Sinusoidal Modulating signals

Firing Pulses to the Converter

Modulation Index

Compare

21

PROD

1

2

PROD1

2

PROD1

2

a

b

c

p4

p1

Compare

21

Compare

21

p6

p30

1

p2

p50

1

0

1

-1

-1

-1

0

1

0

1

mi

0

1

steprc rv

++

-

c

1.035

c0

1

2

1.5

0.6

11440

1440

1440

1440


59

of these sinusoidal waveforms generated by sine wave generator. The resultant

waveforms are then multiplied by modulation index and compared with triangular

waveforms.

Figure 4.8: Angle delta block

4.5.3.2 VSC1 active power control

The active power measured from the system is compared to the reference value of

active power; the resultant error is then passed through the PI-controller (Fig. 4.9). The

output of PI-controller is angle delta which is used to create a phase shift in the firing

pulses of the converter.

Figure 4.9: Active power control block for direct control method

f(u)1 ?s

f(u)1

f(u)1

sin0

sin1

sin2

SUM1

2

delta

f(u)

sin0

sin1

sin2

delta

Active power controller

Angle Delta

PI controller

Kp

Ki

out_ini

u

out

c

1

+-

+

P_ref_rec

?s

++

-

c1.65

c10.75

0

1

P_pu_rec?s

100000000f(s)

fs1

P_pu_rec

P_rec


60

4.5.3.3 VSC1 AC filter bus voltage control

AC voltage measured at the filter bus of VSC1 is converted into per unit which is then

compared with the reference value; the resultant error is then fed to PI-controller (Fig.

4.10). The output of this controller is a modulation index which is used as a multiplier for

the sinusoidal waveforms (sin0, sin1, sin2) as shown in Fig. 4.8.

Figure 4.10: AC voltage control block at VSC1 for direct control

4.5.3.4 VSC2 DC voltage control

At VSC2, DC link voltage is controlled by the angle delta. The DC voltage measured

at the DC terminals of VSC2 is compared with the reference value and the resultant error

is then fed to the PI-controller (Fig. 4.11). The output of PI-controller is angle delta

which is then fed to the delta block.

Modulation Index

AC Bus voltage controller

PI controller

Kp

Ki

out_ini

u

out

c

0.83

step

+-

+

V_ref_rec

?s

++

-

c0.4

c10.5

V_bus_r


61

Figure 4.11: DC voltage control block at VSC2 for direct control

4.5.3.5 VSC2 AC filter bus voltage control

The AC voltage measured at the VSC2 filter bus is converted to per unit quantity

which is compared to the reference AC voltage. The resultant error is then fed to the PI-

controller (Fig. 4.12). The output of PI-controller is modulation index which is fed to

PWM block.

Figure 4.12: AC voltage control at VSC2 for direct control

Angle Delta

DC Link Voltage controller

PI controller

Kp

Ki

out_ini

u

out

c

1

0

+-

+

Vdc_ref_inv

?s

++

-

c1.65

c10.75

Vdc_inv


Modulation Index

PI controller

Kp

Ki

out_ini

u

out

c

0.89

step

+-

+

V_ref_inv

?s

++

-

c10.5

c0.25

V_bus_i


62

The sinusoidal waveforms coming out of delta block are multiplied by modulation

index to control the amplitude of the sine waveforms which are then compared with the

triangular waveforms. Figure 4.13 shows the direct control system (rectifier controller

and inverter controller) modeled in previous sections using EMTP-RV.

Figure 4.13: Direct control system for VSC-HVDC

RECTIFIER CONTROL BLOCK

Active power controller

Angle Delta

Modulation Index


INVERTER CONTROL BLOCK

DC Link Voltage controller


Modulation Index

Angle Delta

f1f2f3f4f5f6

a

b

c

mi

p6p5p4p3p2p1

PWM

PULSE

pwmpulse_invPI controller

Kp

Ki

out_ini

u

out

c

1

+-

+?s

P_ref_rec

++

-

c

0.65

c

10.75

PI controller

Kp

Ki

out_ini

u

out

c

0.83

0

+-

+?s

V_ref_rec

++

-

c

0.4

c

10.5

0

1.2

?s

0.7

1

mi_

rec

sin0

sin1

sin2

delta

Delta_Block_1

i1i3i4

i2i5i6

o1o2o3

o4o5o6

DEV8

timedelay

1

100000000

?sP_pu_rec

f(s)

fs1

f1f2f3f4f5f6

a

b

c

mi

p6p5p4p3p2p1

PWM

PULSE

pwmpulse_inv

c

1

0

+-

+?s

Vdc_ref_inv

++

-

c

1.65

c

10.75

PI controller

Kp

Ki

out_ini

u

out

c

0.83

0

+-

+?s

V_ref_inv

++

-

c

10.5

c

0.25

sin0

sin1

sin2

delta

Delta_Block_2

-1

i1

i3i4i2

i5i6

o1

o2o3o4

o5o6

DEV7

timedelay

1.5

?s

0.7

1

lim

1

PI controller

Kp

Ki

out_ini

u

out

rectifier_firing

inverter_firing

V_bus_r

a

Vdc_inv

b

c

V_bus_i

P_rec


63

4.5.4 Vector control system modeling

According to the mathematical model of VSC-HVDC and vector control technique

provided in chapter 3, EMTP-RV blocks are developed and explained in this sub-section.

On the sending end, active and reactive power between filter bus and VSC1 are controlled

whereas on receiving end, DC voltage at VSC2 and reactive power flow between filter

bus and VSC2 are controlled.

4.5.4.1 Modulation Index and Angle delta Block

The modulation index Mi and the angle δ are calculated from the mathematical

representation of the controller output voltage components Vcd and Vcq in synchronously

rotating reference frame. The voltage (Vdc_rec and Vdc_inv) on the DC side of the

converter is measured and converted to per unit value. The angle δ is added to the phase

angle of the control voltages generated locally by the sinusoidal waveform generator.

Equation (4.1) shows the mathematical representation of modulation index Mi and angle

δ,

(4.1)

The aforementioned quantities are realized by using the control block shown in Fig.

4.14 where three sinusoidal modulating waveforms (sina, sinb, sinc) are generated for 3-

phase SPWM.


64

Figure 4.14: Modulation Index and Angle Delta Block

4.5.4.2 Active power control at VSC1

The active power control block consists of outer (Fig. 4.15) and inner current (Fig.

4.16) control loops.

Figure 4.15: Outer current controller loop for active power control block

SINEWAVE GENERATOR

PROD1

2

SUM1

2

SQRT1 DIV

1

2

f(u)1

f(u)1

f(u)1

DIV1

2

Vcq

Vcd

Vdc_rec

mi

sina

sinb

sinc

f(u)

ATAN1

PROD1

2

SUM1

2

Vcd

Vdc_rec

mi

sina

sinb

sinc

Outer current controller whichcontrols the Active power at VSC-1

++

-

error_p_rec

PI controller

Kp

Ki

out_ini

u

out

1.25

-1.25

11

100000000

?s

P_pu_rec

c10

c0.5

f(s)

0

+-

+

P_ref_rec

?s

c0.45

c0.15

P_rec

P_rec

Id_reference


65

Active power reference (P_ref_rec) is compared with the measured active power

(P_rec) of the AC system and the resultant error (error_p_rec) is passed through a PI-

controller. As explained in sub-section 3.5.2, the d-component of current controls the

active power flow in the system so the output of the PI controller is a reference current

(Id_reference). This reference current is compared with the d-component of actual AC

system current (Id_rec) and the generated error gives the voltage (error_vd_rec) after

passing through PI-controller. According to the vector control strategy explained in

chapter 3, this voltage is compared with system voltage (vd_rec) and cross coupling term

to get the required controller output. The output of inner current control loop is a voltage

component Vd_out_rec.

Figure 4.16: Inner current controller loop for active power control block

Inner current Id controller

c75

++

-

error_id_rec

0

.10

4

PI controller

Kp

Ki

out_ini

u

out

c5

3.14

?s

-3.14

1

vd_out_rec

++

+

-

error_vd_recc0

vd_rec

id_rec

Id_reference

vd_rec


66

4.5.4.3 Reactive power control at VSC1

The reactive power controller consists of outer (Fig 4.17) and inner current control

loops (Fig 4.18). As explained in the sub-section 3.5.3, the q-component of the current

controls the reactive power flow in the system. In the outer current control loop, reactive

power reference (Q_ref_rec) is compared with the measured reactive power (Q_rec). The

error is than passed through the PI-controller which gives output in the form of reference

current (Iq_reference) for inner current control loop. This reference current is compared

with the q-component of actual AC system current (Iq_rec) and the generated error gives

the voltage (error_vq_rec) after passing through PI-controller. According to the vector

control strategy explained in chapter 3, this voltage is compared with system voltage

(vq_rec) and cross coupling term to get the required controller output. The output of inner

current control loop is a voltage component Vq_out_rec.

