Supervisory Control Scheme for FACTS and HVDC Based Damping of Inter-Area Power Oscillations in Hybrid AC-DC Power Systems

Transcription

1 Supervisory Control Scheme for FACTS and HVDC Based Damping of Inter-Area Power Oscillations in Hybrid AC-DC Power Systems A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences 2015 MELIOS HADJIKYPRIS School of Electrical and Electronic Engineering

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3 Table of Contents Table of Contents... 3 List of Figures... 8 List of Tables List of Abbreviations Abstract Declaration Copyright Statement Acknowledgements The Author Introduction Power System Stability Classification of Stability Small Signal Stability Transient Stability Power System Oscillations Role of Inter-area Oscillations in Power System Blackouts Wide Area Measurement Systems FACTS Devices HVDC Systems LCC-HVDC VSC-HVDC Research Aims and Objectives Main Contributions of this Research Outline of the Thesis

4 1.10 References Power System Modelling and Analysis Techniques Modelling Power System Components Synchronous Machines Generator Excitation Systems Speed Governors Power System Stabilizers Transmission Lines Transformers Loads Multi-Machine Power Network Analysis Bus Classification Network Equations Network Reduction Power System Analysis techniques Power System Linearization Modal Analysis Test Power System Power System Modelling Modal Analysis Chapter Summary References FACTS and HVDC Systems: A Stability Perspective Modal Analysis for Control System Design Power System Transfer Function Residue Based Damping Controller Design Power Oscillation Damping Control with TCSC

5 3.2.1 TCSC Modelling System Performance without TCSC-POD Controller Residue Based TCSC-POD Controller Design System Performance with TCSC-POD Controller Power Oscillation Damping Control with SVC SVC Modelling System Performance without SVC-POD Controller Residue Based SVC-POD Controller Design System Performance with SVC-POD Controller Power Oscillation Damping Control with HVDC Link VSC-HVDC Transmission System Modelling System Performance without VSC-HVDC POD Controller Residue Based VSC-HVDC POD Controller Design System Performance with VSC-HVDC POD Controller Simultaneous Operation of FACTS and HVDC Systems: An Uncoordinated Approach Power System Performance Chapter Summary References System Identification History of System Identification in Power Systems Application of System Identification Probing Signal MIMO Subspace System Identification Model Validation Chapter Summary References

6 5 LQG Controller Design Modal Linear Quadratic Gaussian (MLQG) Control Wide Area Supervisory Control Scheme Loop Transfer Recovery Small Signal Stability Analysis Chapter Summary References Performance Measures of the Novel Supervisory Controller Small Signal Disturbance Large Signal Disturbance Signal Transmission Time Delays Reduced System Inertia Chapter Summary Thesis Summary Thesis Conclusions Contributions Future Developments Appendix A: Two-Area Test Power System Network Appendix B: Two-Area Test Power System Network with TCSC Appendix C: Two-Area Test Power System Network with SVC Appendix D: Two-Area Test Power System Network with VSC-HVDC Link Appendix E: Linear State Space Power System Model Appendix F: Linear Quadratic Gaussian Controller Parameters Appendix G: Publications from the Thesis G.1 Submitted International Journal Publications G.2 International Conference Publications G.3 Submitted International Conference Publications

7 7 Final Word Count: 43,268

8 List of Figures Figure 2.1: Closed-loop system composition of synchronous generator, excitation system and PSS Figure 2.2: Block diagram of the IEEE ST1A static exciter Figure 2.3: Block diagram of the steam governor-turbine system Figure 2.4: Classical Power System Stabilizer (PSS) structure Figure 2.5: Equivalent π-circuit representation of a transmission line with lumped parameters Figure 2.6: Per unit equivalent circuit of a two-winding transformer Figure 2.7: Standard equivalent circuit of a two-winding transformer Figure 2.8: Voltage and current phasors representation on orthogonal co-ordinate system (a, b) Figure 2.9: Mode shape of a local oscillatory mode Figure 2.10: Mode shape of an inter-area oscillatory mode Figure 2.11: Two-area test power system network Figure 2.12: Oscillatory modes of the two-area test power system network Figure 3.1: Closed-loop system configuration with negative feedback control Figure 3.2: Classical block diagram of a power oscillation damping (POD) controller Figure 3.3: Shift of the eigenvalue caused by POD feedback control action Figure 3.4: Two-area test power system network with a TCSC device Figure 3.5: Structure of a typical TCSC Figure 3.6: Ideal model of a TCSC for power system stability studies Figure 3.7: Equivalent circuit diagram of a transmission corridor with a TCSC Figure 3.8: Block diagram of basic TCSC control Figure 3.9: Oscillatory modes of the two-area test power system with a TCSC Figure 3.10: Generators rotor speed deviation responses to the small disturbance without TCSC-POD control Figure 3.11: System frequency response to the small disturbance without TCSC-POD control

