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SUPERVISORY HYBRID CONTROL OF A WIND ENERGY CONVERSION AND

BATTERY STORAGE SYSTEM

by

Muhammad Shahid Khan

A dissertation submitted in conformity with the requirements

for the degree of Doctor of Philosophy

Graduate Department of Electrical and Computer Engineering

University of Toronto

Copyright by Muhammad Shahid Khan, 2008


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SUPERVISORY HYBRID CONTROL OF A WIND ENERGY CONVERSION AND

BATTERY STORAGE SYSTEM

Doctor of Philosophy

2008

Muhammad Shahid Khan

Graduate Department of Electrical and Computer Engineering

University of Toronto

ABSTRACT

This thesis presents a supervisory hybrid controller for the automatic operation and control of a

wind energy conversion and battery storage system. The supervisory hybrid control scheme is

based on a radically different approach of modeling and control design, proposed for the subject

wind energy conversion and battery storage system.

The wind energy conversion unit is composed of a 360kW horizontal axis wind turbine

mechanically coupled to an induction generator through a gearbox. The assembly is electrically

interfaced to the dc bus through a thyristor-controlled rectifier to enable variable speed operation

of the unit. Static capacitor banks have been used to meet reactive power requirements of the

unit. A battery storage device is connected to the dc bus through a dc-dc converter to support

operation of the wind energy conversion unit during islanded conditions. Islanding is assumed to

occur when the tiebreaker to the utility feeder is in open position. The wind energy conversion

unit and battery storage system is interfaced to the utility grid at the point of common coupling

through a 25km long, 13.8kV feeder using a voltage-sourced converter unit. A bank of static

(constant impedance) and dynamic (induction motor) loads is connected to the point of common

coupling through a step down transformer.

A finite hybrid-automata based model of the wind energy conversion and storage system has

been proposed that captures the different operating regimes of the system during grid-connected

and in islanded operating modes. The hybrid model of the subject system defines allowable


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operating states and predefines the transition paths between these operating states. A modular

control design approach has been adapted in which the wind energy conversion and storage

system has been partitioned along the dc bus into three independent system modules. Traditional

control schemes using linear proportional-plus-integral compensators have been used for each

system module with suitable modifications where necessary in order to achieve the required

steady state and transient performance objectives. A supervisory control layer has been used to

combine and configure control schemes of the three system modules to suite the requirements of

system operation during any one operating state depicted by the hybrid model of the system.

Transition management strategies have been devised and implemented through the supervisory

control layer to ensure smooth inter-state transitions and bumpless switching among controllers.It has been concluded based on frequency domain linear analysis and time domain

electromagnetic transient simulations that the proposed supervisory hybrid controller is capable

of operating the wind energy conversion and storage system in both grid-connected and in

islanded modes under changing operating conditions including temporary faults on the utility

grid.


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Dedicated to my mother who passed away during the course of this work.

May Allah rest her gentle soul in eternal peace.

Aameen.


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ACKNOWLEDGEMENTS

I would like to express my gratitude to Prof. M.R. Iravani for his guidance and support in

bringing this work to completion. Thanks are also due to those friends and faculty support staff

who made my stay at the University of Toronto more comfortable and pleasant. Financial

support from Prof. M. R. Iravani and from the School of Graduate Studies in the form of research

grants, open fellowship awards and Roger fellowship awards are gratefully acknowledged.

Continuous encouragement and support from my parents and other family members has been

instrumental in completing this work.


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TABLE OF CONTENTS

ABSTRACT ................................................................................................................................................. iiACKNOWLEDGEMENTS.......................................................................................................................... v

TABLE OF CONTENTS............................................................................................................................. vi

LIST OF TABLES....................................................................................................................................... ix

LIST OF FIGURES ...................................................................................................................................... x

NOMENCLATURE................................................................................................................................xviii

ABBREVIATIONS................................................................................................................................... xix

1. INTRODUCTION ................................................................................................................................. 1

1.1 BACKGROUND...................................................................................................................... 1

1.2 PROBLEM STATEMENT ...................................................................................................... 21.3 STUDY SYSTEM.................................................................................................................... 31.4 RESEARCH OBJECTIVES..................................................................................................... 71.5 LIMITATIONS........................................................................................................................ 91.6 THESIS OUTLINE................................................................................................................ 10

2. OPERATION AND CONTROL ......................................................................................................... 12

2.1 OPERATION OF THE STUDY SYSTEM ........................................................................... 122.1.1 Power Management ......................................................................................................... 13

2.1.1.1 Steady State Power Management ....................................................................................... 132.1.1.2 Transient Power Management ............................................................................................14

2.1.2 Load Management ........................................................................................................... 14

2.2 STATE TRANSITION DIAGRAM....................................................................................... 152.3 CONTROL DESIGN .............................................................................................................17

2.3.1 Modular Design Philosophy ............................................................................................ 19

2.3.2 Supervisory Control......................................................................................................... 21

2.4 PERFORMANCE SPECIFICATIONS.................................................................................. 232.4.1 Steady State Specifications.............................................................................................. 23

2.4.2 Transient Specifications................................................................................................... 23

2.5 METHODOLOGY................................................................................................................. 242.6 SUMMARY ........................................................................................................................... 25

3. MODELING AND CONTROL OF SYSTEM MODULES................................................................ 26

3.1 MODULE: WIND ENERGY CONVERSION UNIT............................................................ 263.1.1 Modeling and Control...................................................................................................... 26

3.1.1.1 Control Structure................................................................................................................ 273.1.2 Sensitivity Analysis ......................................................................................................... 29

3.1.2.1 Operating Point Sensitivity.............. ........... .......... ........... ........... .......... ........... .......... ......... 293.1.2.2 Parametric Sensitivity.............. .......... ........... .......... ........... .......... ........... .......... ........... ....... 32

3.1.3 Simulation Studies........................................................................................................... 38

3.1.3.1 Response to Step Changes..................................................................................................38


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3.1.3.2 Performance under Dynamic Wind Conditions........ .......... ........... ........... .......... ........... ..... 413.2 MODULE: VSC-UTILITY GRID ......................................................................................... 43

3.2.1 Control Structure .............................................................................................................43

3.2.2 Sensitivity Analysis ......................................................................................................... 463.2.2.1 Operating Point Sensitivity.............. ........... .......... ........... ........... .......... ........... .......... ......... 463.2.2.2 Parametric Sensitivity.............. .......... ........... .......... ........... .......... ........... .......... ........... ....... 48


3.2.3.1 Steady State Performance...................................................................................................553.2.3.2 Dynamic Performance ........................................................................................................55

3.3 MODULE: BATTERY STORAGE AND DC-DC CONVERTER....................................... 593.3.1 Modeling and Control...................................................................................................... 59

3.3.2 Sensitivity Analysis ......................................................................................................... 60

3.3.2.1 Operating Point Sensitivity.............. ........... .......... ........... ........... .......... ........... .......... ......... 603.3.2.2 Parametric Sensitivity.............. .......... ........... .......... ........... .......... ........... .......... ........... ....... 63

3.3.3 Simulation Studies........................................................................................................... 673.4 SUMMARY AND CONCLUSIONS..................................................................................... 69

3.4.1 Module: Wind Energy Conversion Unit.......................................................................... 69

3.4.2 Module: VSC-Utility Grid............................................................................................... 71