Figure 4.17: Outer current controller loop for reactive power control block

Outer current controller whichcontrols the Reactive power at VSC-1

+-

+?s

Q_ref_rec

PI controller

Kp

Ki

out_ini

u

out

c0.1

c10

++

-

error_q_rec

1

100000000

?s

Q_pu_rec 1

-1

1

c

0

C11c

-0.25

f(s)

0

Q_rec

Iq_reference

Q_rec


67

Figure 4.18: Inner current controller loop for reactive power control block

4.5.4.4 DC voltage control at VSC2

The VSC2 controller supervises the DC link voltage of the VSC-HVDC transmission

system. DC voltage controller consists of outer control loop (Fig 4.19) where the

reference DC voltage (Vdc_ref_inv) is compared with actual DC link voltage (Vdc_inv)

and error is fed to the PI-controller which generates the output in the form of reference

current (Id_reference).

Figure 4.19: Outer current controller for DC voltage control system

Inner current Iq controller

c2

3.14

?s

-3.14

1

Vq_out_rec

PI controller

Kp

Ki

out_ini

u

out ++

-

+

error_vq_rec

++

-

error_iq_rec

0.1

04

c

50

c0

iq_rec

vq_recIq_reference

vq_rec

Outer current controller whichcontrols the DC voltage at VSC-2

++

-

error_p_inv

PI controller

Kp

Ki

out_ini

u

out

1

0.8

1.2

c

1

c

25

c

1

+-

+?s

Vdc_ref_inv

0

c

1

Vdc_invVdc_inv

Id_reference


68

This reference current is compared to the actual d-current component (id_inv) of the

AC system in inner current control loop (Fig 4.20). The output is in the form of voltage

which is then compared to d-component of system voltage (vd_inv) and cross-coupling

term to get the output in the form of voltage (vd_out_inv).

Figure 4.20: Inner current controller for DC voltage control system

4.5.4.5 Reactive power control at VSC2

The reactive power flowing between VSC2 and neighboring AC system is controlled

by reactive power controller at VSC2. The reactive power controller consists of outer

(Fig 4.21) and inner current (Fig 4.22) control loops as well.


+-

-

error_id_inv

0.1

04

PI controller

Kp

Ki

out_ini

u

out

vd_out_inv

1

-3.14

?s

3.14

++

+

-

error_vd_inv

c C2

35

c

1

c

3

vd_inv

id_inv

Id_reference

vd_inv

iq_inv

id_inv


69

Figure 4.21: Inner current controller loop for reactive power control block

Figure 4.22: Outer current controller loop for reactive power control block

In the outer current control loop, reactive power reference (Q_ref_inv) is compared

with the measured system reactive power (Q_inv). The error is than passed through the

PI-controller which gives output in the form of reference current (Iq_reference). This q-


+-

+

Q_ref_inv

?s

c

20

++

-

error_q_inv

1

Q_pu_inv?s

1000000001

-1

1

c

0

c

-0.25

f(s)

0

PI controller

Kp

Ki

out_ini

u

out

c

0.25

Q_inv

Iq_reference

Q_inv


vq_out_inv

1

-3.14

?s

3.14

PI controller

Kp

Ki

out_ini

u

out ++

-

+

error_vq_inv

++

-

error_iq_inv

0.1

04

c25

c

0.5

c

2

vq_inv

iq_inv

Iq_reference

iq_inv

vq_inv

id_inv


70

component of the current controls the reactive power flow in the system so it is used as a

reference current for the inner current controller and compared with q-component of

measure AC current (iq_inv). The error (error_iq_inv) gives voltage after passing through

PI-controller. This voltage is then compared to the q-component of voltage (vq_inv) and

cross coupling term to get the controller output voltage (vq_out_inv).

4.5.5 PI-Controller Gains

The PI-controller gains for all four controllers are provided in Table 4-3. From the

table it is evident that the gains of the inner current controllers are higher than outer

current controller to ensure the stability of the system. Proportional gain KP helps to reach

the reference value quickly whereas the integral gain KI ensures zero steady state error.

Table 4-3 VSC-HVDC PI-controller gains

Controllers

Outer current controller

gains

Inner current controller

gains

KP KI KP KI

Active power (VSC1) 0.15 10 5 75

Reactive power (VSC1) 0.1 10 2 50

DC voltage (VSC2) 1 25 3 35

Reactive power (VSC2) 0.25 20 2 25

Figure 5.23 shows the power system modeling of proposed VSC-HVDC simulation

model in EMTP-RV. Figure 5.24 shows the rectifier control block and Fig 5.25 shows the

inverter control block of proposed VSC-HVDC system model with vector control

strategy.


71

Figure 4.23: VSC-HVDC model created using EMTP-RV software package

RECTIFIER CONTROL BLOCK INVERTER CONTROL BLOCK

DC TRANSMISSION SYSTEMVOLTAGE SOURCE CONVERTER (VSC) -1 VOLTAGE SOURCE CONVERTER (VSC) -2 RECEIVING END AC TRANSMISSION SYSTEMSENDING END AC TRANSMISSION SYSTEM

+

AC1

230kVRMSLL /_0

FIL

TE

RS

RLCFILTERS

+

230kVRMSLL /_0

AC2P ic

60Hz

P_rec+

467mH

?v

L1

Qic

60Hz

Q_rec

i(t)?s

I_inv

DC2 Pc

Pb

PaDC1

fir

ing

33

VSC2

i(t)

?s

I_rec

P ic

P_inv

60Hz

FIL

TE

RS

RLCFILTERS

1 2

230/49

YY2

+

C37

0.075uF

+

L2?v

467mH

+

C1

0.075uF

Q ic

Q_inv

60HzDC2Pc

Pb

Pa DC1

fir

ing

33

VSC1

v (t)

V_source_rec

v (t)?s

V_conv_recv (t)

V_conv_inv

?s v (t)

V_source_inv

1 2

49/230

YY1

rectifier_firing

P_rec

Q_rec

Vdc_rec

vd_rec

iq_rec

vq_rec

id_rec

1

rectifier_controllerI_c_inv

I_b_inv

I_a_inv

V_bus_invvd_inv

iq_inv

vq_inv

id_inv

DEV2

conversion

inverter_firing

Q_inv

Vdc_inv

vd_inv

iq_inv

vq_inv

id_inv

2

Inverter_controller

BA C

Fault_1

p1

p2

p3

p4

Vd

c_

re

c

Vd

c_

inv

I_c_rec

I_b_rec

I_a_rec

vd_recV_bus_rec

iq_rec

vq_rec

id_rec

DEV11

BA C

DEV1

Fault_2

I_a_rec

I_a_rec

I_c_rec

I_c_rec

I_b_rec

I_b_rec

P_inv

V_source_rec

I_b_inv

I_b_inv

I_a_inv

I_a_inv

I_c_inv

I_c_inv

c

b

a

V_conv_inv

c

b

aV_conv_rec

V_source_inv

V_bus_rec

cba

V_bus_rec

V_bus_invabc

V_bus_inv

P_rec

P_rec

Q_rec

Q_rec Q_inv

Q_inv

Vdc_rec

Vdc_rec

Vdc_inv

Vdc_inv

rectifier_firing

inverter_firing


72

Figure 4.24: Rectifier Control Block of Proposed VSC-HVDC model



Inner current Iq controller

Outer current controller whichcontrols the Active power at VSC-1

f1f2f3f4f5f6

a

b

c

mi

p6p5p4p3p2p1

PWM

PULSE

DEV1

pwmpulse_rec

c75

++

-

error_id_rec

0

.10

4

PI controller

Kp

Ki

out_ini

u

out

c5

c2

3.14

?s

-3.14

1

Vq_out_rec

PI controller

Kp

Ki

out_ini

u

out ++

-

+

error_vq_rec

++

-

error_iq_rec

0.1

04

c

50

+-

+?s

Q_ref_rec

3.14

?s

-3.14

1

vd_out_rec

++

-

error_p_rec

PI controller

Kp

Ki

out_ini

u

out

PI controller

Kp

Ki

out_ini

u

out

c0.1

c10

++

-

error_q_rec

++

+

-

error_vd_rec

1

100000000

?s

Q_pu_rec

1.25

-1.25

1

1

-1

1

1

100000000

?s

P_pu_rec

c

0

C11

c10

c0.5

c-0.25

f(s)

f(s)

0

0

+-

+

P_ref_rec

?s

c0.45

c0.15

c0

c0

Vcq

Vcd

Vdc_recmi

sin0

sin1

sin2

DEV2

Mi_delta_reci1

i3i4i2

i5i6

o1

o2o3o4

o5o6

timedelay

DEV12

P_rec

Q_rec

Vdc_rec

vd_rec

iq_rec

vq_rec

id_rec

rectifier_firing

Iq_reference

P_rec

Q_rec

Id_reference

Vdc_rec

vd_rec

iq_rec

vq_rec

id_rec

rectifier_firing


73

Figure 4.25: Inverter control block for proposed VSC-HVDC model

Outer current controller whichcontrols the DC voltage at VSC-2




f1f2f3f4f5f6

a

b

c

mi

p6p5p4p3p2p1

PWM

PULSE

pwmpulse_inv

DEV4

+-

-

error_id_inv

0.1

04

PI controller

Kp

Ki

out_ini

u

out

vq_out_inv

1

-3.14

?s

3.14

PI controller

Kp

Ki

out_ini

u

out ++

-

+

error_vq_inv

++

-

error_iq_inv

0

.10

4

c25

vd_out_inv

1

-3.14

?s

3.14

++

-

error_p_inv

PI controller

Kp

Ki

out_ini

u

out

++

+

-

error_vd_inv

1

0.8

1.2

c C2

35

c

1

c

25

c

1

+-

+?s

Vdc_ref_inv

0

c

1

c

0.5

+-

+

Q_ref_inv

?s

c

20

++

-

error_q_inv

1

Q_pu_inv?s

1000000001

-1

1

c

0

c

-0.25

f(s)