9 Figure 3.12: Inter-area active power flow (as recorded on line 3) response to the small disturbance without TCSC-POD control Figure 3.13: Block diagram of TCSC supplementary POD control Figure 3.14: Closed-loop feedback control with two input signals Figure 3.15: Illustration of estimating the residue s phase angle Figure 3.16: Residue s phase angle estimation for TCSC-POD controller design Figure 3.17: System s oscillatory modes against the gain K d of the TCSC-POD control Figure 3.18: Generators rotor speed deviation responses to the small disturbance with TCSC-POD control Figure 3.19: Inter-area active power flow (as recorded on line 3) response to the small disturbance with TCSC-POD control Figure 3.20: Generators rotor speed deviation responses to the large disturbance with TCSC-POD control Figure 3.21: Inter-area active power flow (as recorded on line 3) response to the large disturbance with TCSC-POD control Figure 3.22: Two-area test power system network with a SVC device Figure 3.23: Structure of a typical SVC Figure 3.24: Ideal model of a SVC for power system stability studies Figure 3.25: Block diagram of basic SVC control Figure 3.26: Oscillatory modes of the two-area test power system with a SVC Figure 3.27: Generators rotor speed deviation responses to the small disturbance without SVC-POD control Figure 3.28: System frequency response to the small disturbance without SVC-POD control Figure 3.29: Inter-area active power flow (as recorded on line 3) response to the small disturbance without SVC-POD control Figure 3.30: Block diagram of SVC supplementary POD control Figure 3.31: Residue s phase angle estimation for SVC-POD controller design Figure 3.32: System s oscillatory modes against the gain K d of the SVC-POD controller Figure 3.33: Generators rotor speed deviation responses to the small disturbance with SVC-POD control

10 Figure 3.34: Inter-area active power flow (as recorded on line 3) response to the small disturbance with SVC-POD control Figure 3.35: Generators rotor speed deviation responses to the large disturbance with SVC-POD control Figure 3.36: Inter-area active power flow (as recorded on line 3) response to the large disturbance with SVC-POD control Figure 3.37: Two-area test power system network with a VSC-HVDC link Figure 3.38: VSC-HVDC transmission system modelling Figure 3.39: VSC-HVDC system: DC voltage control Figure 3.40: VSC-HVDC system: Active power control Figure 3.41: VSC-HVDC system: Reactive power control Figure 3.42: Oscillatory modes of the two-area test power system with a VSC-HVDC link Figure 3.43: Generators rotor speed deviation responses to the small disturbance without VSC-HVDC POD control Figure 3.44: System frequency response to the small disturbance without VSC-HVDC POD control Figure 3.45: Inter-area active power flow (as recorded on line 3) response to the small disturbance without VSC-HVDC POD control Figure 3.46: Block diagram of VSC-HVDC supplementary POD control Figure 3.47: Residue s phase angle estimation for VSC-HVDC POD controller design Figure 3.48: System s oscillatory modes against the gain K d of the VSC-HVDC POD controller Figure 3.49: Generators rotor speed deviation responses to the small disturbance with VSC-HVDC POD control Figure 3.50: Inter-area active power flow (as recorded on line 3) response to the small disturbance with VSC-HVDC POD control Figure 3.51: Generators rotor speed deviation responses to the large disturbance with VSC-HVDC POD control Figure 3.52: Inter-area active power flow (as recorded on line 3) response to the large disturbance with VSC-HVDC POD control