3.4.3 Module: Battery Storage and DC-DC Converter............................................................. 72

4. SUPERVISORY HYBRID CONTROL .............................................................................................. 74

4.1 HYBRID CONTROL SYSTEMS.......................................................................................... 744.2 HYBRID MODEL OF THE STUDY SYTEM...................................................................... 75

4.2.1 Finite Hybrid Automata................................................................................................... 76

4.3 SUPERVISORY HYBRID CONTROL OF THE STUDY SYSTEM................................... 79

4.3.1 Supervisory Control Requirements.................................................................................. 79

4.3.2 Supervisory Hybrid Control Philosophy.......................................................................... 80

4.3.3 Hybrid Control of VSC: Valve Switching Control.......................................................... 82

4.3.4 Control Transition Management...................................................................................... 87

4.3.4.1 State Initialization.................. .......... ........... .......... ........... ........... .......... ........... .......... ......... 884.3.4.2 Parameter Scheduling.........................................................................................................90

4.3.5 Mode Transition Management......................................................................................... 90

4.3.5.1 Synchronization..................................................................................................................914.3.5.1.1 Signal Transfer .................................................................................................... 92

4.3.5.2 On-grid to Off-grid Transition........... ........... .......... ........... .......... ........... .......... ........... ....... 924.4 FLOW CHART...................................................................................................................... 934.5 SUMMARY ........................................................................................................................... 94

5. SYSTEM OPERATION UNDER NORMAL CONDITIONS............................................................ 95

5.1 STUDY CASES ..................................................................................................................... 955.2 WIND ENERGY CONVERSION UNIT-UTILITY GRID................................................... 96

5.2.1 Control Scheme ............................................................................................................... 96


5.2.2.1 Steady State Performance...................................................................................................99


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5.2.2.2 Dynamic Performance ........................................................................................................995.3 WIND ENERGY CONVERSION UNIT-STORAGE......................................................... 108

5.3.1 Control Scheme ............................................................................................................. 108

5.3.2 Simulation Studies......................................................................................................... 110

5.4 STORAGE-UTILITY GRID................................................................................................ 1115.4.1 Control Structure ........................................................................................................... 111


5.5 STORAGE-VSC-LOAD...................................................................................................... 1175.5.1 Control Structure ........................................................................................................... 117


5.6 WIND ENERGY CONVERSION UNIT-STORAGE-UTILITY GRID ............................. 1235.6.1 Control Structure ........................................................................................................... 123


5.7 SUMMARY AND CONCLUSIONS................................................................................... 130

6. SYSTEM OPERATION INVOLVING STATE TRANSITIONS .................................................... 132

6.1 STUDY CASES ................................................................................................................... 1336.2 SYSTEM STARTUP AND STANDBY.............................................................................. 1356.3 STATE TRANSITIONS ...................................................................................................... 137

6.3.1 Off-Grid Mode of Operation ......................................................................................... 137

6.3.2 On-Grid Mode of Operation.......................................................................................... 141

6.3.2.1 State Transitions during Normal Operation.......... ........... ........... .......... ........... .......... ....... 1416.3.2.2 State Transitions during Temporary Fault Conditions ........... ........... .......... ........... .......... 145

6.4 MODE TRANSITIONS....................................................................................................... 149

6.4.1 Pre-planned Transitions................................................................................................. 1496.4.1.1 Synchronization................................................................................................................1496.4.1.2 On-Grid to Off-Grid Transitions ...................................................................................... 153

6.5 SUMMARY AND CONCLUSIONS................................................................................... 159

7. CONCLUSIONS................................................................................................................................ 161

7.1 OVERVIEW......................................................................................................................... 1617.2 CONCLUSIONS.................................................................................................................. 1637.3 CONTRIBUTIONS.............................................................................................................. 1647.4 FUTURE WORK ................................................................................................................. 165

APPENDIX A........................................................................................................................................... 166

APPENDIX B........................................................................................................................................... 170APPENDIX C........................................................................................................................................... 175

APPENDIX D........................................................................................................................................... 189

APPENDIX E ........................................................................................................................................... 206

REFERENCES ......................................................................................................................................... 223


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LIST OF TABLES

TABLE Page

Table 3.1-1: WECU; Steady state operating points.................................................................... 29

Table 3.1-2: WECU; Eigenvalues corresponding to the steady state operating conditions in

Table 3.1-1. .......................................................................................................... 30

Table 3.1-3: WECU; Mode association of state variables and participation factors for case 232

Table 3.2-1: VSC-Utility Grid; Steady state operating points.................................................... 46

Table 3.2-2: VSC-Utility Grid; Eigenvalues corresponding to the steady state operating points

identified in Table 3.2-1....................................................................................... 47

Table 3.2-3: VSC-Utility Grid; Eigenvalues and mode association for case 2........................ 49

Table 3.2-4: VSC-Utility Grid; Dominant modes, mode association of state variables and

participation factors for case 2 .......................................................................... 49

Table 3.3-1: Battery Storage and dc-dc converter (Boost mode of operation); Poles and RHP

zeros at different steady state operating points .................................................... 61

Table 3.3-2: Battery Storage and DC-DC Converter (Buck mode of operation); Eigenvalues at

different steady state operating points ................................................................. 62

Table 5.1-1: Study Cases and Objectives of the Performance Investigation; System Operation

Under Normal Conditions.................................................................................... 95

Table 6.1-1: Study Cases; System Operation Involving State Transitions............................... 133


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LIST OF FIGURES

FIGURE Page

Figure 1.3-1: Wind energy conversion and battery storage system.............................................. 6

Figure 2.2-1: State Transition Diagram (STD) of the wind energy conversion and storage

system .................................................................................................................. 17

Figure 2.3-1: Supervisory hybrid control structure..................................................................... 18

Figure 2.3-2: Schematic diagram of system module Wind Energy Conversion Unit.............. 21

Figure 2.3-3: Schematic diagram of system module VSC-Utility Grid................................... 21

Figure 2.3-4: Schematic diagram of system module Storage and dc-dc converter.................. 21

Figure 3.1-1: WECU; Proposed current-controlled speed regulation scheme............................ 28

Figure 3.1-2: WECU; Root loci corresponding to different steady state operating points......... 31

Figure 3.1-3: WECU; Close-up of the root loci near to the origin; sensitivity with respect to the

steady state operating point.................................................................................. 31

Figure 3.1-4: WECU; Root locus of mode 1 for variations in the values of the control

parameters between 0 and 2 per unit in steps of 0.1 per unit [0: .1: 2] ................ 33

Figure 3.1-5: WECU; Root locus for mode 2 for variations in the values of the control


Figure 3.1-6: WECU; Root locus for mode 3 for variations in the values of the control


Figure 3.1-7: WECU; Root locus of mode 4 for variations in the values of the control


Figure 3.1-8: WECU; Sensitivity of mode 1 and 4 with respect to the LPF time constant iw

(from 3.0ms to 75.0ms in steps of 3.0ms)............................................................ 37

Figure 3.1-9: WECU; Response to step changes in wind speed, 1) wind speed 2) dc output

current and rectifier current limitation 3) optimum, reference and actual speed of

the generator......................................................................................................... 39