0

PI controller

Kp

Ki

out_ini

u

out

c

2

c

3

c

1

c

0.25

Vcq_inv

Vcd_inv

Vdc_invmi

sin0

sin1

sin2

DEV6

Mi_delta_inv

i1i3

i4i2i5

i6

o1o2

o3o4o5

o6

timedelay

DEV14

Q_inv

Vdc_inv

vd_inv

vq_inv

inverter_firing

iq_inv

id_inv

Iq_reference

Id_reference

Q_inv

Vdc_inv

Vdc_inv vd_inv

iq_inv

iq_inv

vq_inv

id_inv

id_inv

inverter_firing


74

4.6 Summary

In this chapter, all the components of the VSC-HVDC system have been explained

regarding required rating and the modeling technique adopted. Modeling of AC and DC

Power system, transformers, AC filters, IGBT-diode unit and VSC is done here. Two

control methods; direct control and vector control method are explained and detailed

modeling of control blocks is provided. For vector control technique, inner and outer

current control loops for all four controllers are modeled and proportional and integral

gains of PI-controllers for all controllers are also provided in this chapter. The VSC-

HVDC system model developed in EMTP-RV software package is provided here.

75

Chapter 5

Simulation Results and Analysis

5.1 Introduction

This chapter focuses on the VSC-HVDC performance assessment during steady state

and dynamic state; step changes in active power, reactive power/AC voltage and DC

voltage, and active and reactive power flow reversals using the EMTP-RV simulation

model. The simulation results with direct control and vector control techniques are

provided and comparison is done regarding independent control of quantities (Active

power, reactive power/AC voltage). Table 5-1 presents the system parameters and ratings

of the VSC-HVDC system. Since the control systems of the VSC-HVDC use high

frequency (1440 Hz) sinusoidal PWM method, the VSC-HVDC model is simulated with

a time step of 5 µs. By using a small time step, it is possible to observe the system

response comprehensively during system start-up, step changes and fault applications.

However, this does slow down the simulation process and generates a huge amount of

data for signal processing purposes.

Table 5-1: Parameters of VSC-HVDC Simulation Model

Parameters Rating

System Power Level 100 MVA

Frequency 60 Hz

Converter Transformer Primary voltage 230 kV

Converter Transformer Secondary voltage 49 kV

DC Voltage ± 40 kV

Switching Frequency of PWM 1440 Hz

Source Impedance (Inductance) 176.1 Ω (467 mH)

DC Cable Length 75 km

AC Filter Rating 70 MVAR

Chapter 5 Simulation Results and Analysis

76

5.2 Steady state operation-Direct control method

The steady-state operational performance of the VSC-HVDC system with direct

control scheme is investigated by considering various operating conditions and reference

values of the system quantities.

5.2.1 Case study-1

Case study-1 presents the operation of VSC-HVDC system in an active power flow

state with constant AC and DC voltages. The active power of 1 pu flows from VSC1 to

VSC2 (Fig. 5.1a) whereas the filter bus AC voltages at both ends (Fig. 5.1b & 5.1c) of the

system and the DC voltage are maintained constant at 1 pu (Fig. 5.1d).

Figure 5.1: Steady-state operation of VSC-HVDC with direct control method (a) 1 pu active power

flow from VSC1 to VSC2 (b) 1 pu AC bus voltage at VSC1 filter bus (c) 1 pu AC bus voltage at VSC2

filter bus (d) 1 pu DC link voltage


77

It is evident that system attains all the reference values within the time of 1 s. There

are some transients seen in active power, AC voltages and DC voltage during the system

start-up which decays within 0.4 s.

5.2.2 Case-study-2

Figure 5.2a shows the scenario of 1 pu active power flow from VSC2 to VSC1 with

filter bus AC voltages at either end (Fig. 5.2b & 5.2c) and DC voltage (Fig. 5.2d)

maintained at 1 pu. Again, it is seen that the system operates with same stability as in the

previous case which proves the ability of VSC-HVDC to operate in bi-directional power

flow mode. Active power reaches the reference value within 1 s.

Figure 5.2: Steady-state operation of VSC-HVDC with direct control method (a) 1 pu active power

flow from VSC2 to VSC1 (b) 1 pu AC bus voltage at VSC1 filter bus (c) 1 pu AC bus voltage at VSC2

filter bus (d) 1 pu DC link voltage


78

The DC voltage has nearly 50 % overshoot before reaching the reference value and the

filter bus AC voltages at VSC1 and VSC2 track the reference value within 1s.

5.3 Step changes-Direct control method

In this case, step changes in active power, DC voltage and AC voltages are

implemented in all four VSC-HVDC controllers. Two of these disturbances are applied at

the converter VSC1 which controls (a) the active power between AC system and DC

transmission system and (b) AC voltage at VSC1 filter bus and the other two disturbances

are applied at the VSC2 which controls (c) the DC voltage and (d) AC voltage at VSC2

filter bus. By applying steps in all four controllers at different times, VSC-HVDC

system’s capability of recovery after disturbances is studied.

As shown in Fig. 5.3a, initially active power of 0.5 pu flows from VSC2 to VSC1. A

negative step of 25% is applied at 1.5 s for duration of 0.5 s in active power controller at

VSC1. First, the active power follows the reference value with little overshoot and

settling time of 0.15 s. Second, it can be seen that the change in active power affects the

AC voltages on both AC systems by creating some transients when the step is applied

and a clear effect in the DC voltage is also observable.

Next, in Fig. 5.3b, initially AC voltage at VSC1 filter bus is at 1 pu. A negative step of

10% is applied at 2.5 s for duration of 0.5 s in the AC voltage at VSC1 filter bus. It can

be seen that the change in AC voltage affects neither the active power on same side nor

the AC voltage on other side of the VSC-HVDC system and DC link voltage.


79

In Fig. 5.3c, initially AC voltage at VSC2 filter bus is at 1 pu. At 3.5 s negative step of

10% is applied for 0.5 s in the AC voltage at VSC2 filter bus. The step in AC voltage at

VSC2 affects neither the active power and AC voltage at VSC1 nor the DC voltage at

VSC2.

In Fig.5.3d, initially the DC voltage is maintained constant at 1 pu which is followed

by a negative step of 10% applied at 4.5 s for 0.5 s in DC voltage controller at VSC2.

First, the transition is observed to be rapid, well controlled with overshoot of about 10%

and settling time of 0.5 s. Second, it can be seen that the change in DC voltage affects

the AC voltages on both sides of the VSC-HVDC system. The active power at VSC1 gets

some disturbance due to step in DC voltage controller at VSC2.

Figure 5.3: Dynamic-state operation of VSC-HVDC with direct control method (a) 0.5 pu active

power flow from VSC2 to VSC1 (b) 1 pu AC bus voltage at VSC1 filter bus (c) 1 pu AC bus voltage at

VSC2 filter bus (d) 1 pu DC link voltage


80

Sequence of events:

Time Action

1.5-2 sec 25 % negative step in active power which flows from VSC2 to VSC1

2.5-3 sec 10 % negative step in reactive power flowing from AC system to VSC1

3.5-4 sec 10 % negative step in reactive power flowing from VSC1 to AC system

4.5-5 sec 10 % negative step in DC link voltage

5.4 Active power flow reversal-Direct control method

In Fig. 5.4a, the initial reference value of the power flow is at +0.5 pu in the direction

from VSC2 (rectifier mode) to VSC1 (inverter mode). At 1.5 s, the reference value is

reversed from +0.5 pu to -0.5 pu. It can be seen that the active power follows the

reference rapidly without any oscillation and reaches the new reference of -0.5 pu within

0.4 s. During this power reversal, some transients are observed in AC voltages at both

ends as shown in Fig. 5.4b and Fig. 5.4c and DC voltage has large overshoot of nearly 40

% which stabilizes within 0.5 s.

In Fig. 5.4b, initially the AC voltage at VSC1 filter bus is at 1 pu. At 2.5 s, the

reference value is changed to 0.9 pu. The AC voltage follows the reference value quickly

and stabilizes within 0.25 s without any kind of oscillations. It is observed that this

change does not create any effect on active power at VSC1 or AC voltage and DC voltage

at VSC2.

In Fig. 5.4c, initially the VSC2 filter bus AC voltage reference is at 1 pu. At 3.5 s, the

reference value is changed to 1.1 pu and as seen, the VSC2 filter bus AC voltage follows

the reference value within 0.2 s without any transients.


81

Next, Fig. 5.4d shows the variation of DC voltage throughout the process of changes

applied to other controllers in the system as shown in Fig 5.4a, 5.4b and 5.4c. During

active power flow direction change, there is a overshoot of 35 % in the value of DC

voltage. The change in AC voltage at both ends does not have any effect on DC voltage.

