CHAPTER 1INTRODUCTION

1.1 INTRODUCTION

As commercial and industrial customers become more and more reliant on high-

quality and high-reliability electric power, utilities have considered approaches that

would provide different options or levels of premium power for those customers who

require something more than what the bulk power system can provide. Insufficient power

quality can be caused by (1) failures and switching operations in the network, which

mainly result in voltage dips, interruptions, and transients and (2) network disturbances

from loads that mainly result in flicker (fast voltage variations), harmonics, and phase

imbalance.

Momentary voltage sags and interruptions are by far the most common

disturbances that adversely impact electric customer process operations in large

distribution systems. In fact, an event lasting less than one-sixtieth of a second (one-

cycle) can cause a multimillion-dollar process disruption for a single industrial customer.

Several compensation devices are available to mitigate the impacts of momentary voltage

sags and interruptions. When PQ problems are arising from nonlinear customer loads,

such as arc furnaces, welding operations, voltage flicker and harmonic problems can

affect the entire distribution feeder. Several devices have been designed to minimize or

reduce the impact of these variations. The primary concept is to provide dynamic

capacitance and reactance to stabilize the power system. This is typically accomplished

by using static switching devices to control the capacitance and reactance, or by using an

injection transformer to supply the reactive power to the system.

Custom power is formally defined as the employment of power electronic or static

controllers in distribution systems rated up to 38 kV for the purpose of supplying a level

of reliability or power quality that is needed by electric power customers who are

sensitive to power variations. Custom power devices or controllers, include static

switches, inverters, converters, injection transformers, master-control modules and

energy-storage modules that have the ability to perform current-interruption and voltage-

regulation functions within a distribution system. Each custom power device can be

1

considered to be a type of power-conditioning device. In general, power-conditioning

technology includes all devices used to correct end-user problems in response to voltage

sags, voltage interruptions, voltage flicker, harmonic distortion and voltage-regulation

problems.

The solution to the above power quality problem is to use Flexible AC

Transmission Systems (FACTS)[2] and Custom Power products like DSTATCOM

(Distribution Static synchronous Compensator), DVR (Dynamic Voltage Restorer),

UPQC (Unified Power Quality Conditioner) etc. These devices deal with the issues

related to power quality using similar control strategies and concepts. Basically, they are

different only in the location in a power system where they are deployed and the

objectives for which they are employed.

Most of the electricity produced today is generated in large generating stations,

which is then transmitted at high voltage to the load centers and transmitted to consumers

at reduced voltage through local distribution systems. In contrast with large generating

stations, distributed generation (DG) produce power on a customer's site or at a local

distribution network. Distribution generation has started gaining importance all over the

world and can become the answer for increasing power failure some times during fault

occurrences. Power failure leads power interruption leading to insecure and unreliable

power system. Conventionally, power plants have been large, centralized units. A new

trend is developing toward distributed energy generation, which means that energy

conversion units are situated close to energy consumers, and smaller ones substitute large

units. In the ultimate case, distributed energy generation means that single buildings can

be completely self-supporting in terms of electricity, heat, and cooling energy.

The relation between distributed generation and power quality is an ambiguous

one. On the one hand, many authors stress the healing effects of distributed generation for

power quality problems. For example, in areas where voltage support is difficult,

distributed generation can contribute because connecting distributed generation generally

leads to a rise in voltage in the network. At the same time if any faults are occurring in

the system the DG must be capable of providing the power supply without any problems

to the customers. Hence in order to ensure the reliability of power to the customers it is

2

necessary to install some compensating devices such that these devices provides the

required reactive power to the generators during fault instants such that the reactive

power drawn from the supply will be nil and the other customers will not be affected.

1.2 PROBLEM DEFINITION

The main objective of the thesis is to show that using DISTRIBUTION

STATCOM (DSTATCOM) it is possible to reduce the voltage fluctuations like sag and

swell conditions in distribution systems. The DSTATCOM which can be used at the PCC

for improving power quality is modeled and simulated using proposed control strategy

and the performance is compared by applying it to a radial distribution system with and

without DSTATCOM. DSTATCOM is applied to a simple Distributed Generation

system consisting of AC generators like Induction and Synchronous Generators and the

system is analyzed by applying faults at various points. Finally the best generator to be

installed with DSTATCOM is chosen.

1.3 OUTLINE OF THE THESIS

The complete thesis is divided into 8 chapters. In this, chapter 1 discusses about

the power quality and the available solutions followed by literature survey for

DSTATCOM and for Distributed Generation. in chapter 2.

Chapter 3 of the thesis summarizes the solutions to different power quality

problems in the form of custom power devices and also in this chapter the power quality

problems in DG are discussed.

Chapter 4 describes about DSTATCOM and its operating principle. In chapter 5,

the modeling part of DSTATCOM along with the voltage regulation technique is shown.

In Chapter 6, the modeled DSTATCOM is applied to a distribution system with

the two loads switched at different times for sag condition and source voltage is increased

to a particular time for swell condition. And the waveforms show that the DSTATCOM

improves the terminal voltage to 1pu in sag and swell conditions. Also the Chapter 6

shows the simulation results simulated with the DG and DSTATCOM. The Simulation is

completed using MATLAB / SIMULINK version 7.0.1.

3

CHAPTER 2

LITERATURE SURVEY

The last decade has seen a marked increase on the deployment of end user

equipment that is highly sensitive to poor quality control electricity supply. Several large

industrial users are reported to have experienced large financial losses as a result of even

minor lapses in the quality of electricity supply [1]- [3]. A great many efforts have been

made to remedy the situation, where the solutions based on the use of latest power

electronic technology figure prominently. Indeed custom power technology, the low

voltage counterpart of the more widely known flexible as transmission system (FACTS)

technology, aimed at high voltage power transmission applications, has emerged as a

credible solution to solve many of the problems relating to continuity of supply at the end

user level. The various power quality Problems at the Distribution level are voltage sag

and swells, fluctuations, harmonics, flickering etc [7].

Recently, various power electronic technology devices have been proposed

especially to be applied to medium voltage networks, generally named custom power.

Custom power concept introduced by N.G.Hingorani [1] has been proposed to ensure

high quality of power supply in distribution networks using power electronics devices.

Additionally, various custom power devices are based on the voltage source converter

technology introduced by N.G.Hingorani and L.Gyugyi [2].

At present, wide range of very flexible controllers, which capitalize on newly

available power electronics components, are emerging for custom power applications.

Among these the Distribution static Compensator (DSTATCOM) and dynamic voltage

restorer (DVR), both of them based on the VSC principle given by L.Gyugyi [2], and the

SSTS are the controllers which have received the most attention. The modeling and

analysis of these custom power devices has applied for the study of power quality by

Olimpo Anaya-Lara and E Acha [4] presenting comprehensive results to assess the

performance of each device as a potential custom power application. The different

control techniques of DSTATCOM are discussed in papers [8]-[12]. Sung- Min Woo,

Dae- wook kang, Woo-Chol Lee, Dong-Seok Hyun, have demonstrated a new control

4

technique for reducing effect of Voltage Sag and Swell with DSTATCOM [8]. In this

Thesis the DSTATCOM is simulated with Voltage Regulation Technique [9].