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Figure 3.1-10: WECU; Response to step changes in wind speed, 1) generator reactive power

consumption and excitation capacitor bank switching event 2) generator output

power 3) angular displacement between the generator and the turbine rotors.... 39

Figure 3.1-11: WECU; Response to step changes in the dc bus voltage, 1) dc bus voltage 2)

rectifier output current 3) angular displacement of the generator with respect to

the wind turbine ................................................................................................... 40

Figure 3.1-12: WECU; Operation during dynamic wind speed conditions, 1) wind speed 2)

optimum, reference and actual speed of the generator and capacitor bank

switching events 3) Reference and actual output dc current............................... 42

Figure 3.2-1: VSC-Utility Grid; Single line schematic and control structure ............................ 45Figure 3.2-2: VSC-Utility Grid; Plot of the eigenvalues (32, 33) corresponding to operating

conditions from full load in rectifier mode ( dI = -1.0 p.u.) to full load in inverter

mode ( dI = 1.0 p.u.) ............................................................................................ 48

Figure 3.2-3: VSC-Utility Grid; Loci of the eigenvalues corresponding to mode 1 for variations

in pdK and idK of the outer dc regulator ............................................................ 50

Figure 3.2-4: VSC-Utility Grid; Loci of mode 1 for variations in piK and iiK of the inner

current regulators ................................................................................................. 51

Figure 3.2-5: VSC-Utility Grid; Plot of the positive eigenvalues corresponding to Mode 2 for

variations in the parameters of the current and dc voltage regulators ................. 52

Figure 3.2-6: VSC-Utility Grid; Plot of the eigenvalue of mode 2 for variations in the

parameters vfb and ivK ....................................................................................... 53

Figure 3.2-7: VSC-Utility Grid; Traces of the eigenvalue of mode 2 for variations in piK and

iiK of the inner current regulators....................................................................... 53

Figure 3.2-8: VSC-Utility Grid; Trace of the eigenvalue associated with mode 3..................... 54

Figure 3.2-9: VSC-Utility Grid; Response to load switchings and step changes in reference

voltages of the dc bus and the load bus, 1) dc current (disturbance) 2) & 3)

active and reactive terminal currents of the VSC........................................... 57


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Figure 3.2-10: VSC-Utility Grid; Response to load switchings and step changes in reference

voltages of the dc bus and the load bus, 1) reference and actual three phase rms

voltage at the load bus 2) reference and actual dc bus voltage........................... 58

Figure 3.3-1: Battery Storage and DC-DC Converter; Control structure and schematic diagram

.............................................................................................................................. 60

Figure 3.3-2: Battery Storage and DC-DC Converter; Loci of the eigenvalues 2 and 3,4

corresponding to the steady state operating points given in Table 3.3-2............. 62

Figure 3.3-3: Battery Storage and DC-DC Converter (Boost Mode); Eigenvalue trace with

respect to the proportional gain pdcK .................................................................. 63

Figure 3.3-4: Battery Storage and DC-DC Converter (Boost Mode); Eigenvalue trace with

respect to the integral gain idcK .......................................................................... 64

Figure 3.3-5: Battery Storage and DC-DC Converter (Buck Mode); Root loci with respect to the

proportional constant pdcK ................................................................................. 65

Figure 3.3-6: Battery Storage and DC-DC Converter (Buck Mode); Root locus with respect to

the integral constant idcK .................................................................................... 66

Figure 3.3-7: Battery Storage and DC-DC Converter (Boost Mode), 1) dc output and inductor

current 2) dc bus voltage..................................................................................... 67

Figure 3.3-8: Battery Storage and DC-DC Converter (Buck Mode), 1) injected at the dc bus

and inductor current 2) dc bus voltage................................................................ 68

Figure 3.3-9: Battery Storage and DC-DC Converter, 1) reference and actual dc bus voltage in

boost mode 2) reference and actual dc bus voltage in buck mode...................... 69

Figure 4.2-1: Finite Hybrid Automata (FHA) of the wind energy conversion and storage system

.............................................................................................................................. 78

Figure 4.3-1: Hybrid switching control of the VSC ................................................................... 84

Figure 4.3-2: Operation of the VSC with only SPWM and with hybrid valve switching control,

1) a phase voltage at the PCC, instantaneous value and amplitude 2) fault current

3) reference and actual phase a converter current............................................. 86

Figure 4.3-3: Operation of the VSC under hybrid valve switching control, 1) cycle-to-cycle

based instantaneous duty ratio and the average duty ratio over a power cycle 2)


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instantaneous switching frequency and the average frequency over a power cycle

.............................................................................................................................. 87

Figure 4.3-4: Re-initialization of inner current regulators for smooth transition from HSVM to

SPWM based valve-switching control................................................................. 89

Figure 4.3-5: Re-initialization of inner current regulators, 1) duration of the OCC operation and

orthogonal components of the converter terminal voltage for resetting of inner

current regulators 2) duration of the OCC operation and the orthogonal

components of the terminal current of the converter........................................... 90

Figure 4.3-6: Frequency control during synchronization ........................................................... 91

Figure 4.4-1: Simplified flow chart for software implementation of the supervisory hybridcontrol scheme ..................................................................................................... 93

Figure 5.2-1: Single line schematic and control structure of the study system in the operating

state #7 consisting of the two system modules i) WECU and ii) VSC Utility

Grid ...................................................................................................................... 97

Figure 5.2-2: Operating State # 7; System Dynamic Performance for Step Changes in Wind

Speed, 1) wind speed 2) rotor optimal, reference and actual speed................. 101

Figure 5.2-3: Operating State # 7; System response to step changes in wind speed, 1) rectifier

output current and active current output of the VSC 2) VSC reactive current

output 3) VSC total current output and HSVM on duration............................. 101

Figure 5.2-4: Operating State # 7; System response to step changes in wind speed, 1) dc bus

voltage 2) load bus rms voltage ........................................................................ 102

Figure 5.2-5: Operating State # 7; System Operation under Load Transients, 1) load bus rms

voltage 2) dc bus voltage load 3) rectifier output current................................ 103

Figure 5.2-6: Operating State # 7; Control performance under load transients, 1) VSC active

current 2) VSC reactive current 3) VSC total current and HSVM on duration

............................................................................................................................ 104

Figure 5.2-7: Operating State # 7; System response during dynamic wind speed and load

transients, 1) wind speed 2) generator optimum, reference and actual speed.. 106

Figure 5.2-8: Operating State # 7; System Control during dynamic input wind speed and load

transients, 1) load bus rms voltage 2) dc bus voltage ...................................... 107


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Figure 5.2-9: Operating State # 7; System response to dynamic wind conditions and load

transients, 1) & 2) VSC active and reactive current components 2) space

vector magnitude of the VSC current ................................................................ 107

Figure 5.3-1: Single line diagram and control schematic of the study system in the operating

state #4 consisting of the two system modules of (a) wind energy conversion unit

(b) battery storage and dc-dc converter.............................................................. 109

Figure 5.3-2: Operating State # 4; System response to changes in wind speed, 1) dc bus voltage

2) generator L-L terminal voltage and capacitor switching events 3) wind speed

............................................................................................................................ 111

Figure 5.4-1: Single line diagram and control schematic of the study system in operating statenumber #9 consisting of the two system modules of (a) battery storage and dc-dc

converter (b) VSC-Utility Grid ......................................................................... 112