Figure 5.4: Steady-state operation of VSC-HVDC with direct control method (a) Initially 0.5 pu

active power flow from VSC2 to VSC1 (b) Initially 1 pu AC bus voltage at VSC1 filter bus (c) Initially

1 pu AC bus voltage at VSC2 filter bus (d) DC link voltage is maintained constant at 1 pu

Sequence of events:

Time Action

1.5 sec Active power flow direction changes from +0.5 pu to -0.5 pu

2.5 sec Filter bus voltage (VSC1) value changes from 1 pu to 0.9 pu

3 sec Filter bus voltage (VSC2) value changes from 1 pu to 1.1 pu

DC voltage is maintained constant at 1 pu


82

5.5 Steady state operation -vector control method

The steady-state operational performance assessment of the VSC-HVDC system

comprises of vector control technique is investigated by considering various operating

conditions and reference values of system quantities. The system modeled here is bi-

directional so active power and reactive powers can flow in both directions according to

the system requirements and operating conditions.

5.5.1 Case study-1

Case study-1 presents the operation of VSC-HVDC system in an active power flow

state only. The active power of 0.45 pu flows from VSC2 to VSC1 (Fig 5.5a) whereas the

reactive power flow at both ends (Fig 5.5b & 5.5d) of the system is zero and the DC

voltage is maintained at 1 pu (Fig 5.5c). It is evident that system attains all the reference

values within the time of 0.4 s. There are some transients seen in active power, reactive

powers and DC voltage during the system start-up with decays within 0.3 s.

Active power and DC voltage get the overshoot of nearly 25 % and 50 % respectively

which may be reduced by further optimizing the gains of PI-controllers. The reactive

powers at both ends achieve the reference values within 0.4 s after initial negative and

positive transients along the reference value.


83

Figure 5.5: Steady-state operation of VSC-HVDC (a) 0.45 pu active power flow from VSC2 to VSC1

(b) Reactive power of 0 pu at VSC1 (c) DC voltage of 1 pu in DC link (d) Reactive power of 0 pu at

VSC2

5.5.2 Case study-2

Figure 5.6a shows the situation of 0.45 pu active power flow from VSC1 to VSC2 with

zero reactive power flow at either end (Fig 5.6b & 5.6d) and 1 pu DC voltage (Fig 5.6c).

Again, it is seen that the system operates with same stability as in the previous case

which proves the ability of VSC-HVDC to operate in bi-directional power flow mode.

Active power reaches the reference value within 0.4 s without any oscillations The DC

voltage has nearly 40 % overshoot before reaching the reference value and the reactive

powers at VSC1 and VSC2 follow the reference value within 0.2 s.


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Figure 5.6: Steady-state operation of VSC-HVDC (a) 0.45 pu active power flow from VSC1 to VSC2

(b) Reactive power of 0 pu at VSC1 (c) DC voltage of 1 pu in DC link (d) Reactive power of 0 pu at

VSC2

5.5.3 Case study-3

This case presents the full load operation of the system with active power flow of 1

pu from VSC1 to VSC2 (Fig 5.7a) along the HVDC transmission system. At VSC1, the

reactive power (Fig 5.7b) of 0.65 pu flows from converter to AC system which shows the

capacitive mode of operation of converter. At VSC2, the reactive power (Fig 5.7d) of 0.45

pu flows from AC system to converter which shows the inductive mode. The DC voltage

(Fig 5.7c) remains constant at 1 pu followed by an overshoot of 45 % during system start

up. The dissimilar amount of reactive power flows in different direction at both ends of

VSC-HVDC. In this case study, the independent control of reactive power is explained


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which is of importance when VSC-HVDC supplying the passive networks or connecting

weak AC networks.

Figure 5.7: Full load active power flow (a) 1 pu active power flow from VSC1 to VSC2 (b) 0.65 pu

reactive power flow from VSC1 to AC system (c) DC voltage of 1 pu in DC link (d) 0.45 pu reactive

power flow from AC system to VSC2

5.5.4 Case study-4

This case presents the full load operation of the system with active power flow of 1

pu from VSC2 to VSC1 (Fig 5.8a) along the HVDC transmission system. At VSC1, the

reactive power (Fig 5.8b) of 0.25 pu flows from converter to AC system. At VSC2, the

reactive power (Fig 5.8d) of 0.5 pu flows from converter to AC system. The DC voltage

(Fig 5.8c) attains the reference value of 1 pu with in 0.5 s. This case study explains the


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converter operation in the inductive mode, where both the converters VSC1 and VSC2

absorb the reactive power from the system.

Figure 5.8: Full load active power flow (a) 1 pu active power flow from VSC2 to VSC1 (b) 0.25 pu

reactive power flow from VSC1 to AC system (c) DC voltage of 1 pu in DC link (d) 0.5 pu reactive

power flow from VSC2 to AC system

5.6 Step changes -vector control method

In this case, step changes in active power, DC voltage and reactive power are

implemented in all four VSC-HVDC controllers. Two of these disturbances are applied at

the converter VSC1 which controls (a) the active power between AC system and DC

transmission system and (b) reactive power at VSC1. Two of the disturbances are applied

at the VSC2 which controls (c) the DC voltage and (d) reactive power at VSC2. In this


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one test, in reality, four separate disturbances are implemented to understand the system’s

ability to recover during disturbances.

As shown in Fig. 5.9a, initially active power of 0.5 pu flows from VSC2 to VSC1. A

negative step of 25% is applied at 1 s for duration of 0.6 s in active power controller at

VSC1. First, the transition is observed to be well controlled with little overshoot and

settling time of 0.35 s. Second, it can be seen that the change in active power does not

affect the reactive powers on either side of the AC systems though there are some

transients observed when the step is applied. A noticeable effect in the DC voltage is

observable and the size of the DC capacitor employed will have an impact on this.

Next, in Fig. 5.9b, initially reactive power of 0.5 pu flows from AC system to VSC1.

A negative step of 25% is applied at 2.35 s for duration of 0.6 s in the reactive power

controller at VSC1. First, the transition is observed to be well controlled with little

overshoot and settling time of 0.35 s. Second, it can be seen that the change in reactive

power affects neither the active power on same side nor the reactive power on other side

of the AC system. This clearly demonstrates the independence of the P and Q controllers

at the VSC1 end.

In Fig.5.9c, initially the DC voltage is maintained constant at 1 pu which is followed

by a positive step of 10% applied at 3.7 s for 0.6 s in DC voltage controller at VSC2.

First, the transition is observed to be rapid, well controlled with overshoot of about 5%

and settling time of 0.15 s. Second, it can be seen that the change in DC voltage does

affect the reactive powers on either side of the AC systems. Naturally, the size of DC

capacitor will be a significant element in controlling this effect.


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Next, in Fig. 5.9d, initially 0.5 pu reactive power flows from VSC2 to VSC1, at 5.05 s

positive step of 25% is applied for 0.6 s in reactive power at VSC2. First, the transition is

observed to be well controlled with little overshoot and 0.15 s settling time. Second, it

can be seen that, there are some transients observed when the step is applied. The steps in

reactive power at both ends do not affect the active power and DC voltage.

Figure 5.9: Step changes in the active power, reactive power and DC voltage (a) 0.5 pu active power

flow from VSC2 to VSC1 (b) 0.5 pu reactive power flow from AC system to VSC1 (c) DC voltage of 1

pu in DC link (d) 0.5 pu reactive power flow from VSC2 to AC system

Sequence of events:

Time Action

1-1.6 sec 25 % negative step in active power which flows from VSC2 to VSC1

2.35-2.95 sec 25 % negative step in reactive power flowing from AC system to VSC1

3.7-4.3 sec 10 % positive step in DC link voltage

5.05-5.65 sec 25 % positive step in reactive power flowing from VSC1 to AC system


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5.7 Active and reactive power reversal -vector control method

In this case study, the reference values of P and Q controllers are changed so as to

reverse the active and reactive power whilst keeping DC voltage constant. The behaviors

of all four controllers are observable in this case.

In Fig. 5.10a, the initial reference value of the power flow is at +0.5 pu in the

direction from VSC2 (rectifier mode) to VSC1 (inverter mode). At 1 s, the reference value

is reversed from +0.5 pu to -0.5 pu. It can be seen that the active power follows the

reference rapidly without any oscillation and reaches the new reference of -0.5 pu within

0.4 s. During this power reversal, some transients are observed in reactive power at both

ends as shown in Fig. 5.10b and Fig. 5.10c and DC voltage has large overshoot of nearly

40 % which stabilizes within 0.3 second. Due to system parameters it is not feasible to

reverse the full power instantaneously.

Fig. 5.10b shows a large change in reactive power at VSC1. Initially the reactive

power reference is at +0.15 pu which flows from AC system to VSC1. At 2 s, the

reference value is reversed to -0.15 pu. The direction change in the flow of reactive

power is stabilized within 0.35 s without any kind of oscillations. It is observed that this

disturbance does not create any kind of effect on active power or reactive power at VSC2.

There is virtually little effect on the DC voltage due to this reversal of reactive power.