The interest in distributed generation has considerably increased due to market

deregulation, technological advances, governmental incentives, and environment impact

concerns [18]. At present, most distributed generation [19] Installations employ induction

and synchronous machines, which can be used in thermal, hydro, and wind generation

plants [18]. Although such technologies are well known, there is no consensus on what is

the best choice under a wide technical perspective. In the paper by Prof.Mrs. P.R.Khatri,

Prof.Mrs. V.S.Jape, Prof.Mrs. N.M.Lokhande, Prof.Mrs. B.S.Motling, [18], they

discussed the main problems associated with DG and also how to interface the DG to the

utility systems.

M. I. Marei, E. F. El-Saadany and M. M. A. Salama, in their work dealt with the

Flexible Distributed Generation proposed a novel control scheme for the nonlinear link

connecting DG to the distribution network using a current controlled Voltage Source

Inverter (VSI).

5

CHAPTER 3

POWER QUALITY ISSUES AND SOLUTIONS IN DISTRIBUTION

SYSTEM

3.1 INTRODUCTION

FACTS use the latest power electronic devices and methods to control

electronically the high-voltage side of the network. Custom Power focuses on low-

voltage distribution, and it is a technology born in response to reports of poor power

quality and reliability of supply affecting factories, offices and homes [1]-[3]. With

Custom Power solutions in place, the end-user will see tighter voltage regulation, near-

zero power interruptions, low harmonic voltages, and acceptance of rapidly fluctuating

and other non-linear loads in the vicinity.

A Custom Power specification may include provision for

No power interruption.

Tight voltage regulation including short duration sags or swells

Low harmonic voltages

Acceptance of fluctuating and non linear loads without effect on terminal

voltage

The family of emerging power electronic devices being offered to achieve these

custom power objectives includes:

Distribution Static Compensator (DSTATCOM) to protect the distribution

system from the effects of a polluting e.g. fluctuating, voltage sags and swells

and non-linear loads.

Dynamic voltage restorer (DVR) to protect a critical load from disturbances

e.g. sags swells, transients or harmonics, originating on the interconnected

distribution system.

Unified Power Quality Conditioner (UPQC) is the combination of series and

shunt APF, which compensates supply voltage and load current imperfections

in the distribution system.

6

Solid State Breaker (SSB) to provide power quality improvement through

instantaneous current interruption there by protecting the sensitive loads from

disturbances that conventional electromechanical breaker cannot eliminate.

Solid- State Transfer switch (SSTS) to instantaneously transfer sensitive

loads from a disturbance on the normal feed to the undisturbed alternate feed.

3.2 Available Custom Power Devices

This section presents an overview of the VSC-based custom power controllers mentioned

above.

3.2.1 Dynamic Voltage Restorer (DVR): The DVR is a powerful controller that is

commonly used for voltage sags mitigation at the point of connection. The DVR employs

the same blocks as the D-STATCOM, but in this application the coupling transformer is

connected in series with the ac system [5]-[6], as illustrated in Fig 3.1. The VSC

generates a three-phase ac output voltage, which is controllable in phase and magnitude.

These voltages are injected into the ac distribution system in order to maintain the load

voltage at the desired voltage reference.

Fig 3.1 Schematic representation of the DVR

The DVR is a solid state dc to ac switching power converter that injects a set of

three single phase ac output voltages in series with the distribution feeder and in

synchronism with the voltages of the distribution system. By injecting voltages of

controllable amplitude, phase angle and frequency (harmonic) into the distribution feeder

in instantaneous real time via a series injection transformer, the DVR can restore the

7

quality of voltage at its load side terminals when the quality of the source side terminal

voltage is significantly out of specification for sensitive load equipment.

The reactive power exchanged between the DVR and distribution system

is internally generated by the DVR without any ac passive reactive components, i.e.

reactors and capacitors. For large variations in the source voltage, the DVR supplies

partial power to the load from a rechargeable energy source attached to the DVR dc

terminal. The DVR, with its three single phase independent control and inverter design is

able to restore line voltage to critical loads during sags caused by unsymmetrical L-G, L-

L, L-L-G, as well as symmetrical three phase faults on adjacent feeders or disturbances

that may originate many miles away on the higher voltage interconnected transmission

system.Connection to the distribution network is via three single-phase series

transformers there by allowing the DVR to be applied to all classes of distribution

voltages. At the point of connection the DVR will, within the limits of its inverter,

provide a highly regulated clean output voltage.

3.2.2 Unified Power Quality Conditioner (UPQC): The Universal Power Quality

Conditioner (UPQC) is a more complete solution for the power quality problem. The

basic structure of this equipment is shown in shown in Fig 3.2. In this figure, the UPQC

is an association of a series and shunt active filter based on two converters with common

dc link [5], [6]. The series converter has the function to compensate for the harmonic

components

Fig 3.2 Basic Block Diagram of UPQC

8

(Including unbalances) present in the source voltages in such a way that the voltage on

the load is sinusoidal and balanced. The shunt active filter has the function of eliminating

the harmonic components of nonlinear loads in such a way that the source current is

sinusoidal and balanced. This equipment is a good solution for the case when the voltage

source presents distortion and a harmonic sensitive load is close to a nonlinear load as

shown in Fig3.2.

.3.2.3 Solid State Transfer Switch (SSTS): The SSTS consists of two three-phase static

switches, each constituted in turn by two anti-parallel thyristors per phase. Normally, the

static switch on the primary source is fired regularly, while the other one is off. In the

event of a voltage disturbance, the SSTS [6] is used to transfer the load from the

preferred source to an alternative healthy source. This results in a very effective way of

mitigating the effects of both interruptions and voltage dips by limiting their duration as

seen by the load.

Fig 3.3 Basic Block diagram of SSTS

The SSTS can be used very effectively to protect sensitive loads against voltage

sags, swells and other electrical disturbances. The SSTS ensures continuous high-quality

power supply to sensitive loads by transferring, within a time scale of milliseconds, the

load from a faulted bus to a healthy one. The basic configuration of this device consists

9

of two three-phase solid-state switches, one for the main feeder and one for the backup

feeder. These switches have an arrangement of back-to-back connected thyristors, as

illustrated in the schematic diagram of Fig 3.3.

Each time a fault condition is detected in the main feeder, the control system

swaps the firing signals to the thyristors in both switches, i.e., Switch 1 in the main feeder

is deactivated and Switch 2 in the backup feeder is activated. The control system

measures the peak value of the voltage waveform at every half cycle and checks whether

or not it is within a prespecified range. If it is outside limits, an abnormal condition is

detected and the firing signals to the thyristors are changed to transfer the load to the

healthy feeder.