Figure 5.4-2: Operating State # 9; Response to load switching events, 1) dc bus voltage 2) 3

phase rms voltage at the load bus....................................................................... 114

Figure 5.4-3: Operating State # 9; Response to load switching events, VSC terminal currents,

1) active current component 2) reactive current component 3) maximum

current limit and actual output current of the VSC and OCC duration.............. 114

Figure 5.4-4: Operating State # 9; Response to step changes in the dc current, 1) dc current 2)

dc bus reference and actual voltage 3) reference and actual rms voltage at the

load bus .............................................................................................................. 116

Figure 5.4-5: Operating State # 9; Response to step changes in reference voltage signals, 1)

reference and actual dc bus voltages 2) reference and acutal rms voltages of the

load bus .............................................................................................................. 117

Figure 5.5-1: Single line diagram and control schematic of the study system in the operating

state number #3 consisting of the battery storage and dc-dc converter module

interfaced to the load through the VSC.............................................................. 119

Figure 5.5-2: Operating State # 3; Response to load switching and step changes in external dc

current , 1) reference and actual dc bus voltage 2) reference and actual rms

voltage at the PCC 3) reference and actual rms voltage at the load bus .......... 120


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Figure 5.5-3: Operating State # 3; Response to load switching and step changes in the external

dc current, 1) external dc source current 2) active current component of the VSC

3) reactive current component of the VSC 4) total output current of the VSC. 121

Figure 5.5-4: Operating State # 3; Response to step changes in reference signals, 1) reference

and actual voltages of the dc bus 2) reference and actual rms voltages at the PCC

3) reference and actual rms voltages at the load bus ......................................... 122

Figure 5.6-1: Single line schematic and control structure of the study system in the operating

state #6 (WECU + Storage + Utility) where all the three system modules are in

service ................................................................................................................ 125

Figure 5.6-2: Operating State # 6; Response to variations in wind speed, 1) wind speed 2)optimum, reference and actual speed of the generator 3) output current of the

thyristor-controlled rectifier............................................................................... 128

Figure 5.6-3: Operating State # 6; Response during load switching, wind speed changes and

step changes in the reference voltage signals, 1) dc bus voltage 2) rms voltage at

the load bus ........................................................................................................ 128

Figure 5.6-4: Operating State # 6; Response to load switching and step changes in reference

signals, 1) active current components 2) reactive current components 3)

converter limit, total output current and HSVM on duration............................. 129

Figure 5.6-5: Operating State # 6; Response of the storage element to load switching and

reference step changes, 1) battery terminal voltage 2) battery terminal current

............................................................................................................................ 129

Figure 6.2-1: Startup and Standby Operation, 1) dc bus voltage 2) reference and actual rms

voltage at the PCC 3) phase voltages at the PCC ............................................. 136

Figure 6.2-2: Startup and Standby Operation, 1) orthogonal current components of the VSC 2)

battery terminal voltage 3) reference and inductor current............................... 137

Figure 6.3-1: Off-grid operation; Inter-state transitions among state #2 (Standby), state #3

(Storage + VSC) and state #4 (WECU + Storage), 1) dc bus reference and actual

voltage 2) reference and actual rms voltage at the PCC and ITI performance

limits 3) reference and actual rms voltage at the load bus and ITI performance

limits 4) Operating state of the system.............................................................. 139


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(Storage + VSC) and state #4 (WECU + Storage), 1) orthogonal current

components of the VSC 2) orthogonal current components of the load bus 3)

rectifier output current, total instantaneous current of the load branch and the

output current of the VSC .................................................................................. 140


(Storage + VSC) and state #4 (WECU + Storage), 1) battery terminal voltage 2)

reference and actual battery terminal currents ................................................... 140

Figure 6.3-4: State transitions during on-grid mode of operation, 1) reference and actual dc bus

voltage 2) reference and actual rms voltage at the PCC 3) reference and rmsvoltage at the load bus 4) system operating state and OCC operation intervals142

Figure 6.3-5: On-grid mode of operation; transitions during normal operation, 1) reference and

actual active current component of the converter 2) reference and actual reactive

current component of the converter 3) rectifier output current and total current of

the load branch and the VSC ............................................................................. 144

Figure 6.3-6: On-grid mode of operation; transitions during normal operation, 1) battery

terminal voltage 2) reference and actual battery current................................... 144

Figure 6.3-7: On-grid operating mode; state transitions caused by single line to ground fault, 1)

dc bus and thyristor-controlled rectifier output voltage 2) three phase rms

voltage at the PCC and the upper and lower limits defined by the ITI curve 3)

single phase rms voltage at the PCC and the ITI curve 4) control signals used for

state transition management............................................................................... 147

Figure 6.3-8: On-grid operating mode; State transitions caused by single line to ground faults,

1) VSC active current output 2) VSC reactive current output 3) rectifier

output current, total current of the load branch and output current of the VSC 148

Figure 6.3-9: On-grid operating mode; State transitions caused by single line to ground faults,

1) battery terminal voltage 2) reference and inductor current ......................... 149

Figure 6.4-1: Mode transitions; synchronization, 1) a phase voltage waveforms at the two

sides of the TCB and the synchronization interval 2) reference frequency and


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PLL outputs for the two sides of the TCB 3) reference and actual rms voltages

on the utility side of the TCB, at the PCC and at the load bus .......................... 151

Figure 6.4-2: Mode transitions; synchronization, 1) reference and actual dc bus voltage 2)

reference and actual rectifier current, reference and actual active current

component of the converter 3) reference and actual reactive current of the

converter 4) system operating state and duration of the OCC operation.......... 152

Figure 6.4-3: Mode transitions; no load synchronization, 1) control signals 2) wind energy

conversion and storage system operating states................................................. 152

Figure 6.4-4: Pre-planned on-grid to off-grid mode transition, 1) reference and actual dc bus

voltage 2) reference and actual rms voltage at the PCC 3) reference and actualrms voltage at the load bus 4) operating state................................................... 153

Figure 6.4-5: Pre-planned on-grid to off-grid mode transition, 1) active current component of

the VSC 2) reactive current component of the VSC 3) space vector magnitude

of the load branch and the VSC output current.................................................. 154

Figure 6.4-6: Pre-planned on-grid to off-grid mode transition, 1) battery terminal voltage 2)

battery terminal current...................................................................................... 154

Figure 6.4-7: Un-planned on-grid to off-grid mode transition, 1) reference and actual dc bus

voltage 2) reference and actual rms voltage at the PCC 3) reference and actual

rms voltage at the load bus 4) operating state and OCC operation duration .... 156

Figure 6.4-8: Un-planned on-grid to off-grid mode transition, 1) active current component of

the VSC 2) reactive current component of the VSC 3) space vector magnitude

of the load branch and the VSC output current.................................................. 157

Figure 6.4-9: Un-planned on-grid to off-grid mode transition, 1) battery terminal voltage 2)

battery terminal current...................................................................................... 157


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NOMENCLATURE

Instantaneous quantities are represented by lower case letters e.g., x

Space vector quantities are represented with an underscore e.g., tv

Average and DC quantities are represented by upper case letters e.g., dcV

Small perturbations in a signal are represented by a tilde over the instantaneous symbol

for the signal e.g., dcv~


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ABBREVIATIONS

WECU:

VSC:

IGBT:

BES:

SPWM:

HSVM:

CPM:

RHP:

LHS:

RHS:

OLD:

STD:

FHA:

FOS:

SOS:

OCC:

Wind Energy Conversion Unit

Voltage-Sourced Converter

Insulated-Gate Bipolar Transistor

Battery Energy Storage

Sinusoidal Pulse-Width Modulation

Hysteresis Space Vector Modulation

Current Programmed Mode

Right Hand Plane

Left Hand Side

Right Hand Side

Operating Logic Diagram

State Transition Diagram

Finite Hybrid Automata

Fault Operating State

Synchronization Operating State

Over Current Control


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CHAPTER 1

INTRODUCTION

his chapter introduces the research work reported in this thesis. General background of

the subject has been presented first which is followed by the problem statement. A

description of the study system used in the research reported in this thesis has been presented and

research objectives are outlined. Limitations of the reported work have been pointed out

followed by an outline of the thesis given at the end of the chapter

1.1 BACKGROUNDThe depletion of conventional energy sources e.g. oil and gas and the desire to limit their use

due to environmental concerns has led to the search for increased utilization of renewable energy

sources to meet the ever-growing demand of electrical power [1], [2]. Wind is one of the most

promising among the different types of available renewable energy sources [1]-[3]. During the

past decade considerable research has been carried out to improve wind turbine design and

control for increased power conversion efficiency and availability. Recently, the deregulated

electricity market has also opened the doors for customers owned distributed generation due to

perceived economic and technical benefits [4], [5]. Distributed generators are commonly

connected to the system at distribution voltage levels [6]-[8]. To date, distributed generators are

not operated in islanded conditions due to safety concerns of both personnel and equipment, and

are required to disconnect and shutdown in response to disturbances on the utility grid [9]. The

diversified and distributed nature of the supply system, due to increased penetration of

distributed generators and distributed storage devices, has the potential to increase overall

security and reliability of power supply [10]-[11]. Customized power quality and reliability

levels can be achieved according to the individual needs of the customers by using power

electronic based power processing units [12]-[13]. The diversified nature of the power system

together with the use of power electronics based power processing units can result in a power

system capable of providing the desired level of service under a variety of operating conditions.

Using conventional control methods commonly used in power systems, the control of a

system with limited energy capabilities in the off-grid operating mode is a difficult task [14]. The

T


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situation becomes even more complex if the system contains a renewable energy source such as

wind energy power conversion unit(s), since this type of primary energy source is often

intermittent in availability. It becomes necessary to include a dispatchable energy source, such as

a storage device, in the scheme [15]. The control design of such a system also requires a

radically different approach when operation of these systems is required to weather changes in

operating conditions over a wide range. The operating conditions may include on-grid and off-

grid mode of operation and changes in the combination of the internal energy sources supplying

the load during the two operating modes [16].

In this research work the automatic control and management problem of a wind energy

conversion and battery storage system has been tackled from a supervisory hybrid control point

of view. Hybrid control systems are composed of both discrete and continuous state variables

[17], provide superior performance as compared to conventional control schemes [18]-[20] and

allow the pursuit of multiple control objectives [21]. Hybrid control has found widespread

applications from automotive, manufacturing and process industries to aerospace industry [22]-

[25]. Hybrid control has also found limited application in drives control and power systems [26],

[27].

The supervisory hybrid control scheme presented in this thesis for the wind energy

conversion and battery storage system provides automatic control of the system in islanded and

in grid-connected modes of operation under steady state as well as during transient operating

conditions including accidental state transitions caused by faults in the external supply system.

1.2 PROBLEM STATEMENTConnection of the wind energy conversion units to the utility system through power electronic

converters has the advantage that the units can be operated at variable speeds to maximize

energy capture from the prevailing wind conditions and to alleviate stresses in the drive train

[28]. The converter interface to the utility also shields these units from normal disturbances on

the utility side and provides the ability to transfer the wind-generated power to the utility sidewith improved power quality [29]. However, conventional control schemes employed for the

interfacing power electronic converters do not provide satisfactory operation during temporary

faults on the utility systems and these may also be taken out of service due to the operation of the

switch overload protection during such faults. The majority of faults on the utility system are


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temporary single line to ground faults [30] and these can contribute considerably to the

downtime of these wind conversion systems.

As wind is stochastic in nature therefore permanent faults on the system also cause

shutdown of the wind conversion units even if sufficient wind power is available to serve localloads. It is therefore desirable that the wind conversion system is capable of operation during

islanded conditions. This can be a particularly useful feature in case of rural communities or far-

flung areas with abundant wind energy potential and which are supplied through long radial

feeders. A low cost system in terms of initial investment and environmental impacts besides low

maintenance and operational requirements would be the desirables of a distributed energy system

in such communities.

The following are the desirable characteristics of operation of the subject wind energy

conversion and battery storage system to provide economical, reliable and acceptable quality

electrical power:

1. Automatic system operation

2. Fault ride through capability for temporary faults on the utility feeder

3. Protection and control integration for the power converters

4. Ability to operate in on-grid and off-grid modes

5. A control scheme based on available local system information

6. Control performance that conforms to specifications provided in section 2.4.

There is no available literature on wind energy conversion systems that addresses all the above

desirable control and operational requirements. The research presented in this thesis is focused

on the design of a control scheme for a wind energy conversion and battery storage system that

incorporates the above-mentioned desirable economic, control and operating features.

1.3 STUDY SYSTEMFigure 1.3-1 shows the wind energy conversion and battery storage system used for the research

reported in this thesis. The study system is composed of a Wind Energy Conversion Unit

(WECU) and a battery storage element connected to a common dc bus. The system is interfaced

to the load and the utility grid at the Point of Common Coupling (PCC) through a Voltage-

Sourced Converter (VSC) unit. The complete system parameters are given in appendix A.

The WECU itself consists of a horizontal axis, three-blade wind turbine mechanically

coupled through a gearbox to an induction generator with a squirrel cage rotor construction. The


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wind turbine has a maximum output capability of 360kW (1.2 per unit based on 300kVA base).

The generator has a matching maximum rating of 1.2 per unit and a rated voltage of 690V. The

generator unit is connected to the dc bus through a six-pulse thyristor-controlled rectifier and

delivers wind generated power at 1000 volts on the dc side. Static capacitor banks connected at

the generator terminals meet the reactive power demand of the generator and the thyristor

rectifier. The thyristor based rectifier interface gives the wind energy conversion unit the

capability of variable speed operation for maximum energy capture from the prevailing wind

conditions [31], [32].

The storage element is composed of series and parallel combination of battery banks, which

are connected to the dc bus through a dc-dc converter. The wind energy conversion and battery

storage system is connected to the utility grid and to the load at the PCC through the VSC

interface. The VSC is assumed to be composed of three legs, each containing a pair of Insulated-

Gate Bipolar Transistors (IGBTs). The utility grid has been represented by its Thevenin

equivalent at 132kV level and a long (25km) radial feeder at 13.8kV has been assumed for

connection to the wind energy conversion and battery storage system. The local load is served at

480 volts and is connected to the PCC through a step-up transformer. The load consists of both

constant impedance static and dynamic induction motor loads.