11 Figure 3.53: Two-area test power system network performing parallel operation of TCSC, SVC and a VSC-HVDC link Figure 3.54: Oscillatory modes of the two-area test power system under unsupervised control Figure 3.55: Generators rotor speed deviation responses to the small disturbance with unsupervised control Figure 3.56: Inter-area active power flow (as recorded on line 3) response to the small disturbance with unsupervised control Figure 3.57: Generators rotor speed deviation responses to the large disturbance with unsupervised control Figure 3.58: Inter-area active power flow (as recorded on line 3) response to the large disturbance with unsupervised control Figure 4.1: Identification and validation process for linear state space model estimation Figure 4.2: Local TCSC control integrating a supervisory control signal Figure 4.3: Local SVC control integrating a supervisory control signal Figure 4.4: Local VSC-HVDC active power control integrating a supervisory control signal Figure 4.5: Representation of targeted linear state space model of power system Figure 4.6: Typical PRBS injection signals used during system identification process (the graphs signify the input channels used from local controllers of FACTS devices and HVDC link to accommodate the PRBS signals) Figure 4.7: System s output responses as recorded by generators rotor speed deviations during PRBS probing for system identification purposes Figure 4.8: Electromechanical modes of non-linear power system and estimated linear model - A comparative study (dashed line signifies 5% damping threshold) Figure 4.9: Generator s 1 rotor speed deviation response to the impulse signal applied at P sup (VSC-HVDC inverter) input channel Figure 4.10: Generator s 1 rotor speed deviation response to the impulse signal applied at V sup (SVC) input channel Figure 4.11: Generator s 1 rotor speed deviation response to the impulse signal applied at X sup (TCSC) input channel

12 Figure 4.12: Generator s 1 rotor speed deviation response to impulse signals applied at P sup (VSC-HVDC inverter) and V sup (SVC) input channels Figure 4.13: Generator s 1 rotor speed deviation response to impulse signals applied at P sup (VSC-HVDC inverter) and X sup (TCSC) input channels Figure 4.14: Generator s 1 rotor speed deviation response to impulse signals applied at V sup (SVC) and X sup (TCSC) input channels Figure 4.15: Generator s 1 rotor speed deviation response to impulse signals applied at P sup (VSC-HVDC inverter), V sup (SVC) and X sup (TCSC) input channels Figure 5.1: Standard LQG controller structure integrating an optimal state feedback controller (LQR) and a Kalman filter (state observer) Figure 5.2: Wide area supervisory control scheme configuration Figure 5.3: Loop Transfer Recovery (LTR) process implemented at the plant input for recovering closed-loop system s robustness properties by varying q values Figure 5.4: System s electromechanical modes as obtained by local control and supervisory control A comparative study (dashed line signifies 5% damping threshold) Figure 6.1: Two-area test power system network performing parallel operation of TCSC, SVC and a VSC-HVDC link Figure 6.2: Generators rotor speed deviation responses to the small disturbance under the implementation of local and supervisory control schemes Figure 6.3: Inter-area active power flow (as recorded on line 3) response to the small disturbance under the implementation of local and supervisory control schemes Figure 6.4: HVDC link s active power response to the small disturbance under the implementation of local and supervisory control schemes Figure 6.5: TCSC s reactance response to the small disturbance under the implementation of local and supervisory control schemes Figure 6.6: SVC s reactive power response to the small disturbance under the implementation of local and supervisory control schemes Figure 6.7: Generators rotor speed deviation responses to the large disturbance under the implementation of local and supervisory control schemes Figure 6.8: Inter-area active power flow (as recorded on line 3) response to the large disturbance under the implementation of local and supervisory control schemes

13 Figure 6.9: HVDC link s active power response to the large disturbance under the implementation of local and supervisory control schemes Figure 6.10: TCSC s reactance response to the large disturbance under the implementation of local and supervisory control schemes Figure 6.11: SVC s reactive power response to the large disturbance under the implementation of local and supervisory control schemes Figure 6.12: Generators rotor speed deviation responses to the large disturbance under the implementation of local and supervisory control schemes considering a range of signal transmission time delays applied on supervisory controller s input and output channels (dashed line indicates the case of local control scheme implementation with no time delays involved, while the rest of the lines represent the corresponding time delays applied on input and output channels of the supervisory control scheme) Figure 6.13: Inter-area active power flow (as recorded on line 3) response to the large disturbance under the implementation of local and supervisory control schemes considering a range of signal transmission time delays applied on supervisory controller s input and output channels (dashed line indicates the case of local control scheme implementation with no time delays involved, while the rest of the lines represent the corresponding time delays applied on input and output channels of the supervisory control scheme) Figure 6.14: HVDC link s active power response to the large disturbance under the implementation of local and supervisory control schemes considering a range of signal transmission time delays applied on supervisory controller s input and output channels (dashed line indicates the case of local control scheme implementation with no time delays involved, while the rest of the lines represent the corresponding time delays applied on input and output channels of the supervisory control scheme) Figure 6.15: TCSC s reactance response to the large disturbance under the implementation of local and supervisory control schemes considering a range of signal transmission time delays applied on supervisory controller s input and output channels (dashed line indicates the case of local control scheme implementation with no time delays involved, while the rest of the lines represent the corresponding time delays applied on input and output channels of the supervisory control scheme)