3.2.4 Solid State Breaker (SSB): It offers a solution to several problems originated in

distribution systems since it can act as [6]:

a) Transfer switch by transferring sensitive loads from the normal supply that

experiences a disturbance to an alternate supply unaffected by the disturbance

b) A substation bus-tie switch that is normally open; a fault on one feeder leads to

Opening its circuit breaker, the bus-tie switch will close to serve the loads from other

feeder as soon as the faulty feeder is separated from loads

c) A current limiter that conducts inrush and fault currents for several cycles

Fig 3.4 Block Diagram Of SSB

10

Fig 3.4. Shows a schematic diagram of a SSB acting as a fault current limiter (FCL),

constructed from a number of antiparallel GTO modules; in normal operation GTO

elements are closed, under abnormal condition the breaker detects the rise of both the

steady state current and the rate of current change, di/dt, and opens rapidly. The SSB in

its present form is not likely to replace the conventional circuit breaker. However it has a

number of applications where, used in place of a circuit breaker could provide

uninterrupted power by providing rapid transfer to a secondary feeder or limit reactive in-

rush currents by pulse width modulating the current.

3.3 DISTRIBUTED GENERATION

Most of the electricity produced today is generated in large generating stations,

which is then transmitted at high voltage to the load centers and transmitted to consumers

at reduced voltage through local distribution systems. In contrast with large generating

stations, distributed generation (DG) produce power on a customer's site or at a local

distribution network. The International Energy Agency (IEA) [20] defines distributed

generation as the following: "Distributed generation is a generating plant serving a

customer on-site or providing support to a distribution network, connected to the grid at

distribution level voltages. The technologies include engines, small (and micro) turbines,

fuel cells, and photovoltaic systems [18]. They generate electricity through various small-

scale power generation technologies. Distributed energy resources (DE) refers to a

variety of small, modular power-generating technologies that can be combined with

energy management and storage systems and used to improve the operation of the

electricity delivery system, whether or not those technologies are connected to an

electricity grid.

3.3.1 Advantages of Distributed Generation

Distributed generation has some economic advantages over power from the grid,

particularly for on-site power production [18].

1) On-site production avoids transmission and distribution costs, which otherwise

amount to about 30% of the cost of delivered electricity.

11

2) Onsite power production by fossil fuels generates waste heat that can be used

by the customer. Distributed generation may also be better positioned to use inexpensive

fuels such as landfill gas.

3) End-user perspective: End users who place a high value on electric power can

generally benefit greatly by having back up generation to provide improved reliability.

There are also substantial benefits in high efficiency applications, such as combined heat

and power, where the total energy bill is reduced. End users may also be able to receive

compensation for making their generation capacity available to the power systems in area

where there are potential power shortages.

4) Distribution utility perspective: The distribution utility is interested in selling

power to end users through its existing network of lines and substation. It can be used for

transmission and distribution capacity relief. Thus it can also serve as a hedge against

uncertain load growth and high price hikes on the power market, if permitted by

regulatory agencies.

5) Commercial power producer perspective: Those looking at DG from this

perspective are mainly interested in selling power in the power market. Commercial

aggregators will bid the capacities of the units generated by them. The DG then can be

directly interconnected into the grid or simply serve the load off- grid. However the

perspectives on interconnected DG of typical utility distribution are very conservative in

their approach to planning and operation.

3.3.2 Power Quality problems with DG

The Main Power Quality Issues affected by Distributed Generation are [18].

1. Sustained Interruption: This is the traditional reliability area. Many generators

are designed to provide backup power to the load in case of power interruption. However,

Distributed Generation has the potential to increase the number of interruptions in some

cases.

2. Voltage Regulation: This is often the most limiting factor for how much

Distributed Generation (DG) can be accommodated on a distribution feeder without

making changes.

12

3. Harmonics: There are harmonics concerns with both rotating machines and

inverters, although concern with inverters is less with modern technologies.

4. Voltage Sag: The most common power quality problem is the voltage sag.

3.3.3 Interfacing to the Utility System

While the energy conversion technology may play some role in the power quality,

most power quality issues relates to the type of electrical system interface [18]. However

some notable exceptions are:

1. The power variation from renewable sources such as wind and solar can cause

voltage fluctuations.

2. Some fuel cells and micro turbines do not follow step changes in load and must

be supplemented with battery or flywheel storage to achieve improved reliability.

3. Misfiring of the engine sets can lead to persistent and irritating type of flicker

which is more prominent when magnified by the response of power system.

The main types of electrical system interfaces however are

1) Synchronous machine.

2) Asynchronous or Induction machine.

3) Electronic power inverters.

The most common type of distributed generation employs ac rotating machines i.e.

Induction generator and Synchronous generator.

Though the synchronous machines are most commonly used technology and are

well understood. The machine can follow any load within its designed capability. It is

possible for such machine, which is large enough relative to the capacity of the system at

the PCC to regulate the utility system voltage, which can be a power quality advantage in

certain weak systems. Generators should be sized or designed considerably larger than

the load to achieve satisfactory power quality in isolated operation.

Though it is very simple to interface induction machine to the utility system as no

special synchronizing equipment is necessary. The chief issue however is that a simple

induction generator requires reactive power to excite the machine from the power where

it is connected. Another problem that is prominent in such machines is that the capacitor

bank yields resonance that coincides with the harmonics produced. Most of the DG

13

technologies nowadays have to use electronic power inverter to interface with the

electrical power system. However to achieve better control and to avoid harmonics

problems the inverter technology has changed to switched, pulse width modulated

technologies.

14

CHAPTER 4

DISTRIBUTION STATCOM

4.1 INTRODUCTION

This chapter presents the operating principles of DSTATCOM. The DSTATCOM

is basically one of the custom power devices. It is nothing but a STATCOM but used at

the Distribution level. The key component of the DSTATCOM is a power VSC that is

based on high power electronics technologies.

The Distribution STATCOM is a versatile device for providing reactive

compensation in ac networks. The control of reactive power is achieved via the regulation

of a controlled voltage source behind the leakage impedance of a transformer, in much

the same way as a conventional synchronous compensator. However, unlike the

conventional synchronous compensator, which is essentially a synchronous generator

where the field current is used to adjust the regulated voltage, the DSTATCOM uses an

electronic voltage sourced converter (VSC), to achieve the same regulation task. The fast

control of the VSC permits the STATCOM to have a rapid rate of response.

The DSTATCOM is the solid – state based power converter version of the SVC.