In Fig. 5.10c, initially the reactive power reference at VSC2 is at -0.5 pu which flows

from converter to AC system. At 3 s, the reference value is changed to +0.5 pu and as

seen in the Fig. 5.10c, the reactive power at VSC2 follows the reference value within 0.2 s


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without any transients. Some small transients are observed in the DC voltage with no

effect in active and reactive power at VSC1.

Next, Fig. 5.10d shows the variation of DC voltage throughout the process of changes

applied to other controllers in the system as shown in Fig. 5.10a, 5.10b and 5.10c. During

active power flow direction change, there is a large overshoot of 40 % in the value of DC

voltage. The sudden change in direction of DC current flow is responsible for this large

overshoot which may be reduced by optimizing the controller gains further. The change

in reactive power at both ends does not have much effect on DC voltage though there are

some transients at the time of the step change.

Figure 5.10: Active and reactive power reversal (a) Initial active power flow of 0.5 pu from VSC2 to

VSC1 (b) Initial reactive power of 0.15 pu flow from AC system to VSC1 (c) Initial reactive power of -

0.5 pu flow from VSC2 to AC system (d) DC voltage of 1 pu in DC link


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Sequence of events:

Time Action

1 sec Active power flow direction changes from +0.5 pu to -0.5 pu

2 sec Reactive power flow (VSC1) direction changes from +0.15 pu to -0.15pu

3 sec Reactive power flow (VSC2) direction changes from -0.5 pu to +0.5 pu

DC voltage is maintained constant at 1 pu

5.8 Comparison between direct and vector control technique

The simulation results for step changes of the VSC-HVDC system with direct control

technique and vector control technique are shown in Fig. 5.3 and Fig. 5.9 respectively. As

shown, the step changes are applied in the active power, reactive power /AC voltages and

DC voltage.

In Fig. 5.3, it is evident that the step change in active power strongly creates the

transients in AC voltages at both ends and DC link voltage. Same way, the step in DC

voltage has created large transients in active power and AC voltages at both ends of the

VSC-HVDC system.

In Fig. 5.9, it is seen that the change in active power affects neither the reactive

powers at either end nor the DC voltage at VSC2. The step in DC voltage has created a

small transient in reactive power flow at VSC2 whereas there is no effect on the active

power and reactive power at VSC1.

In case of direct control method; due to coupling between electrical quantities, change

in one quantity strongly affects the other quantity. Whereas in case of vector control


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technique, the coupling is removed so change in one parameter does not create any effect

on other parameter in the system. This is a major advantage of vector control method

compared to direct control method. As described earlier, in direct control method,

modulation index and angle delta are controlled directly whereas in vector control

technique modulation index and angle delta is derived using de-coupled feed forward

control loops. By using vector control, the coupling between these electrical quantities is

removed so change in one quantity does not affect the other quantity.

5.9 Effect of DC capacitance on VSC-HVDC performance

The DC side capacitors play a vital role on the system performance. The role of DC

capacitors is described in section 2.2.5. The charging and discharging of DC capacitors

according to the current flow and hence the power flow takes place in the DC

transmission system. The step changes in active power, reactive power and DC voltages

are applied for different values of DC capacitors. Figure 5.11 shows the results with the

two 200µF DC capacitors connected in series across the DC terminals of VSC at both

ends. It is seen that even though the system operates in stably, the transients in active

power, reactive powers at both ends and DC voltage are very high which may lead the

system to instability.


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Figure 5.11: Step changes with the DC capacitance of 100µF at each terminal

As the value of DC capacitor increases, the transients in all four quantities are

reduced. Fig. 5.12, 5.13 and 5.14 show the simulation results with DC capacitor size of

200µF, 300µF and 400µF respectively. The transients in 400µF DC capacitance are least

compared to any other lower size capacitor.



94




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5.10 Summary

In this chapter, the VSC-HVDC system modeled in chapter 4 is simulated using

EMTP-RV software package. Two different control techniques; direct and vector control

are considered for simulation studies. The simulation studies regarding the various

operating conditions, reference value changes and reversal of active and reactive power

are carried out and analyzed. The comparison of two control techniques regarding the

operational performance during step changes is done here. The size of DC capacitor

affects the system performance, and a comparative study with four capacitor values was

performed.

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Chapter 6

Fault Analysis

6.1 Introduction

The VSC-HVDC model developed in EMTP-RV in chapter 4 is studied regarding

symmetrical and asymmetrical faults on sending and receiving end AC networks and

fault on DC transmission system. Vector control strategy is implemented during this

analysis. The different electrical quantities like voltage, current, active power and

reactive powers at both AC sides and voltage and current in DC link are studied to

understand the system behavior under the fault conditions. From this analysis, the ability

of VSC-HVDC system to remain in stability and to decouple two connected AC systems

is determined.

6.2 Fault analysis at VSC1

Various symmetrical and asymmetrical faults at the VSC1 converter transformer high

voltage (HV) filter bus were created. The starting conditions are implemented such as

active power of 0.45 pu flows from VSC2 to VSC1, the reactive power flow at either end

is zero and the DC voltage is at 1 pu.

Chapter 6 Fault Analysis

97

6.2.1 Single line to ground (SLG) fault at VSC1

A SLG fault in phase A is created at 0.75 s for 6 cycles (100 ms) at HV side of VSC1

converter transformer. Figures 6.1a, b, c and d show the phase A voltage and current

waveforms at VSC1 and VSC2 AC filter bus respectively. During the fault, phase A

voltage at VSC1 becomes nearly zero and high amplitude fault current flows with

oscillations.

Figure 6.1: SLG fault at VSC1 converter transformer


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The phase A voltage at VSC2 is stable during this fault which shows no effect of the

fault on the unfaulted side of the HVDC transmission system. The phase A current on

VSC2 is stable though it gets some disturbance which stabilizes within 4 cycles. The

distortion can be seen in DC link voltage and current (Fig. 6.1e & 6.1f) which reflects

slow response to the fault and follows the reference value within 0.4 s. When the fault is

removed, the voltage at VSC1 stabilizes within 5 cycles. The fault impedance of 1mH and

5 Ω is utilized during this fault condition.

6.2.2 Asymmetrical double line to ground (DLG) fault on AC filter bus

at VSC1

A DLG fault in phase A and B is created at 0.75 s for 6 cycles at the VSC1 converter

transformer HV side. Figures 6.2a, b, c and d show the phase A and B voltage and current

waveforms at AC filter bus of VSC1 and VSC2 respectively. During the fault, it is seen

that the phase A and B voltages at VSC1 reach nearly 40 % of their rated value and high

fault currents flow from phase A and B to ground. The voltages at VSC2 are stable

during this fault which shows the ability of VSC-HVDC to decouple systems during fault

conditions on neighboring AC systems. The currents at VSC2 show some distortion

which decay within 5 cycles. The DC link voltage oscillates between 1.1 pu and 0.8 pu

before returning to the reference value of 1 pu. When the fault is removed, the voltages at

VSC1 stabilize within 5 cycles. A fault impedance of 1mH and 65 Ω was utilized during

this fault condition.


99

Figure 6.2: DLG fault at VSC1 converter transformer

6.2.3 Symmetrical three lines to ground (TLG) fault on AC filter bus at

VSC1

A TLG fault was created at 0.75 s for 6 cycles at the VSC1 converter transformer HV

side. The voltage and current waveforms of all the phases at AC filter bus of VSC1 and

VSC2 are shown in Fig. 6.3a, b, c and d respectively. It is seen that during the fault, the

voltages at the VSC1 becomes nearly 60 % of rated value and distorted fault currents flow

to ground. The VSC-HVDC operates as a shock absorber during the faults so the

voltages at VSC2 are stable during this fault. The currents at VSC2 get minor disturbance


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which decays within 10 cycles. When the fault is removed, the voltages at VSC1 stabilize

within 8 cycles. DC link voltage (Fig. 6.3e) reaches to 1.1 and 0.85 pu respectively

during the fault application. DC current (Fig. 6.3f) follows the disturbance with small

transients and reaches to the original pre-fault value within 0.2 s. The fault impedance of

1mH and 130 Ω is utilized during this fault situation.

Figure 6.3: TLG fault at VSC1 converter transformer


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6.3 Fault analysis at VSC2

Different symmetrical and unsymmetrical faults are produced at VSC2 converter

transformer HV bus. The active power of 0.45 pu flows from VSC2 to VSC1, reactive

power flow is zero at either end and DC link voltage is maintained at 1 pu during all the

fault conditions considered here.

6.3.1 Single line to ground fault at VSC2

Single line to ground fault in phase A is created at 0.75 s for 6 cycles at HV side of

the VSC2 converter transformer. Figures 6.4a and 6.4c show the voltage waveforms at

AC filter bus of VSC2 and VSC1 respectively. It is seen that during the fault, the voltage

at the VSC2 becomes nearly 15 % of rated value whereas the AC bus voltage at VSC1 is

stable during this fault which shows no effect of the fault on the other side of the HVDC

transmission system. After the fault is removed, the voltage at VSC2 becomes stable

within 5 cycles.

The distortion can be seen in DC link voltage and current (Fig. 6.2e & 6.2f) which

reflects slow response to the fault and follows the reference value within 0.4 s. A fault

impedance of 1mH and 60 Ω is utilized during this fault situation.