The storage device shown in Figure 1.3-1 can be used for transient power support of the

wind energy conversion unit during islanded operation. The storage can be used in combination

with the wind energy conversion unit or on its own to cater to the local load demand in the

absence of the utility supply. The presence of the storage device gives the system the desirable

characteristic of dispatchability. The use of a squirrel cage induction machine reduces system

initial cost and subsequent operational and maintenance requirements [33]. The use of a battery

storage device is environmentally benign and apart from some initial investment has no

significant operational and maintenance costs. The study system therefore provides a very

attractive solution to the power quality and reliability problems of isolated rural communities.

The following are the main features of the proposed system configuration:

1. Use has been made of an induction generator with squirrel cage construction. The output of

the generator is regulated through a thyristor-controlled rectifier. This means less initial

investment and low operation and maintenance costs of the wind energy conversion unit

making it economically more attractive.


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2. The inclusion of a storage element on the dc bus side has a number of advantages than if it

is connected to the system at some other location:

a. The same power electronic interface to the utility grid is utilized for the wind energy

conversion unit and the battery storage element thus reducing the overall cost of the

system.

b. Simple system control: Active and reactive power sharing control has been avoided

which would have been required if the storage were connected at some other point to

the system. This would have been necessary in order to control system frequency and

to regulate node voltages in the system, respectively [33].


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Figure 1.3-1: Wind energy conversion and battery storage system


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1.4 RESEARCH OBJECTIVESAs described in chapter 2, section 2.1, the wind energy conversion and battery storage system

has two operating modes i) on-grid and ii) off-grid. Within these two basic operating modes a

number of operating states could be identified in which the system has different state spacecompositions based on the traditional definition of a state space involving state variables of the

system.

The primary objective of this thesis is to present a simplified approach based on hybrid

control theory to tackle the control design and analysis problem of the study system. Based on

the proposed approach a supervisory hybrid control scheme for the wind energy conversion and

battery storage system shown in Figure 1.3-1 has been presented. The supervisory hybrid

controller will operate the wind energy conversion and storage system in both on-grid and in off-

grid mode of operations, in steady state and during transient system operating conditions as

described in section 2.3.2. The supervisory controller for the study system will use local

information to steer its operation along pre-specified transition routes in response to different

system events. The transition routes are given by the hybrid automata shown in Figure 4.2-1.

The following tasks have been identified to achieve the above stated objective:

1. To develop a systematic approach towards the hybrid automata based hybrid modeling and

control of the wind energy conversion and battery storage system.

The hybrid automata of a system describes the allowable operating states of the system and

transition paths between these operating states that the system will follow in response to some

discrete events. An operating state of the system will be based on the possible combinations of

the energy sources in the system (and not based on its state space composition). This point is

further elaborated in Appendix B in which a State Transition Diagram (STD) has been

developed from the Operating Logic Diagram (OLD) of the study system. The STD depicts

only those operating states in which the system is capable of steady state operation. The STD

will be complemented with transient states as described in CHAPTER 4, to develop the finalFinite Hybrid Automata (FHA) for the wind energy conversion and battery storage system. The

transient states are the temporary operating states of the system during its transition from one

stable operating state in the on-grid operating mode to another in the off-grid mode of operation

and vice versa.


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2. To devise control schemes based on the traditional control techniques using Proportional-

plus-Integral (PI) compensators for the three system modules i.e. the wind energy

conversion unit, the battery storage and dc-dc converter and the VSC-utility grid

independent of each other as described in section 2.3. The operation of the three system

modules under the proposed control schemes will be investigated separately for stability

and performance evaluation. Operational stability and selection of proper control

parameters for the three system modules will be based on eigenvalue sensitivity analysis

with respect to operating point and with respect to control parameters, respectively [35]-

[43].

3. To combine and configure the control schemes developed for the three system modules of

WECU, the battery storage and dc-dc converter and the VSC-utility grid respectively, to

control operation of the wind energy conversion and battery storage system during the

permissible operating states as depicted by the state transition diagram developed in

Appendix B.

4. To devise control schemes for operation of the wind energy conversion and storage system

during the transient operating states depicted in the hybrid automata of the system which

is shown in Figure 4.2-1.

5. To develop suitable transition management strategies to minimize system transients caused

by state transitions and switching of the associated control schemes.

6. To develop supervisory control scheme for the wind energy conversion and storage system

to perform the following actions:

a. Oversee control transfer between different candidate controllers and control

schemes in response to changing operating states.

b. Manage transitions between different operating states. In other words to

reconfigure the wind energy conversion and storage system according to the

system events.

c. Provide suitable reference and feedback signals to the active controllers.

d. Provide control reset and initialization signals to the candidate controllers.

e. Perform power and load management in the wind energy conversion and storage

system.

f. Implement state and control transition management strategies.


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The supervisory controller will infer the changing operating states of the system based on

information from selected indicators. Referring to Figure 1.3-1, the open or closed state of the

TCB (tiebreaker) will indicate the operating mode of the wind energy conversion and storage

system (i.e., on-grid or off-grid mode). The availability of the utility grid will be decided based

on the single phase rms voltages on the utility side of the TCB when these are within the normal

operating range specified by ANSI C84.1- 1995 [44]. Islanding detection algorithms have not

been implemented and islanding has been assumed as a known event. Availability of the battery

storage is assumed to be known a priori (this information could come from an energy

management system for the battery storage). Availability of the wind energy conversion unit will

be inferred from the dc output current of the thyristor-controlled rectifier. Fault on the utility side

will be determined from the peak value of the phase voltages on the utility side of the TCB. For

this purpose the algorithm described in reference [45] will be implemented. Status of the load

(connected or disconnected) will be inferred from the load breaker (LCB) shown in Figure 1.3-1.

In the absence of any theoretical guarantees for the control stability of the wind energy

conversion and storage system, digital time domain simulations of the nonlinear system will be

performed using PSCAD/EMTDC to investigate system operation under the proposed

supervisory hybrid control scheme both for stability and for performance evaluation [46]. This

will also require that suitable performance criteria be specified for the performance assessment

of the study system. Performance specifications have been described in section 2.4.

1.5 LIMITATIONSThe following are the main limitations of this thesis:

1. Linear integral and proportional-plus integral compensators have been used.

2. Parametric sensitivity analysis has been performed with respect to a single control variable

at a time; no inter-parametric sensitivity analysis has been attempted.

3. Aspects concerning economics of the system management have not been treated in this

research.

4. System protection issues are not of primary concern.

5. Neither system nor control parameters have been optimized.


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1.6 THESIS OUTLINEChapter 2 gives details of operation of the wind energy conversion and storage system and

describes management of the system from a hybrid control point of view. Different possible

operating states are enumerated. Load and power management strategies are described andperformance criteria are presented. The chapter also gives a methodology for the control design

of the wind energy conversion and storage system and provides a strategy for evaluating stability

and performance of the system in its different operating states.

Chapters 3 provides control schematics and results of the linear and nonlinear analysis of the

three system modules of the study system i) the wind energy conversion unit consisting of the

wind turbine, the induction generator, static capacitor banks and the thyristor-controlled rectifier

ii) the storage element consisting of the battery storage and the dc-dc converter and iii) the VSC-

utility grid system module, respectively. Each system module contains one energy source and is

treated in the following order:

1. Module: Wind Energy Conversion Unit

2. Module: VSC-Utility Grid

3. Module: Battery Storage and DC-DC Converter

In this chapter associated control schemes for the three system modules have been elaborated

whereas mathematical models have been presented in the appendices.