Operating as a shunt – connected SVC, its capacitive or inductive output currents can be

controlled independently from its connected AC bus voltage. Because of the fast-

switching characteristic of power converters, the DSTATCOM provides much faster

response as compare to SVC. DSTATCOM is a shunt connected, reactive compensation

equipment, which is capable of generating and or absorbing reactive power whose output

can be varied so as to maintain control of specific parameters of the electric power

system. DSTATCOM provides operating characteristics similar to a rotating synchronous

compensator without mechanical inertia, due to the DSTATCOM employ solid state

power switching devices it provides rapid controllability of the three phase voltages, both

in magnitude and phase angle.

In addition, in the event of a rapid change in system voltage, the capacitor voltage

does not change instantaneously; therefore the DSTATCOM reacts for the desired

responses. For example, if the system voltage drops for any reason, there is a tendency

for the DSTATCOM inject capacitive power to support the dipped voltages.

15

4.2 Operating Principle of the DSTATCOM

Basically, the DSTATCOM system is comprised of three main parts: a VSC, a set

of coupling reactors and a controller. The basic principle of a DSTATCOM installed in a

power system is the generation of a controllable ac voltage source by a voltage source

inverter (VSI) connected to a dc capacitor (energy storage device). The ac voltage source,

in general, appears behind a transformer leakage reactance. The active and reactive power

transfer between the power system and the DSTATCOM is caused by the voltage

difference across this reactance. The DSTATCOM is connected to the power networks at

a PCC, where the voltage-quality problem is a concern. All required voltages and

currents are measured and are fed into the controller to be compared with the commands.

The controller then performs feedback control and outputs a set of switching signals to

drive the main semiconductor switches (IGBT’s, which are used at the distribution level)

of the power converter accordingly. The basic diagram of the DSTATCOM is illustrated

in Fig 4.1.

Fig 4.1 Block Diagram of the voltage source converter based DSTATCOM

The ac voltage control is achieved by firing angle control. Ideally the output

voltage of the VSI is in phase with the bus (where the DSTATCOM is connected)

voltage. In steady state, the dc side capacitance is maintained at a fixed voltage and there

is no real power exchange, except for losses. The DSTATCOM differs from other

16

reactive power generating devices (such as shunt Capacitors, Static Var Compensators

etc.) in the sense that the ability for energy storage is not a rigid necessity but is only

required for system unbalance or harmonic absorption.

There are two control objectives implemented in the DSTATCOM. One is the ac

voltage regulation of the power system at the bus where the DSTATCOM is connected

and the other is dc voltage control across the capacitor inside the DSTATCOM. It is

widely known that shunt reactive power injection can be used to control the bus voltage.

In conventional control scheme, there are two voltage regulators designed for these

purposes: ac voltage regulator for bus voltage control and dc voltage regulator for

capacitor voltage control. In the simplest strategy, both the regulators are proportional

integral (PI) type controllers. Thus, the shunt current is split into d-axis and q-axis

components. The reference values for these currents are obtained by separate PI

regulators from dc voltage and ac-bus voltage errors, respectively. Then, subsequently,

these reference currents are regulated by another set of PI regulators whose outputs are

the d-axis and q-axis control voltages for the DSTATCOM.

4.3 Principle of Voltage Regulation

1) Voltage Regulation without Compensator:

Consider a simple circuit as shown in Fig 4.2. It consists of a source Voltage E, V

is the voltage at a PCC and a load drawing the current Il. Without a voltage compensator

[8], the PCC voltage drop caused by the load current Il, shown in fig as V,

,

IVSsoVIS *** ,

From above equation,

V

QjPI ll

l

*

so that,

17

xr

lslslsls

llss

VVV

QRPXj

V

QXPRV

jQPjXRV

)()(

))((

The voltage change has a component Vr in phase with V and component Vx, which are

illustrated in Fig 4.2(a). It is clear that both magnitude and the phase of V, relative to the

supply voltage E, are functions of the magnitude and phase of the load current namely the

voltage drop depends on both the real and reactive power of the load. The component V

is rewritten as

ssss XjIRIV

Fig 4.2 A Simple Circuit for demonstrating the voltage regulation principle.

Fig 4.2 (a) Phasor diagram for uncompensated

18

2) Voltage regulation with DSTATCOM:

Now consider a compensator connected to the system. It is as shown in Fig 4.2(b)

shows vector diagram with voltage compensation. By adding a compensator in parallel

with the load, it is possible to make E=V by controlling the current of the

compensator.

Is =Ir + Il

Where Ir is the compensating current.

Fig 4.2(b) Phasor diagram for voltage regulation with compensation

CHAPTER 5

19

MODELING OF DSTATCOM

5.1 INTRODUCTION

The Fig 5.1 shows the basic structure of a six-pulse DSTATCOM to a load bus in

a power system where Rp represents the 'ON' state resistance of the switches including

transformer leakage resistance, Lp is transformer leakage inductance and the switching

losses are taken into account by a shunt dc-side resistance Rdc. A VSI resides at the core

of the DSTATCOM. It generates a balanced and controlled three-phase voltage Vp. The

voltage control is achieved by firing angle control of the VSI. Under steady state, the dc-

side capacitor possesses fixed voltage Vdc, and there is no real power transfer, except for

losses. Thus, the ac-bus voltage remains in phase with the fundamental component of Vp.

However, the reactive power supplied by DSTATCOM is either inductive or capacitive

depending upon the relative magnitude of fundamental component of Vp with respect to

Vt. If |Vt| > |Vp| the VSI draws reactive power from the ac-bus whereas if |Vt| < |Vp|, it

supplies reactive power to the ac-system. This is the basic principle of DSTATCOM.

Fig 5.1 Basic DSTATCOM connected to a load in a distribution system

The sending end source is assumed to be a strong system with high short circuit ratio and

low impedance. Thus, the source voltage is treated as a constant source irrespective of

variations in load current. The equivalent circuit of the above system is shown is figure

below:

20

5.2 Equivalent circuit of the above system with DSTATCOM

5.2 Modeling of DSTATCOM in d-q frame

The dynamic equations governing the instantaneous values of the three-phase

voltages across the two sides of DSTATCOM [9] and the current flowing into it are given

by:

ptppp VVidt

dLR

T

cbap iiiiwhere )( ,

T pc pb pa p

T tc tb ta t V V V V and V V V V ) ( ) (

.

Under the assumption that the system has no zero sequence components, all currents and

voltages can be uniquely represented by equivalent space phasors and then transformed

into the synchronous d-q-o frame by applying the following Park’s transformation (q is

the angle between the d-axis and reference phase axis).

2

1

2

1

2

1

)3

2sin()

3

2sin(sin

)3

2cos()

3

2cos(cos

3

2

T

21

This transformation is called as Park's Transformation. And after transforming to d-q axis

the equations in two phases are given by,

c

b

a

q

d

V

V

V

T

V

V

V

0 ,

c

b

a

q

d

i

i

i

T

i

i

i

0 .Thus the transformed dynamic equations are

) (

1 pd td

p pq pd

p

p pd V V L

i i L

R

dt

di

-----------------(1)

)(1

pqtqp

pdpqp

ppq VVL

iiL

R

dt

diand

-----------------(2),

)( dtdwhere , is the angular frequency of the source voltage.