14 Figure 6.16: SVC s reactive power response to the large disturbance under the implementation of local and supervisory control schemes considering a range of signal transmission time delays applied on supervisory controller s input and output channels (dashed line indicates the case of local control scheme implementation with no time delays involved, while the rest of the lines represent the corresponding time delays applied on input and output channels of the supervisory control scheme) Figure 6.17: Generators rotor speed deviation responses to the large disturbance under the implementation of local and supervisory control schemes considering a range of system inertia values and signal transmission time delays of 200ms applied on supervisory controller s input and output channels (dashed line represents the case of local control scheme implementation with system inertia of H=3.3s and no time delays involved, while the rest of the lines represent the corresponding system inertia values under the implementation of supervisory control scheme considering signal transmission time delays of 200ms) Figure 6.18: Inter-area active power flow (as recorded on line 3) response to the large disturbance under the implementation of local and supervisory control schemes considering a range of system inertia values and signal transmission time delays of 200ms applied on supervisory controller s input and output channels (dashed line represents the case of local control scheme implementation with system inertia of H=3.3s and no time delays involved, while the rest of the lines represent the corresponding system inertia values under the implementation of supervisory control scheme considering signal transmission time delays of 200ms) Figure 6.19: HVDC link s active power response to the large disturbance under the implementation of local and supervisory control schemes considering a range of system inertia values and signal transmission time delays of 200ms applied on supervisory controller s input and output channels (dashed line represents the case of local control scheme implementation with system inertia of H=3.3s and no time delays involved, while the rest of the lines represent the corresponding system inertia values under the implementation of supervisory control scheme considering signal transmission time delays of 200ms) Figure 6.20: TCSC s reactance response to the large disturbance under the implementation of local and supervisory control schemes considering a range of system inertia values and signal transmission time delays of 200ms applied on 14

15 supervisory controller s input and output channels (dashed line represents the case of local control scheme implementation with system inertia of H=3.3s and no time delays involved, while the rest of the lines represent the corresponding system inertia values under the implementation of supervisory control scheme considering signal transmission time delays of 200ms) Figure 6.21: SVC s reactive power response to the large disturbance under the implementation of local and supervisory control schemes considering a range of system inertia values and signal transmission time delays of 200ms applied on supervisory controller s input and output channels (dashed line represents the case of local control scheme implementation with system inertia of H=3.3s and no time delays involved, while the rest of the lines represent the corresponding system inertia values under the implementation of supervisory control scheme considering signal transmission time delays of 200ms) Figure A.1: Two-area test power system network Figure A.2: Block diagram of the IEEE ST1A static exciter used for synchronous generators terminal voltage control Figure A.3: Block diagram of the steam governor-turbine system used for synchronous generators rotor speed control Figure A.4: Block diagram of the PSS system used for synchronous generators control Figure B.1: Two-area test power system network with TCSC device Figure B.2: Block diagram of the IEEE ST1A static exciter used for synchronous generators terminal voltage control Figure B.3: Block diagram of the steam governor-turbine system used for synchronous generators rotor speed control Figure B.4: Block diagram of the PSS system used for synchronous generators control Figure B.5: Block diagram of the TCSC device with supplementary POD controller Figure C.1: Two-area test power system network with SVC device Figure C.2: Block diagram of the IEEE ST1A static exciter used for synchronous generators terminal voltage control

16 Figure C.3: Block diagram of the steam governor-turbine system used for synchronous generators rotor speed control Figure C.4: Block diagram of the PSS system used for synchronous generators control Figure C.5: Block diagram of the SVC device with supplementary POD controller 189 Figure D.1: Two-area test power system network with VSC-HVDC link Figure D.2: Block diagram of the IEEE ST1A static exciter used for synchronous generators terminal voltage control Figure D.3: Block diagram of the steam governor-turbine system used for synchronous generators rotor speed control Figure D.4: Block diagram of the PSS system used for synchronous generators control Figure D.5: Block diagram of the internal current controllers (d-axis and q-axis) for both rectifier and inverter of the VSC-HVDC system Figure D.6: Block diagram of the DC voltage controller (rectifier) of the VSC-HVDC system Figure D.7: Block diagram of the reactive power controller (rectifier) of the VSC- HVDC system Figure D.8: Block diagram of the reactive power controller (inverter) of the VSC- HVDC system Figure D.9: Block diagram of the active power controller (inverter) of the VSC- HVDC system with supplementary POD controller