Chapter 4 deals with the subject of the hybrid supervisory control of the wind energy

conversion and battery storage system. It provides background information into the hybrid

control systems. In this chapter a finite hybrid automata of the wind energy conversion and

storage system has been presented which depicts system operating states and the uni- and bi-

directional links between these states which the system is required to follow while moving from

one state to another. The chapter introduces the concept of transition management used for the

supervisory hybrid control of the wind energy conversion and battery storage system.

Chapter 5 provides stability and performance evaluation of the operation of the study system

during all the operating states under normal operating conditions as depicted by the hybrid

automata of the system shown in Figure 4.2-1. In all these operating states two or more of the

system modules are interacting with each other under normal load switching conditions. Control

schemes have been developed for the operation of the wind energy conversion and battery

storage system in each of the normal operating state. These control schemes have been derived


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from the control schemes proposed for the three system modules of the study system and also

incorporate elements of transition management strategies described in chapter 4. Results of the

stability and performance evaluation of the study system using digital time domain simulations

of the nonlinear system in PSCAD/EMTDC have been presented in this chapter. The following

normal operating states have been considered:

1. Operating State: Wind Energy Conversion Unit-Utility Grid

2. Operating State: Storage-Utility Grid

3. Operating State: Storage-VSC-Load

4. Operating State: Wind Energy Conversion Unit -Storage

5. Operating State: Wind Energy Conversion Unit -Storage-Utility Grid

Chapter 6 provides simulation results for the operation of the wind energy conversion and

storage system during state transitions under the proposed supervisory hybrid control scheme.

The operation of the study system has been investigated for stability and performance, during

state transitions, through time domain simulations in PSCAD/EMTDC environment. Worst case

scenarios for transitions between the permissible operating states of the study system given by

the proposed hybrid automata have been considered.

Chapter 7 concludes the research reported in this thesis. It provides conclusions based on the

reported work and lists thesis contributions. Future research opportunities have been identified at

the end of the chapter.

Appendices are given at the end of the thesis:

1. System parameters are given in appendix A.

2. Appendix B provides a method to determine operating states of the system using simple

Boolean logic to arrive at a State Transition Diagram (STD) for the study system.

3. Appendix C gives modeling details of the wind energy conversion unit.

4. Appendix D provides modeling details of the VSC-Utility grid system.

5. Appendix E details modeling of the battery storage and dc-dc converter.

References are provided at the end of the thesis.


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CHAPTER 2

OPERATION AND CONTROL

n this chapter issues concerning the operation and hybrid control of the wind energy

conversion and storage system have been treated. Methodology for the control design,

selection of control parameters, criterion for performance evaluation and a methodology for

performance verification have been the main subjects of this chapter.

2.1 OPERATION OF THE STUDY SYSTEMReferring to Figure 1.3-1, the wind turbine driven by the lifting force of the wind blowing acrossthe blades drives the mechanically coupled induction generator thereby converting the kinetic

energy of the wind into electrical energy. The electrical output power of the generator is

controlled and delivered at the dc bus through the thyristor rectifier. The generated power could

be delivered to the grid, used for battery charging, or else it could be partly delivered to the grid

and partly stored in the battery storage device for later use.

In the grid-connected mode, the load in the wind energy conversion and storage system can

be served by the energy sources internal to the system as well as by the external utility supply.The load can be supplied with power from the utility feeder with or without any contribution

from the local energy sources and proper system operation can be ensured during both steady

state and dynamic operating conditions. In the isolated mode of operation, the VSC alone

transfers the available power from the wind energy conversion unit and the battery storage at the

desired voltage level and frequency. Power management is required in both grid connected and

during islanded operation in order to control power contribution from the energy sources in the

wind energy conversion and storage system and to direct it to proper sinks.

The two energy sources in the wind energy conversion and battery storage system are

limited in capacity and also a maximum capacity of 1.3 per unit has been assumed for the VSC,

therefore during islanded operation quality power can be assured only with a limited amount of

dynamic motor load connected at the load bus. Some load management is therefore required for

proper off-grid operation regardless of the level of availability of the energy sources in the wind

energy conversion and battery storage system.

I


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The objective of the load management and the power management strategies is to achieve power

balance between the energy sources and the loads at any given time. However power

management is also concerned with the load sharing among the active energy sources in the

system and achieves the objective by continuously controlling power contributions from the

different energy sources in the system. Load management on the other hand, is provided to assist

in power management by ensuring that only that much power is demanded that can be supplied

by the available energy sources within the power quality constraints. Load management is a

discrete phenomenon while power management may involve discrete actions in that one or the

other energy source in the system may be taken in or out of service.

2.1.1 POWER MANAGEMENTThe power management strategy is based on the assumption that the storage will be used as a

backup supply and to provide steady state and transient power support during islanded mode of

operation. Provisions will be made to use the battery storage while the system is operating on-

grid for maintaining the dc bus voltage during severe disturbances including faults on the utility

feeder. Also the wind energy conversion unit will be operated to follow the maxim output curve

of the turbine for maximum power extraction from the prevailing wind.

2.1.1.1 Steady State Power ManagementThe steady state power management is concerned with power delivery from the energy sourcesin the wind energy conversion and battery storage system over longer periods of time usually

called energy management, in both grid connected and in isolated mode of operation.

The following strategy will be used during the grid-connected mode of operation:

1. The WECU will be operated to extract maximum power from the prevailing wind

conditions at all times.

2. All the wind-generated power will be transferred directly to the utility side when the storage

element is fully charged.

3. All or part of the wind-generated power will be used for charging the battery storage when

required.

4. In the absence of wind-generated power, utility supply will be used for charging the storage

element if necessary.

The following strategy will be used in the islanded mode of operation:


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1. Battery storage will be used to cater to the load demand together with WECU when the

power output of the latter is not enough to meet the load demand.

2. When the power output of the WECU exceeds the load demand then the excess power will

be used for charging the batteries.

3. In the absence of wind power generation, storage alone will meet the load power demand.

The case where the batteries are fully charged and the power output of the wind energy

conversion unit exceeds load demand has not been treated in this thesis. One solution in such a

scenario would be to operate the wind energy conversion unit at sub optimal level to match its

output to that of the load demand. Another strategy however is to use some dummy load (for

example some heating load) preferably connected to the dc bus to be able to extract maximum

power from the wind at all times.

2.1.1.2 Transient Power ManagementTransient power management is concerned with the momentary power imbalance in the system

during the course of its operation.

During grid-connected mode of operation, variations in power from the wind energy

conversion unit caused by variations in wind speed as well as variations in load power demand

will be reflected in the amount of power delivered by the utility feeder. In other words, utility

supply will be relied upon for transient power support during grid-connected operation of thewind energy conversion and storage system. Battery storage support during grid-connected

operation will only be used when dc bus voltage variations exceed certain limits in order to

ensure control stability of the system. This point is further explained in section 4.3.2.

In the isolated mode of operation, power transients from WECU and load power variations

will be absorbed by the storage element. In the absence of wind power generation, storage alone

will provide the required transient power support besides meeting steady state power demand.