For an effective dc-voltage control, the input power should be equal to the sum of

load power (if any) and the charging rate of capacitor voltage on an instantaneous basis

[9]. Thus, by power balance between the ac input and the dc output,

dc

dcdcdc

ppqpdpqtqpdtd

R

V

dt

dVCV

RiiiViVp

2

22 )(2

3

----------------(3),

dc

dc

dc

ppqpdpqtqpdtddc

CR

V

CV

RiiiViV

dt

dVHence

)(

2

3,

22

---------(4).

The above equation models the dynamic behavior of the dc-side capacitor voltage.

The equation (1), (2) and (4) together describe the dynamic model of DSTATCOM. The

22

Voltage regulation control strategy for DSTATCOM is concerned with the control of ac-

bus and dc-bus voltage on both sides of DSTATCOM. The dual control objectives are

met by generating appropriate current reference (for d- and q-axis) and, then, by

regulating these currents in the DSTATCOM. PI controllers are conventionally employed

for both the tasks while attempting to decouple the d- and q-axis current regulators.

The DSTATCOM current (ip) is split into real (in phase with ac-bus

voltage) and reactive components. The reference value for the real current is decided so

that the capacitor voltage is regulated by power balance. The reference for reactive

component is determined by ac-bus voltage regulator. As per the strategy, the original

currents in d - q frame (ipd, ipq) are now transformed into another frame, d1-q1 frame,

where d1-axis coincides with the ac-bus voltage (Vt), as shown in Fig4.5. Thus, in d1-q1

frame, the currents ipd1 and ipq

1represent the real and reactive currents and they are given

by,

Fig 5.3 Phasor Diagram showing d-q and d1-q1 frame.

23

tpdtpqpq

tpqtpdpd

iii

iii

sincos

sincos

1

1

Now for DSTATCOM current control, the equations (1), (2) and (4) are modified as,

)(1

111

1

pdtP

pqpdp

ppd VVL

iiL

R

dt

di

-----------(5)

)(1

111

1

pqP

pdpqp

ppq VL

iiL

R

dt

di

------------------(6)

tpdtpqpq

tpqtpdpd

VVV

VVVwhere

sincos

sincos

1

1

----------------------------------(7)

The VSI voltages are controlled as follows,

111

111 )(

dptpqppd

qppdppq

uLViLV

uLiLV

By using the equation (7) in (5) and (6), the equations will be modified to,

11

1

11

1

qpqp

ppq

dpdp

ppd

uiL

R

dt

di

uiL

R

dt

di

-------------------(8)

Also the dc bus voltage dynamic voltage equation is given by

dc

dcpqpd

pdt

CR

V

CVdc

RpiiiV

dt

dVdc

)(

2

3 111

22

-------

(9)

Now the control signals ud1 and uq

1 are determined by selecting the proper values for the

Kp and Ki’s used in the control technique.

5.3 DSTATCOM VOLTAGE REGULATION TECHNIQUE

24

The DSTATCOM improves the voltage sags and swell conditions and the ac

output voltage at the customer points is improved, thus improving the quality of power at

the distribution side In this thesis the voltage controller technique [14] (also called as

decouple technique) is used as the control technique for DSTATCOM. The method is

already discussed in the previous topic. This control strategy uses the dq0 rotating

reference frame because it offers higher accuracy than stationary frame-based techniques

[2]. In this VABC are the three-phase terminal voltages, Iabc are the three-phase currents

injected by the DSTATCOM into the network, Vrms is the root-mean-square (rms)

terminal voltage, Vdc is the dc voltage measured in the capacitor, and the superscripts

indicate reference values. Such a controller employs a phase-locked loop (PLL) to

synchronize the three phase voltages at the converter output with the zero crossings of the

fundamental component of the phase-A terminal voltage. The block diagram of a

proposed control technique is shown in Fig 4.6. Therefore, the PLL provides the angle

to the abc-to-dq0 (and dq0-to-abc) transformation. There are also four proportional-

integral (PI) regulators.

25

Fig 5.4 Block Diagram of DSTATCOM Control

The first one is responsible for controlling the terminal voltage through the

reactive power exchange with the ac network. This PI regulator provides the reactive

current reference Iq*, which is limited between +1pu capacitive and -1pu inductive.

Another PI regulator is responsible for keeping the dc voltage constant through a small

active power exchange with the ac network, compensating the active power losses in the

transformer and inverter. This PI regulator provides the active current reference Id*. The

other two PI regulators determine voltage reference Vd*, and Vq

*, which are sent to the

PWM signal generator of the converter, after a dq0-to-abc transformation. Finally, Vabc*

are the three-phase voltages desired at the converter output.

26

CHAPTER 6

Test Systems and Simulation Results

6.1 Test System for Distribution system

6.1.1 Introduction

Basically, DSTATCOM consists of PWM voltage source inverter circuit and a

DC capacitor connected at one end.. In the distribution voltage level, the switching

element is usually the integrated gate bipolar transistor (IGBT), due to its lower switching

losses and reduced size. Moreover, the power rating of custom power devices is relatively

low. Consequently, the output voltage control may be executed through the pulse width-

modulation (PWM) switching method. IGBT based PWM inverter is implemented using

Universal bridge block from Power Electronics subset of Sim Power Systems. RC

snubber circuits are connected in parallel with each IGBT for protection. Such a model

consists of a six-pulse voltage-source converter using IGBTs/diodes, a 3000Vdc

capacitor, a PWM signal generator with switching frequency equal to 3 kHz, After

modeling of DSTATCOM, It is applied to a simple radial distribution line consisting of

different loads. The single line diagram of the radial distribution system to be tested is

shown in Fig 61. Refer to Appendix A for the complete details of the system.

Fig 6.1 Single Line Diagram of the system used.

27

6.1.2 Test Details:

The Test System details: Table 6.1 The test system details of single line diagram used

Input Voltage 11kv, 50Hz.

Source Impendence 0.968, 0.03H

Line impedance 0.4 , 0.003H

DC Voltage 3000V.

Capacitor 2000F.

Load1 0.5MW, 0.2MVAr

Load2 0.10MW, 0.05MVAr

6.1.3 Testing the DSTATCOM:

To verify the performance of the DSTATCOM, a variable load is connected at

bus 2 and the substation voltage is also changed during the simulation. The sequence of

events simulated is explained as follows. Initially, there is no load connected at bus 2. At

t=200ms, the switch S1 is closed so that load1 is applied and at t=500ms, the switch S2 is

closed i.e load2 is applied too; both switches remain closed until the end of the

simulation. During these events, the terminal voltage of bus 2 decreases showing the

effect of sags and, at t=800ms, the substation voltage is increased to, the terminal voltage

of bus 2 also rises, showing the swell condition.