102

Figure 6.4: SLG fault at VSC2 converter transformer

6.3.2 Asymmetrical double line to ground fault on AC filter bus at VSC2

Asymmetrical double line to ground fault in phase A and B is created at 0.75 s for 6

cycles at HV side of the VSC2 converter transformer. Figures 6.5a and 6.5c show the

voltage waveforms of phase A and B at AC filter bus of VSC2 and VSC1 respectively. It

is seen that during the fault, the voltages of phase A and B at the VSC2 becomes nearly 70

% with distortion whereas the voltage VSC1 is stable during this fault. When the fault is

removed, the voltage at VSC2 becomes stable within 5 cycles. The disturbance is seen in


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DC current during fault application (Fig 6.5f). The fault impedance of 1mH and 135 Ω is

utilized during this fault situation.

Figure 6.5: DLG fault at VSC2 converter transformer

6.3.3 Symmetrical three lines to ground fault on AC filter bus at VSC2

Symmetrical three lines to ground fault is created at 0.75 s for 6 cycles at HV side of

the VSC2 converter transformer. Figures 6.6a and 6.6c show the voltage waveforms of all

the phases at AC filter bus of VSC2 and VSC1 respectively. It is seen that during the fault,

the voltages at the VSC2 becomes nearly 50 % with distortion. As shown in the figure,


104

the voltage VSC1 is stable during this fault which shows no effect of the fault on the other

side of the HVDC transmission system.

When the fault is removed, the voltage at VSC2 becomes stable within 5 cycles. The

DC link voltage shows the variation from 0.85 pu to 1.15 pu before reaching the

reference value of 1 pu. The fault impedance of 1mH and 170 Ω is utilized during this

fault condition.

Figure 6.6: TLG fault at VSC2 converter transformer


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6.4 Fault at DC transmission system

During the fault in DC transmission system, the DC voltage on the faulted terminal

reduces due to continuous flow of the DC fault current. The DC capacitors also feed this

fault current which depends on the fault impedance. Since DC breakers are not available

to interrupt this fault current so the AC breakers on both the AC systems must be opened

to remove this fault. This process is slow comparatively so the complete system must be

shut down. The detailed analysis of system behavior during DC fault is out of scope of

this thesis.

6.5 Summary

This chapter provides the VSC-HVDC system performance evaluation under different

symmetrical and asymmetrical faults on VSC1 and VSC2 AC filter bus and DC

transmission system. The waveforms for electrical quantities like voltage, current, active

power, reactive power, DC voltage and current are presented and analyzed during

application of faults to understand the behavior of proposed VSC-HVDC transmission

system.

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Chapter 7

Conclusions and Future work

7.1 Conclusions

The development of VSC in last decade generated tremendous research interest in its

power system applications. The VSC-HVDC uses the IGBTs and SPWM, which makes it

possible to control the magnitude and phase angle of the converter AC output voltage.

This flexibility of control allows for a number of advantages of VSC-HVDC. In order to

fully utilize the capability of the VSC-HVDC, two control algorithms of the VSC-HVDC

are investigated and the performance is tested under different situations in this thesis. The

direct control and vector control strategies which control active power, AC

voltage/reactive power and DC voltage, are presented and evaluated for the VSC-HVDC

transmission system which connects two weak AC grids. The results for both control

techniques show that the system response is fast, high quality AC voltages and currents

can be obtained, and the active and reactive power can be controlled precisely. However,

the direct control suffers from coupling problems between active/reactive power controls.

Chapter 7 Conclusions and Future work

107

The major contributions of this thesis are as follows:

Development of a detailed model of a 2-level, 3-phase, 6-switch VSC which is

used to build VSC-HVDC transmission system. The model includes

representation of valves, snubber circuits, and passive components.

Implementation of SPWM technique for generating firing pulses for VSC

converter.

Implementation of direct control and vector control strategies which includes

design of four different controllers; two controllers at sending end (active power

and AC voltage/reactive power controller) and two controllers at the receiving

end (DC voltage and AC voltage/reactive power).

Design and implementation of VSC-HVDC transmission system connected to

weak AC grids in EMTP-RV in order to understand the operational behavior of it

under steady-state, dynamic and fault operating conditions.

The simulation results for VSC-HVDC model with direct control strategy show the

satisfactory operation during steady-state and dynamic conditions. Due to coupling

between electrical quantities (voltage and currents), changes in one quantity affect the

other quantity which may lead the system to instability during abnormal conditions (i.e.

faults and sudden load changes).

On the other hand, vector control technique implements the feed forward closed loop

control technique so the coupling between electrical quantities is removed. This is a

major advantage of vector control method when compared to direct control method. The


108

vector control strategy is implemented with VSC-HVDC and comprehensive simulation

results show that

The system response is of the order of 0.75 s which is comparatively fast.

The active power and reactive powers in both AC systems can be controlled

independently.

During dynamic tests i.e. 25 % negative step in active and reactive power at

VSC1; 25 % positive step in reactive power and 10 % positive step in DC link

voltage at VSC2, the system response is fast and well controlled.

The active power and reactive power reversal analysis shows the bi-directional

transfer of power capability of VSC-HVDC without large oscillations. At the

instant of active power reversal, the DC voltage increases sharply which depends

on the amount of the active power flow.

During asymmetrical faults at VSC1 and VSC2 AC filter buses, DC voltage drops

and shows some oscillation, the effect on the unfaulted AC side depends on the

active power flow during the application of the fault.

Symmetrical 3-phase faults at both AC sides of HVDC link show the satisfactory

operation of the system. The fault impedance is comparatively higher than single

phase and double phase to ground fault.

The fault application on positive pole of DC link shows that the power can still be

transmitted by unfaulted pole of DC link. The DC capacitors play major role

during faults on DC transmission system and provide the necessary voltages

during these conditions.


109

The effect of different DC capacitor sizes on active power, reactive power and

DC voltage is also studied which shows that as the capacitor size increases, the

transients in active and reactive power and AC and DC voltages are reduced. The

cost is the limiting factor for the capacitor size employed in actual installations.

7.2 Future work

The following topics emanating from this research are considered for future work:

Study of proposed VSC-HVDC model during the fault applications at AC and DC

systems, in case of a unbalanced AC system.

Design of VSC-HVDC transmission system using multi-level VSC like Neutral

point clamped and Flying capacitor topologies.

Study of VSC-HVDC and its control systems connected to passive network at

receiving end.

Multi-terminal VSC-HVDC system operation and grid integration of renewable

energy sources like wind and solar farms.

Implementation of active AC filters to VSC-HVDC system.

110

References

[1]. HVDC Transmission: yesterday and today, by Willis Long and Stig Nilsson,

IEEE Power and Energy Magazine March/ April 2007.

[2]. Nikolas Flourentzou, Vassitios G. Agelidis, Georgios D.Demetriades, “VSC-

Based HVDC Power Transmission Systems: An Overview,” IEEE Transactions

on Power Electronics, vol. 24, No.3, March 2009.

[3]. B R Anderson, L Xu, K T G Wong, “Topologies for VSC transmission,” AC-DC

Power Transmission, IEEE Conference Publication no. 485 © IEEE 2001,

28-30 November 2001.

[4]. Gregory Reed, Masatoshi Takeda, “Advantages of Voltage Source Converter

based Design concepts for FACTS and HVDC link Applications,” IEEE Power

Engineering Society General Meeting 2003, vol. 3.

[5]. K. Erikson, “Operational experience of HVDC light,” Seventh International

Conference on AC-DC Power Transmission pp. 205-210, November 2001.

[6]. Jih-sheng Lai, Fang Zheng Peng, “Multilevel converters-A new breed of power

converters,” IEEE Transactions on Industry Applications, vol.32 No.3, May/June

1996.

[7]. D.M. Larruskain, I. Zamora, A.J. Mazón, O. Abarrategui, J. Monasterio,

“Transmission and Distribution Networks: AC versus DC,” Published in May

2005, by Solarec Egypt, Egyptian Solar Research Center.

[8]. Johan Setreus, Lina Bertling, “Introduction to HVDC Technology for reliable

Electrical Power Systems,” in Proc.10th

International Conference on

Probabilistic Methods applied to Power Systems, 2008 pp: 1-8.

[9]. Björn Jacobson, Paulo Fischer, De Toledo, Gunnar Asplund, “500 Mw City

Center In feed with Voltage Source Converter Based HVDC,” 40th

Meeting of

Study Committee B4 and Colloquium on Role of HVDC, FACTS and Emerging

Technologies in Evolving Power Systems, Bangalore, India, July 2005.

[10]. S. Cole, D. Van Hertem, I. Pardon and R. Belmans, “Randstad HVDC: A Case

Study of VSC HVDC Bulk Power Transmission in a Meshed Grid,” Security And

Reliability Of Electric Power Systems CIGRE Regional Meeting, June 18-20,

2007 / Tallinn, Estonia.

[11]. Guanjun Ding, Ghangfu Tang, Zhiyuan He and Ming Ding, “New Technologies

of Voltage Source Converters (VSC) for HVDC Transmission System based on

VSC,” IEEE Power and Energy Society General Meeting - Conversion and

Delivery of Electrical Energy in 21st Century, 2008.