2.1.2

LOAD MANAGEMENT

The amount of dynamic load that the wind energy conversion and storage system is able to

supply, in the islanded mode of operation without compromising power quality depends on the

capacity of the energy sources internal to the system. Also the VSC, which will transfer the

available energy from the internal sources to the load side, has a limited capacity (1.3 per unit).

Therefore a limited amount of load could be supplied during islanded operation of the system.


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The inrush current at startup of a motor load (predominantly induction motors) which contains a

large reactive component causes voltage dips which in turn causes these loads to lose rotational

speed and in turn demand increased reactive current. This situation slows down the system

voltage recovery and may exceed converter rating and the available power capability of the

energy sources in the wind energy conversion and storage system. A similar situation exists

when motor load is subjected to transient voltage disturbances.

In this thesis a simple load management strategy has been adapted in which the higher rated

motor load ML2 (Figure 1.3-1) will be disconnected following system transition from grid-

connected to islanded mode of operation. The transients caused by the rest of the load are within

the handling capability of the VSC and the energy sources in the system.

2.2 STATE TRANSITION DIAGRAMReferring to Figure 1.3-1, the wind energy conversion and storage system has two basic

operating modes:

1. Grid Connected Mode

2. Islanding Mode

The system has the following two possible operating conditions in each mode of operation:

1. Steady State

2. Transient

In steady state conditions, load and generation are in perfect balance. Under transient conditions

however, there exists a transitory imbalance between the load and generation and the system

settles down to a new equilibrium condition if it remains stable. System transients can be broadly

classified as:

1. Pre-planned Transients

2. Accidental Transients

Pre-planned transients are those resulting from intentional switching e.g. shutting down of the

utility feeder for maintenance work. Accidental transients on the other hand are random in nature

and are caused by system faults and switching in or out of the load as well as the power factor

correction capacitors, among others.


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Both pre-planned and accidental transients can cause the combination of active energy sources in

the system to change leading to a change of the system operating state. Transient system

conditions accompanied by a change in the system operating state will be called a transition

while the dynamic conditions where the operating state of the system remains unchanged will

simply be called transients. A transition may be a result of a permanent system fault e.g. on the

utility feeder, followed by a disconnection from the utility (accidental transition) in which case

the system will switch from the grid connected mode to the isolated mode of operation. The

transition from one mode to the other may also be a pre-planned event e.g. due to maintenance

work on the feeder.

The wind energy conversion and storage system can have finitely many operating states and

in the absence of any analytical tool, in the context of hybrid control systems, the hybrid control

design for the wind energy conversion and storage system and its evaluation for stability and

performance is a very difficult task. It is therefore necessary to curtail the number of states that

the supervisory hybrid control scheme has to manage. Figure 2.2-1 gives the State Transition

Diagram (STD) of the study system with a limited set of operating states for which a supervisory

hybrid control scheme will be devised. Appendix B gives a systematic procedure for determining

the possible operating states of a system with multiple energy sources.

Referring to the STD in Figure 2.2-1, during grid connected mode when storage needs to be

built up, the system enters into the state WECU + Storage + Utility and falls back to the state

WECU + Utility when the storage element is toped up to a pre-specified level. In the VSC +

Utility operating state where only utility is supplying the load; the VSC will be used only for

voltage support at the PCC. Full converter capacity could be utilized for reactive power support

during this mode of operation. During islanding operation, there are three steady state operating

conditions namely:

1. The standby state in which the dc bus is energized using the battery storage but the load

bus is kept disconnected.

2. The storage operating state where the battery storage alone is supplying the load.

3. The WECU + Storage operating state where the output from the wind energy conversion

unit and the storage are used together to meet the load demand.

During the latter two operating states load may or may not be present. The operating state in the

off-grid mode where only wind energy conversion unit is active is not sustainable due to the


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absence of external transient power support and is therefore not represented. In the start up

operating state, the dc bus voltage will be energized in a controlled manner using the battery

storage and its voltage will be stabilized at the nominal value of 1.0 per unit (1000V).

Figure 2.2-1: State Transition Diagram (STD) of the wind energy conversion and storage system

2.3 CONTROL DESIGNControl of the wind energy conversion and storage system using linear compensators and

Sinusoidal Pulse-Width Modulation (SPWM) based switching of the VSC necessarily cannot

provide for adequate control during accidental state transitions caused by faults on the utility

feeder and may not ensure performance requirements during transient operating conditions (e.g.

motor load switching) within each operating state. Considering that a reasonable limit has been

imposed on the output current of the VSC (1.3 p.u. = 1.2 p.u from WTU + 0.1 p.u. margin for

transient conditions as also for reactive power support; combined load is rated at 1.0 p.u.), it will

not be possible to contain its output current below the limit (1.3 p.u.) during a fault (particularly

a close up fault) on the utility feeder (also possibly during motor load switching) and the over

current protection of the VSC associated with its switching elements will take the unit out

causing interruption in service. It is therefore necessary to handle the management of the wind

energy conversion and storage system from a hybrid control perspective [19], [47] and [48].

The hybrid control problem is usually concerned either with switching between a number of


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controllers for a single plant for optimum operation under changing operating conditions or

control of a plant whose state space composition is subject to change due to some discrete time

events or both. In a hybrid control structure the supervisory controller is at the top level of the

hierarchy and interacts both with the unit level regulators as well as the regulated plant(s). Figure

2.3-1 adapted from [49], illustrates the supervisory control structure. The thick solid lines

represent all the external inputs to the plant and all the outputs of the plant while thin solid lines

represent a subset of the plant outputs and the set of (control signal) inputs from the primary

regulators. The dotted lines represent information flow (monitoring and control signals) to and

from the supervisory control layer.

Figure 2.3-1: Supervisory hybrid control structure

The control action by the supervisory layer on the primary regulator may involve switching

between different controllers or updating control parameters for a single regulator or may

involve a combination of the two. Control action on the plant itself may include taking in or out asubsystem (module) of the plant.

The control of the wind energy conversion and storage system will involve all the above-

mentioned aspects of a supervisory hybrid control scheme. Control action on the plant will

involve taking in or out one or more of the energy sources in the wind energy conversion and

battery storage system including the utility supply. The different operating states given by the


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STD of the wind energy conversion and storage system (the plant) correspond to different state

space compositions as a result of the supervisory control actions on the system. This is further

explained in the subsequent sections and in CHAPTER 4 in the context of supervisory hybrid

control of the study system.

2.3.1 MODULAR DESIGN PHILOSOPHYThe wind energy conversion and storage system is a Multiple-Input-Multiple-Output (MIMO)

system and the control design could be tackled as such. As explained in section 2.2, the wind

energy conversion and battery storage system has two possible operating modes (on-grid and off-

grid). There are a number of possible operating states within the two basic modes of operation of

the system. Control design for each operating state using MIMO control approach represents a

considerably difficult design task. As pointed out in the previous section, control and mode

transitions will still have to be considered under a supervisory control layer (section 2.3.2).

The complexity associated with the control design of the study system using MIMO control

philosophy can be avoided by following the approach proposed in [50]. According to the

proposed approach the problem of modeling and control design for a complex system can be

simplified if the system could be partitioned along a suitable axis. The STD of Figure 2.2-1

points to such a partitioning axis i.e., the dc bus. It can be seen from

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