6.1.4 Simulation Results:

The DSTATCOM along with the Distribution System is simulated in MATLAB /

SIMULINK Software of Version 7.0.1. and the diagram is shown in Fig 6.2 below. For

the detailed circuit refer to Appendix A.

28

Fig 6.2.The Single line diagram implemented in MATLAB.

Fig 6.3 Terminal Voltage of Bus2 in per unit.The three-phase rms value of the line voltage Vab of bus2 for the events previously

described is shown in Fig 6.3. In the absence of the DSTATCOM, the terminal voltage

varies considerably, but such variations are minimized in the presence of the

DSTATCOM.

29

Furthermore, the reactive and active power injected by the DSTATCOM into the

network is shown in Fig6.4, where the consumption of active and reactive powers by the

DSTATCOM is represented by positive values and the generation by negative values.

Fig 6.4 Active (P) and Reactive (Q) Powers injected by the DSTATCOMThe voltage and current waveforms of the phase A after connecting the DSTATCOM in

to the network are shown in Fig 6.5 and Fig 6.6. Clearly we can observe from the figure

that the voltage sag and swell conditions are compensated with DSTATCOM.

Fig 6.5 Terminal voltage of Bus2 Va in pu with DSTATCOM

30

Fig 6.6 Current Ia in pu injected by DSTATCOM into the network.

Fig 6.7 Dc capacitor voltage

31

Fig 6.8 Three phase currents injected by DSTATCOM in to the network.

The 3 phase current injected by the DSTATCOM into the network is shown in Fig 6.8

where as the respective two phase currents i.e. Id and Iq are shown in Fig 6.9 and 6.10

respectively.

32

Fig 6.9 The Id (Active current) injected by DSTATCOM before converting to 3 phase

Fig 6.10 The Iq (Reactive current) injected by DSTATCOM before converting to 3 phase

6.1.5 Conclusion

Voltage sag and swells has emerged as a major concern in the area of power

quality. The voltage sag and swell problems in a 11 kV distribution system is investigated

in this topic. The analysis and simulation of a DSTATCOM application for the voltage

flicker mitigating are presented and discussed. The three-phase rms value of the line

voltage of bus2 for the events previously described is shown in Fig6.3. In the absence of

the DSTATCOM, the terminal voltage varies considerably, but such variations are

minimized in the presence of the DSTATCOM. The dynamic behavior of the dc voltage

is also shown in Fig 6.7. Furthermore, the reactive and active power injected by the

DSTATCOM into the network is shown in Fig 6.4, where the consumption of active or

reactive power by the DSTATCOM is represented by positive values and the generation

by negative values.

33

Hence, by the application of DSTATCOM in to the network the voltage sag and

swell conditions are improved and the voltage is recovered to approximately 1pu voltage.

6.2 Test System with Distributed Generation

6.2.1 Introduction

In this section, the model is developed in MATLAB / SIMULINKS. All the

network components were represented by three-phase models. The distribution feeders

were modeled as series RL impedances. The three-phase transformers were simulated

taking into account the core losses (T circuit). The single-line diagram of the test network

is shown in Fig. 6.2.1. Such network comprises a 132 kV, 50 Hz, sub transmission

system with short-circuit level of 1500 MVA, represented by a Thevenin equivalent

(Sub), which feeds a 33 kV distribution system through two 132/33 kV, / Yg,

transformers. This test system is taken from [16]. An AC generator with capacity of 30

MW is connected at bus 7, which is connected to the network through a 33 / 0.69 kV, /

Yg, transformer. This machine can represent one generator [16] in a thermal generation

plant as well as an equivalent of various generators in a wind or small hydro generation

plant. There is a three-phase capacitor bank of 10 MVAr (selected as one third of the

capacity of generator) connected at the generator terminal plant. In some cases, such a

generator was simulated as an induction generator and in other one as a synchronous

generator. Moreover, there is a DSTATCOM with a capacity of 5 MVAr connected at

bus 5 through a 33/2- kV / Yg transformer. AC Generator Models: The dynamic

behavior of the induction generator was represented by a sixth-order three-phase model

(available in SIMULINK) in the d–q rotor reference frame [Appendix B]. In the cases

simulated without a DSTATCOM, a three-phase capacitor bank was connected to the

terminals of the induction generator, which was adjusted to keep the terminal voltage at 1

p.u. during steady state (usually the capacity of 3 phase capacitor bank is selected as one

third of the capacity of the generator). The mechanical power was considered constant

(i.e., the primer mover and governor effects were neglected). The synchronous generator

was represented by an eighth-order three-phase model (available in SIMULINK) in the

d–q rotor reference frame [Appendix B]. Such a generator was considered equipped with

34

an automatic voltage regulator (AVR) represented by the IEEE –Type 1 model. The

mechanical power was considered constant i.e. the regulator and primary mover

dynamics are neglected.

6.2.2 Test System:

The test system for simulating the DSTATCOM along with AC generators is

shown in Fig 6.2.1. It has, AC generator (G), which includes either of Induction

generator, or synchronous generator.

Fig 6.2.1 Test System with DG and DSTATCOM

The test system consists of a substation, which has the ratings of 132kV and the

short circuit capacity of 1500 MVA. The voltage 132kV is stepped down to 32kV by two

transformers of capacity 100MVA which are connected in / Yg configuration. All the

transmission line parameters and transformer ratings are shown in Appendix B. The

loads are always connected to the system. The DSTATCOM is connected in parallel to

the bus5 with a transformer 33/2 kV. The capacity of DSTATCOM used is 10MVA. At

bus 5 the system voltage is stepped down to 690v using a / Yg transformer. At bus6

the generator is connected. The capacity of generator is 30 MW. The detailed parameters

of the generators are shown in Appendix B.

For the simulation, first the Induction generator is connected and the system is

analyzed and again the synchronous generator is connected and the system is analyzed.

35

Throughout the simulation the loads are connected to the system. The faults applied are

(1) 3 phase to ground fault and single phase fault at bus 4 and fault is cleared by tripping

the branch 2-4 and (2) 3 phase to ground fault at middle of branch 4-5 and fault is cleared

without tripping the branch.

6.3 Simulation with DG

6.3.1 With fault at bus 4 and cleared by tripping line 2-4:

Case (a): DSTATCOM with Induction Generator:

The simulation with induction generator is divided in to two parts: one with a 3-phase

fault and another with a single-phase fault.

1) With Three Phase fault and fault clearance time=0.15s:

A three phase to ground short circuit is applied at bus4 at t=10. 5secs. and eliminated at

t=10.65 by tripping branch 2-4 of the circuit shown in Fig 6.1. The generator terminal

voltage responses for this fault are shown in Fig 6.3.1. It can be verified from the figures

that the two simulations i.e. with DSTATCM and without DSTATCOM are stable since

the voltage is at the acceptable level but with the lower value in without DSTATCOM.