111

[12]. ADSP-21990: Reference frame conversions © Analog devices Inc, January 2002.

[13]. Guibin Zhang, Zheng Xu, Ye Cai, “An equivalent model for simulating VSC

based HVDC,” Transmission and Distribution Conference and Exposition, 2001

IEEE/PES, vol. 1, pp: 20-24.

[14]. Guibin Zhang, Zheng Xu, “Steady-state model for VSC based HVDC and its

controller design,” IEEE Power Engineering Society Winter Meeting 2001,

pp: 1085-1090.

[15]. Wang Yan, Zhao Shu-Zhen, Huangfu Cheng, Ruan Jiang-Jun, “Dynamic model

and control of voltage source converter based HVDC,” IEEE Power and Energy

Engineering Conference 2009, APPEEC 2009, pp.1-5, 2009.

[16]. Parkhideh B., Bhattacharya S., “A practical approach to controlling the back to

back voltage source converter system,” 34th

Annual Conference of IEEE on

Industrial Electronics, 2008, IECON 2008, pp: 514-519.

[17]. Yin Ming, Li Gengyin, Li Guangkai, Liang Haifeng, Zhou Ming, “Modeling of

VSC-HVDC and its active power control scheme,” International Conference

on Power System Technology, POWERCON 2004, Singapore,21-24

November 2004.vol. 2,pp. 1351-1355.

[18]. Gengyin Li, Ming Yin, Ming Zhou, Chengyong Zhao, “Modeling of VSC- HVDC

and control strategies for supplying both active and passive systems,” IEEE

Power Engineering Society General Meeting, 2006.

[19]. M.M.Zakaria Moustafa, S.Filizadeh, “Electromagnetic transient simulation of a

back-to-back voltage source converter based transmission scheme,” Canadian

Conference on Electrical and Computer Engineering 2007, pp.1570-1573.

[20]. M.M.Zakaria Moustafa, S.Filizadeh, “Simulation of a VSC transmission scheme

supplying passive load,” 34th

Annual conference of IEEE on industrial

electronics, 2008, pp.942-946.

[21]. Cuiqing Du., Sannino A, Bollen M.H.J., “Analysis of the control algorithms of

Voltage-Source Converter HVDC,” Power Tech 2005, IEEE Russia. Publication

year: 2005, page(s):1-7.

[22]. Chandra Bajracharya, Marta Molinas, Jonare Suul,Tore M Undeland,

“Understanding of tuning techniques of converter controllers for VSC-

HVDC,” NORPIE 2008, Nordic Workshop on Power and Industrial Electronics,

June 9-11,2008.

[23]. Temesgen M.Haileselassie, Marta Molinas, Tore Undeland, “Multi-terminal

VSC-HVDC system for integration of offshore wind farms and green

112

electrification of platforms in the north sea,” NORPIE 2008, Nordic workshop on

power and industrial electronics, June 9-11, 2008.

[24]. Ahmed Mahjoub, Ravindra Mukerjee, “Modeling of controller for voltage

sourced converter based HVDC transmission system,” 2nd

IEEE International

Conference on Power and Energy (PECON 08), December 1-3, 2008, Johor

Baharu, Malaysia.

[25]. S. Douangsyla, P. Indarack, A. Kanthee, M. Kando, S. Kittiratsatcha and V.

Kinnares, “Modeling for PWM voltage source converter controlled power

transfer,” Intl. Symposium on Communications and Information Technologies

2004, Sapporo, Japan, October 26-29, 2004.

[26]. D.Jovcic, L.A.Lamont, L.Xu, “VSC transmission model for analytical studies,”

IEEE Power Engineering Society General Meeting 2003, Vol.3, 2003.

[27]. K.R.Padiar, Naresh Prabhu, “Modeling, control design and analysis of VSC based

HVDC transmission systems,” International Conference on Power System

Technology 2004 POWERCON 2004, Singapore, 21-24 November 2004.

[28]. Peter W. Lehn, Stephne Podrucky, “Small Signal modeling of Power Electronics

Converters with Resonant Controllers,” International Conference on Power

System Transients IPST 2009, Kyoto, Japan, June 3-6, 2009.

[29]. Jiuping Pan, Reynaldo Nuqui, Le Tang, Per Holmberg, “VSC-HVDC Control and

Application in Meshed AC Networks,” IEEE Power Engineering Society General

Meeting, Pittsburgh, Pennsylvania, July 20-24, 2008.

[30]. Masoud Karimi-Ghartemani and M.Reza Iravani, “A Nonlinear Adaptive Filter

for Online Signal Analysis in Power Systems: Application,” IEEE Transactions

on Power Delivery, Vol.17, No.2, April 2002.

[31]. Vladimir Blasko, and Vikram Kaura, “A New Mathematical Model and Control

of a Three-Phase AC–DC Voltage Source Converter,” IEEE Transactions on

Power Electronics, Vol. 12, No. 1, January 1997, page(s): 116-123.

[32]. Bhim Singh, B. K. Panigrahi, and D. Madhan Mohan, “Voltage Regulation and

Power Flow Control of VSC Based HVDC System,” IEEE International

Conference on Power Electronics, Drive and Energy Systems, PEDES 2006.

[33]. Sonali Dasgupta and Gayatri Agnihotri, “A Control Strategy for a VSC HVDC

System in steady state response,” International Conference on Advances in

Computing, Control, and Telecommunication Technologies, ACT 2009.

113

[34]. Tatsuhito Nakajima, Shoichi Irokawa, “A Control System for HVDC

Transmission by Voltage Sourced Converters,” IEEE Power Engineering Society

Summer Meeting 1999, Publication Year: 1999 , Page(s): 1113 - 1119 vol.2

[35]. Mariusz Cichowlas, Marian P.Kazmierkowski, “Comparison of current control

techniques for PWM rectifiers,” Proceedings of IEEE International

Symposium on Industrial Electronics, 2002.ISIE 2002. Publication Year: 2002,

Page(s): 1259 - 1263 vol.4.

[36]. B. Jacobson, Y. Jiang-Hafner, P. Rey, G. Asplund , “HVDC with Voltage Source

Converters and Extruded cables for up to ±300 kV and 1000MW,” B4-105,

CIGRE, Paris, France 2006.

[37]. F. Schettler, H. Huang, N. Christl, “HVDC transmission system using Voltage

Source Converters-Design and Applications,” IEEE Power Engineering Society

Summer Meeting, 2000, pp: 715-720.

[38]. Joerg Dorn, Hartmut Huang, Dietmar Retzmann, “Novel Voltage-Sources

Converters for HVDC and FACTS Applications,” CIGRE 314, Osaka 2007.

[39]. Michael Bahrman, Abdel-Aty Edris, Rich Haley, “Asynchronous Back-to-Back

HVDC link with Voltage Source Converters,” Presented at Minnesota Power

Systems Conference, Nov 1999, USA.

[40]. Gunnar Asplund , Kjell Eriksson, Hongbo Jiang, Johan Lindberg, Rolf Pålsson

and Kjell Svensson, “DC Transmission Based On Voltage Source Converters,”

International Conference on Large High Voltage Electric Systems, CIGRE 98, vol

4, Paris, France, 1998.

[41]. Stefen G. Johansson, Gunnar Asplund, Erik Jansson and Roberto Rudervall,

“Power System Stability benefits with VSC-DC Transmission Systems,” B4-204,

CIGRE Session, 2004.

[42]. Chang Hsin Chien, Richard W.G. Bucknall, “Analysis of Harmonics in subsea

power transmission cables used in VSC-HVDC Transmission system operating

under steady-state conditions,” IEEE Transactions on Power Delivery, vol.

22,Issue 4, 2007, page(s): 2489-2497.

[43]. Rolf Grunbaum, “Voltage source converters for maintaining power quality and

stability in power distribution,” European Conference on Power Electronics and

Applications, EPE 2005.

[44]. A.Petersson, A.Edris, “Dynamic Performance of the Eagle Pass Back-to-Back

HVDC Light Tie, AC-DC Transmission,” 28-30 November 2001, Conference

Publication No. 485, IEE 2001.

114

[45]. F. Schettler H. Huang N. Christl, “HVDC Transmission Systems using Voltage

Sourced Converters -Design and Applications,” IEEE Power Engineering Society

Summer Meeting, 2000, pp: 715-720.

[46]. Dr. Vijay K. Sood, HVDC and FACTS controllers, 1st ed., New York: KLUWER

Academic Publishers, 2004.

[47]. Chan-Ki Kim, Vijay K. Sood, Gil-Soo Jang, Seong-Joo Lim and Seok-Jin Lee,

HVDC TRANSMISSION Power Conversion Applications in Power Systems,

1st ed., Singapore: John Wiley & Sons (Asia) Pte Ltd, 2009.

[48]. Ned Mohan, Tore M.Undeland and W.P.Robbins, Power Electronics:

Converters, Applications and Design, 2nd

ed., New York, NY: John Wiley and

Sons Inc, 1995.

[49]. Muhammad H.Rashid, Power Electronics Circuits, Devices, and Applications,

3rd

ed., New Jersey: Pearson Prentice Hall, 2004.