Fig 6.3.1 The Induction Generator terminal voltage

36

Fig 6.3.2 The rotor speed of Induction Generator

Fig 6.3.3 The reactive power injected by DSTATCOM in to the network

37

The rotor speed responses are exhibited in Fig. 6.3.2 It can be seen that both the cases

present a good damping, confirming the fact that the transients of induction generators

are very fast. Note that the pre and posfault rotor speeds are different from each other due

to the distinct values of the terminal voltage. Fig 6.3.3 shows the Reactive power injected

by DSTATCOM in to the network at the fault moment.

2) With Three Phase fault and fault clearance time=0.20s

This case is equal to the previous one except the fault clearance time =0.20s. The

terminal voltage responses are presented in Fig 6.3.4. It can be observed that only the

case with the DSTATCOM voltage controller is stable. Without the DSTATCOM, the

system becomes unstable due to a lack of reactive power. In the other situations, the

DSTATCOM acts as a variable reactive power source. However, the reactive power

injections are shown in Fig. 6.3.6, where the consumption of reactive power by the

DSTATCOM is represented by positive values and the generation by negative values. In

the voltage-control mode, the DSTATCOM increases the injection of reactive power

during and after the short-circuit interval.

Fig 6.3.4 The terminal voltage response of Induction Generator

38

Fig 6.3.5 The rotor speed of Induction generator

Fig 6.3.6 Reactive Power Injected by DSTATCOM

39

It can also be seen from the Fig 6.3.5, without DSTATCOM the rotor speed of the

generator exceeds the stability limit where as without DSTATCOM the system remains

stable.

3) With Single Phase fault and fault clearance time =0.2s

This case was simulated by applying a phase-A-to-ground fault bus 4 at t=10.50 second,

which was cleared at t=10.70s by isolating line 2–4.

Fig 6.3.7 The terminal Voltage of induction generator

The terminal voltage responses are shown in Fig.6.3.7. With or Without DSTATCOM the

system is stable, but only with the DSTATCOM, the voltage quickly recovers to

approximately 0.95 p.u. Otherwise, the postfault terminal voltage is equal to 0.85 p.u.

This is due to the fact that the other phases b and c remains excited by the source.

CASE (b): DSTATCOM with Synchronous Generator:

1) With Three Phase fault and fault clearance time=9 cycles:

The same fault as described in above section was simulated (i.e., a three-phase-to-

ground short circuit at bus 4 cleared by tripping line 2–4). Now, the induction generator

was substituted by a synchronous generator. The dynamic behavior of the generator

40

terminal voltage is presented in Fig 6.3.8. It can be seen that the responses for all

situations are very similar. According to Fig. 6.3.9, which presents the rotor speed

responses, the initial damping is slightly worse without the DSTATCOM.

Fig 6.3.8 The Terminal voltage response of Synchronous Generator.

Fig 6.3.9 The rotor speed of Synchronous generator.

41

This is due to the fact that the Synchronous Generator has a separate excitation where as

the Induction Generator does not has the separate excitation. And this excitation takes

care of the required reactive for the generator.

2) With Three Phase fault and fault clearance time =0.20s

The rotor speed responses and the terminal voltage responses are presented in Fig.

6.3.11and in 6.3.10. Moreover, it can be observed that similar damping is obtained from

the cases without and with a DSTATCOM voltage controller. It can be easily observed

that without and with the DSTATCOM, the Synchronous Generator provides the same

results. This is due to the fact that the Synchronous Generator has the Automatic voltage

regulator (AVR). This AVR takes care of the required reactive power for the generator.

Fig 6.3.10 The terminal voltage response of Synchronous Generator.

42

Fig 6.3.11 The rotor speed of Synchronous generator

3) With Single Phase fault and fault clearance time is equal to 0.20 s

A phase-A-to-ground short-circuit at bus 4 was applied at t=10.5s, which was

cleared at t=10.70s by tripping the line 2–4.The dynamic behavior of the terminal voltage

is shown in Fig. 6.3.12. Note that similar behavior is obtained from both the cases. There

is not any enhancement in the system dynamic performance due to the presence of the

DSTATCOM.

43

Fig 6.3.12 Terminal Voltage of Synchronous Generator

6.3.2 With Fault in between buses 4-5 and cleared without line tripping:

All simulations presented in this section were obtained using Matlab/Simulink.

The short circuits simulated were applied in between buses 4-5 from t=10.5s to t=10.7s

and cleared without line tripping. The system loads remained connected to the network

during the contingencies analyzed. The objective of this study is to determine the

influence of a DSTATCOM on the short-circuit currents supplied by ac generators during

faults.

A) With Three Phase to ground fault:

The installation of ac generators may elevate the values of the short-circuit

currents, becoming mandatory to update the protection and/or the network devices. Thus,

in this section, the short-circuit currents supplied by the ac generators during faults are

determined by using simulations. The fault and ground resistances were set equal to

44

0.001 ohm. Fig. 6.3.13 and Fig 6.3.14 presents the dynamic behavior of the currents

supplied by both the induction and synchronous generators (stator current) during a three-

phase-to ground short circuit applied at bus 5 at t=10.5s. It can be seen that the current

response is different for each generator.

Fig 6.3.13 Phase A stator currents of Synchronous generator with and without

DSTATCOM

In the case of the induction generator, although initially the magnitude of the

currents is high, they decrease quickly because this machine has no capacity to provide

sustained short-circuit currents unloaded. Consequently, there is no external excitation

source for the generator, and it becomes unable to produce voltage. In the case of

synchronous generators, it can be observed that the usage of the excitation system as a

voltage regulator permits that the generator supplies sustained short-circuit current.

It can be verified from the Fig 6.3.13 and 6.3.14 that the presence of the

DSTATCOM has no influence on the short-circuit currents provided by the ac generators.

Furthermore, comparing Figs.6.3.13 and 6.3.14, it can be noted that even though the

45

initial value of the short-circuit current provided by the synchronous generator is larger

than the current supplied by the induction generator, the latter decays very quickly.

Fig 6.3.14 Phase A stator current of Induction Generator with and without DSTATCOM

Therefore, an induction generator with a DSTATCOM controlled by voltage may

be a good solution for distribution networks. And if synchronous generators are used,

then there is no need to install a separate DSTATCOM.

46

6.4 Case Study of Agasthyamuzhy substation

6.4.1 Introduction

The following figure shows the structural layout of the substation. For the case

study, Agasthyamuzhy substation is selected. The incoming feeder for the substation

consists of 2lines of 110kV from kunnamangalam. These 110kV lines are stepped down

to 33kV using a 16MVA Yg/ Transformer. Again these 33kV bus is stepped down to

11kV using 10MVA transformer for supplying to local load centers. The outgoing feeder

form the substation consists of two feeders of 11kV each going to Manassery and

Omassery.

Fig 6.4.1 Structural layout of Agasthyamuzhy Sub-Station

47

Also shown in the Figure a 6MW Hydel Power generation located at

Chembukadavu village. The distance from Chembukadavu village to Agasthyamuzhy

station is 25kms. This is a Pico Hydel station since it generates only a small amount of

power. This Hydel station is operated only during rainy season since it has plenty of

water in that season. After the rainy season this hydel station is turned off due to lack of

water. A power of 6MW is generated at 11kv at this station and transmitted at 33kV. In

rainy season the power generated is used for local feeders and also it supplies for the grid

if extra power is generated. The generator used for electricity generation is Synchronous

Generator. However the Induction Generator is also taken and simulated for the

comparison purpose.

The single diagram of Fig 6.2.1 is taken and the complete system is replaced by

the data from Agasthyamuzhi substation. A model is developed in

MATLAB/SIMULINK with Synchronous Generator and DSTATCOM.A 3phase-fault is

applied at near to Generator and cleared with out line tripping. See Appendix(C) for

complete data.

6.4.2 Simulation Results

A three phase to ground fault is applied at t=10.5s in between buses 2-4 and

cleared at t=10.65s without line tripping. Since the hydel power station consists of

Synchronous generators, it will have an exciter system, such that the required reactive

currents are obtained from the excitation system. Hence there is not much any

improvement in the terminal voltage of the generator.

In general, for power generation the Induction generators are also used, mainly in

Wind Generation systems. In this study, the synchronous generator is replaced by the

Induction Generator of the same capacity and the same fault is applied and removed

without line tripping. The terminal voltage of the Induction generator is shown in Fig

6.4.3 below. By observing the Fig 6.4.2 and 6.4.3, the Induction Generators installed with

DSTATCOM, recovers the voltage to 1pu in less time compared to without

DSTATCOM.

48

Fig 6.4.2 The Terminal voltage of the Synchronous Generator

Fig 6.4.3 The terminal Voltage of the Induction Generator

Hence, In Wind generations where the Induction generators are installed, the

DSTATCOM improves the performance of the generator during sag conditions.

49

CHAPTER 7CONCLUSIONS

7.1 Conclusion

Custom power devices like DVR, D-STATCOM, and UPQC can enhance power

quality in the distribution system. Based on the power quality problem at the load or at

the distribution system, there is a choice to choose particular custom power device with

specific compensation. Distribution Static Synchronous Compensator (DSTATCOM) can

compensate the voltage sag and swells conditions. A simple control technique called as

Voltage Regulation Technique is simulated for DSTATCOM control and the same is

applied to the radial distribution system. The Simulation results shows that the

DSTATCOM can compensate the voltage sag and swell conditions caused due to sudden

switching of loads.

The DSTATCOM voltage controller can significantly improve the voltage

stability performance of induction generators without increasing the short-circuit currents

provided by them. A DSTATCOM voltage controller does not introduce significant

improvements in the transient stability of synchronous generators. In fact, the AVR

system of these machines can provide voltage control. In a distribution system suffering

from short-circuit level and stability constraints, the installation of an induction generator

combined with a DSTATCOM voltage controller may be a good choice for distributed

generation expansion since the fault currents are minimized in the case of Induction

generators. Hence, in the cases of Wind Generations where the Induction Generators are

majorly used, it is a good choice to install a DSTATCOM since it can provide the

required reactive support for the system

7.2 Scope for future work

In this thesis work, it is shown that the DSTATCOM can mitigate the

voltage sag and swell conditions. The work can be extended to reduce the source voltage

and source current harmonics supplied due to the non-linear loads. This thesis can also be

extended for multilevel inverters to reduce the harmonic current at the supply side due to

50

loads. This thesis is done for only single generators and can be extended to multi-

connected generators with multi level inverters for DSTATCOM.

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53

APPENDIX

Appendix - A (Data of Distribution System)

Source Details:

Voltage ph-ph (rms) : 11kV

Frequency : 50 Hz

Short Circuit MVA : 12.5

Source Resistance : 0.968

Source Inductance : 0.03H

Distribution Line data:

Length of Line : 1 km.

Positive and Zero Sequence Resistance (in ohms/km):

R1 : 0.1903 R0 : 0.4359

Positive and Zero- Sequence Inductance (in H/km):

L1: 1.249mH L0 : 5.9mH

Loads Data:

Load1 : 0.5 MW and 0.2 MVAr

Load2 : 0.1 MW and 0.05 MVAr

Coupling Transformer: (Yg / )

Rated Power : 100 KVA

Rated Voltage : 11kV / 2kV

Resistance : 0.01 pu

Inductance : 0.02pu

DSTATCOM details:

Power Device used : IGBT

DC side Capacitor : 2000 F

Inverter DC Voltage : 3000V

Switching Frequency : 3000Hz

Appendix – B (Data of Distributed Generation)

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Source Details:

Voltage ph-ph (rms) : 132kV

Frequency : 50 Hz

Short Circuit MVA : 1500MVA

Source Resistance : 0

Source Inductance : 0.0369H

Source Transformer details:

Rated Power : 100MVARated Voltage : 132 / 33kVR1, R2 : 0.005puL1, L2 : 0.02pu

Feeder Details:Table A Feeder Details

BranchResistance in

OhmsInductance in

Henry

2 - 4 2.34 9.9e-3

2 - 3 0.486 5.54e-3

3 - 4 2.6 12e-3

4 - 5 1.3 6e-3

Load Details:Table B Load Details

Load at branch

P (MW) Q (MW)

2 58 12

3 6 2

4 24 5

5 12 3

Generators Data:A) Induction Generator:

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Power : 30MVA

Voltage (rms) : 0.69kV

Stator Resistance : 0.01pu

Stator Inductance : 0.1pu

Rotor Resistance : 0.014pu

Rotor Inductance : 0.098pu

Inertia constant : 1.5s

Mutual Inductance: 3.5 pu

B) Synchronous Generator:

Power : 30MVA

Voltage (rms) : 0.69kV

Stator Resistance : 0.0014pu

Reactances :

Xd=1.4pu, Xd1=0.231pu, Xd

11=0.118

Xq= 1.372pu, Xq1=0.8, Xq

11=0.118, Xl=0.05pu

Inertia constant : 1.5s

Appendix C (Case study data)

Substation details

Voltage incoming = 110kV(2lines from kunnamangalam)

Transformer ratings:

HV = 110kV

LV = 33kV

MVA = 16MVA

Out going feeders = 11kV lines (2 nos)

1) Manassery

2) Omassery

Hydel Power from Chembukadavau

Power generated = 6MW at 11kV

Distance from chembukadavu to Agasthuamuzhi = 25kms.

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