[50]. N.G.Hingorani and L.Gyugyi, Understanding FACTS: Concepts and

Technology of Flexible AC Transmission Systems, New York: IEEE Press, 2000.

[51]. Amirnaser Yazdani and Reza Iravani, Voltage-Sourced Converters in Power

Systems, 1st ed., New Jersey: Wiley, 2010.

115

Appendix A

Per Unit Calculations

The VSC-HVDC transmission system modeled in this thesis is converted in per unit. The

following calculations are given for the conversion to per unit.

Sb = Nominal three phase apparent power of the AC network

= 100 MVA

Vb = Nominal peak phase voltage of AC network

= where is line-to-line RMS voltage

Vb = 187.795 kV

Ib = Nominal peak phase current

=

= 355 Amps

Zb = Base AC impedance

=

116

=529 Ω

ωb = Base angular frequency

= 377 rad/sec

The per unit presentation at the DC side is achieved as follows:

As, = and then the power balance is given by,

The base value for the DC voltage is chosen as,

Then by the power balance equation as above,

And

= 1410.66 Ω

117

Appendix B

Filter Design Calculations

Each individual harmonic component is studied regarding its amplitude and contribution

to total harmonic distortion in voltage and current. From this information, filter rating is

calculated according to the particular harmonic component attenuation and power factor

improvement in the system. The graph of impedance versus harmonic frequency provides the

information to choose the quality factor of particular filter branch. The quality factor of 30 is

considered for design calculations provided here. The low order harmonics which have high

magnitude are mitigated by low-pass (LP) single tuned filters and high frequency harmonics

frequency components are attenuated by high-pass (HP) damped filters.

Table B-7-1: Filter ratings at VSC1

Ratings of the filters connected at HV bus of converter transformer at VSC1

Type

Harmonics

Order

Rating

(MVAR)

Resistance

(Ω)

Inductance

(H)

Capacitance

(µF)

LP 4th

5 22.05 1.754 0.251

LP 5th

5 14.11 1.122 0.251

LP 7th

5 05.51 0.438 0.251

LP 8th

5 02.91 0.240 0.251

HP 6th

50 14.69 0.094 3.000

Table B-7-2: Filter ratings at VSC2

Ratings of the filters connected at HV bus of converter transformer at VSC2

Type

Harmonics

Order

Rating

(MVAR)

Resistance

(Ω)

Inductance

(H)

Capacitance

(µF)

LP 4th

5 22.05 1.754 0.251

LP 5th

5 14.11 1.122 0.251

LP 7th

5 05.51 0.438 0.251

HP 8th

5 02.91 0.240 0.251

HP 6th

25 29.39 0.187 1.500

HP 17th

25 03.00 0.022 1.500

118

(1) MATLAB code for calculations of low-pass single frequency tuned filter RLC

branches

% Low pass filter design program % Filter connection is Wye % frequency = 60 Hz Q=input ('Please enter the filter rating (MVA)'); V=input ('Please enter the system voltage (kV line to line)'); n=input ('Please enter the harmonic order'); Quality=input ('Please enter the quality factor'); f=60; Omega=2*pi*f; C=Q/ (omega*(V^2)) L=1/(C*(n*omega)^2) R= (omega*L)/quality

(2) MATLAB code for calculations of high-pass damped filter RLC branches

% High Pass (HP) Damped Filter design program % Filter connection is Wye % frequency = 60 Hz

Q=input ('Please enter the filter rating (MVA)'); V=input ('Please enter the system voltage (kV line to line)'); n=input ('Please enter the harmonic order'); Quality=input ('Please enter the quality factor'); f=60; Omega=2*pi*f; C=Q/ (omega*(V^2)) L=1/(C*(n*omega) ^2) R=quality*omega*L

119

Appendix C

ABC-dq Conversions

Figure 7: ABC to dq conversion block modeling in EMTP-RV

Figure C 1 shows the modeling of ABC to dq conversion in EMTP-RV. The introduction

of reference frames in the analysis of electrical machines has turned out not only to be useful

in their analysis but also has provided a powerful tool for the implementation of sophisticated

control techniques. The significant breakthrough in the analysis of three-phase AC machines

was the development of reference frame theory. Using these techniques, it is possible to

transform the phase variable machine description to another reference frame. By appropriate

choice of the reference frame, it is possible to simplify considerably the complexity of the

mathematical machine model. These techniques are initially developed for the analysis and

simulation of AC machines; they are now invaluable tools in the digital control. These digital

control techniques are now extended for control of currents also. Over the years, many

different reference frames have been proposed for the analysis but the most commonly used

ones are the stationary reference frame and the rotor reference frame.

f(u)1

f(u)1

Fm6

f(u)1

Fm7

f(u)1

Fm8

f(u)1

f(u)1 f(u)

a

b

cf(u)

1

2

3

4

5

6

f(u)

1

2

3

4

5

6

Fm13

f(u)

1

2

3

d

q

z

SUM1

2

t

cos0

cos1

cos2

sin0

sin1

sin2

d

q

a

b

c

z

omega_t

t

120

The 3-phase AC machines are conventionally modeled using phase variable notation. For

3-phase star connected machine, the phase quantities are not independent variable so the

vector sum of all 3-phase quantities is zero. As a result of this redundancy in the phase

variable representation it is possible to transform the system to an equivalent 2-phase

representation. The transformation from 3-phase to 2-phase quantities is written in matrix

form as: (C.1)

Where γ=2π/3 and the can be any phasor quantity. It is evident that for control of the

current amplitude this form is more suitable than 3-phase representation. The transformation

shown in (C.1) is known as Clarke Transformation. The space vector may be viewed in the

complex plane as shown in Fig C 2.

Xa

X

α

jβ

Xb

Xc

A Axis

B Axis

C Axis

γ

Figure 8: Phasor diagram for relationship between ABC and αβ

121

In case of AC machine theory, the stator space vectors are complex quantities defined in

a reference frame whose real axis is fixed to the magnetic axis of the stator winding. The

corresponding quantities defined for the rotor circuit of a 3-phase AC machine are similarly

stated in a reference frame fixed to the rotor. In the analysis of electrical machines, it is

necessary to adopt a common reference frame for both the rotor and the stator. The second

transformation known as a vector rotation is formulated that rotates space vector quantities

through a known angle. The space vector in this new reference frame is written in matrix

form as:

(C.2)

Where the angle the angle by which the vector rotation is done. This transformation is

called Park’s transformation. The relationship of the real and imaginary components of the

current space vector in the original stationary two-axis reference frame and the new rotating

reference frame is shown in the Fig 63.

122

Xα

X

α

jβ

Xb

Stationary Real

Axis

d

jq

Xβ

Xq

Xd

Stationary

Imaginary Axis

Rotating

Imaginary Axis Rotating Real

Axis

θ

ω

Figure 9: Phasor diagram for relationship between ABC, dq and αβ quantities

From above discussion, it can be said that a balanced 3-phase system in phase variables

may be transformed to an equivalent 2-axis representation that is independent of the angular

position by applying a three-phase to two-phase transformation followed by a vector rotation

by the angular position of the phase quantities.

The two transformations can be combined in one step using following matrix equation.

(C.3)

Equation C.3 can be used for the conversion of ABC-quantity in to dq-quantity directly.

123

Appendix D

EMTP-RV: An Overview

EMTP-RV was originally developed by the Bonneville Power Administration (BPA),

Portland, Oregon. The present version was developed at IREQ, Hydro-Quebec under the

agreement with the Direct Coordination Group (DCG). This software is widely used in power

utilities, research institutes and academic institutions for transients simulation studies around

the world, and is a circuit based power system simulator. This thesis used the software for the

modeling and study of 2 levels, 6 switches VSC based HVDC transmission system. The

EMTP-RV is the enhanced computational engine and EMTP Works is its new Graphical

User Interface (GUI).

Power system networks can be modeled using EMTP-RV to represent practical systems

and large numbers of models are available in EMTP-RV for study purpose. These models

can be used for the steady state and transient state phenomenon simulation in power

networks. The power network can be modeled using voltage or current sources, machines,

multiphase circuits, distributed or lumped parameter line models and switches. The simulated

model can be used to represent a specified power system. There are different subroutines

available in EMTP-RV that solve the mathematical equations and provide solutions to such

models. Another advantage of the EMTP-RV is that it can handle very large and complex

power system networks. Its uses include switching and lighting surge analysis, insulation

coordination and power electronic applications in power systems. Further information about

EMTP-RV can be obtained from the website www.emtp.com.

http://www.emtp.com/

124

Appendix E

List of Publications

1. As a part of this research, the following research paper has been accepted by IEEE

Electrical Power and Energy Conference (EPEC 2010) at Halifax, Nova Scotia,

August 25-27, 2010. Hiteshkumar Patel and V.K. Sood, “Modeling of Voltage

Source Converter based HVDC system in EMTP-RV,”

2. As a part of this research, the following paper is going to be submitted to an

International conference. Hiteshkumar Patel and V.K. Sood, “Comparison of direct

control and vector control methods for Voltage Source Converter based HVDC

transmission system,”

Documents

Modeling of Voltage Source Converter Based HVDC