17 List of Tables Table 2.1: Electromechanical oscillatory modes of the test power system Table 3.1: Electromechanical oscillatory modes of the two-area test power system with a TCSC Table 3.2: Electromechanical oscillatory modes of the two-area test power system with a SVC Table 3.3: Electromechanical modes of the two-area test system with a VSC-HVDC link Table 3.4: Electromechanical modes of the two-area test system under unsupervised control Table 4.1: Electromechanical modes of the non-linear power system Table 4.2: Electromechanical modes of the estimated linear state space model Table 5.1: System s electromechanical modes under local control implementation. 144 Table 5.2: System s electromechanical modes under supervisory control implementation Table A.1: Synchronous machines dynamic data Table A.2: Power generation conditions of the synchronous machines Table A.3: Parameters of the IEEE ST1A static exciter Table A.4: Parameters of the steam governor-turbine system Table A.5: Parameters of the PSS system Table A.6: Transformers data Table A.7: AC transmission lines parameters Table A.8: Loads data Table A.9: Shunt capacitors data Table B.1: Synchronous machines dynamic data Table B.2: Power generation conditions of the synchronous machines Table B.3: Parameters of the IEEE ST1A static exciter Table B.4: Parameters of the steam governor-turbine system Table B.5: Parameters of the PSS system Table B.6: Parameters of the TCSC device

18 Table B.7: Parameters of the supplementary TCSC POD controller Table B.8: Transformers data Table B.9: AC transmission lines parameters Table B.10: Loads data Table B.11: Shunt capacitors data Table C.1: Synchronous machines dynamic data Table C.2: Power generation conditions of the synchronous machines Table C.3: Parameters of the IEEE ST1A static exciter Table C.4: Parameters of the steam governor-turbine system Table C.5: Parameters of the PSS system Table C.6: Parameters of the primary voltage control of SVC device Table C.7: Parameters of the supplementary SVC POD controller Table C.8: Transformers data Table C.9: AC transmission lines parameters Table C.10: Loads data Table C.11: Shunt capacitors data Table D.1: Synchronous machines dynamic data Table D.2: Power generation conditions of the synchronous machines Table D.3: Parameters of the IEEE ST1A static exciter Table D.4: Parameters of the steam governor-turbine system Table D.5: Parameters of the PSS system Table D.6: Operating condition of the VSC-HVDC system Table D.7: Parameters of the internal current controllers (d-axis and q-axis) for both rectifier and inverter of the VSC-HVDC system Table D.8: Parameters of the DC voltage controller (rectifier) of the VSC-HVDC system Table D.9: Parameters of the reactive power controller (rectifier) of the VSC-HVDC system Table D.10: Parameters of the reactive power controller (inverter) of the VSC-HVDC system Table D.11: Parameters of the active power controller (inverter) of the VSC-HVDC system

19 Table D.12: Parameters of the additional POD controller (inverter) of the VSC-HVDC system Table D.13: Transformers parameters Table D.14: AC transmission lines parameters Table D.15: VSC-HVDC transmission line s parameters Table D.16: Loads data Table D.17: Shunt capacitors data Table E.1: Linear state space power system model A matrix-columns: Table E.2: Linear state space power system model A matrix-columns: Table E.3: Linear state space power system model B matrix Table E.4: Linear state space power system model C matrix-columns: Table E.5: Linear state space power system model C matrix-columns: Table E.6: Linear state space power system model C matrix-columns: Table F.1: Elements of the weighting matrix Q m columns Table F.2: Elements of the weighting matrix Q m columns Table F.3: LQR optimal state feedback controller gain matrix K columns: Table F.4: LQR optimal state feedback controller gain matrix K columns: Table F.5: LQR optimal state feedback controller gain matrix K columns: Table F.6: Optimal Kalman filter gain matrix L

20 List of Abbreviations AC ARE AVR COI CPS CSC DC DE FACTS GPS HP HQ HV HVDC IEEE IGBT IPFC LCC LLPRN LQG LQR LTR LV MAPP MAX MIMO MIN MIS MLQG Alternating Current Algebraic Riccati Equation Automatic Voltage Regulator California-Oregon AC Intertie Controllable Phase Shifters Current Source Converter Direct Current Detroit Edison Flexible AC Transmission Systems Global Positioning System High Pressure Hydro Quebec High Voltage High Voltage Direct Current Institute of Electrical and Electronics Engineers Insulated Gate Bipolar Transistor Interline Power Flow Controller Line Commutated Converter Low Level Pseudo-Random Noise Linear Quadratic Gaussian Linear Quadratic Regulator Loop Transfer Recovery Low Voltage Mid-Continent Area Power Pool Maximum Multiple-Input Multiple-Output Minimum Mexican Interconnected System Modal Linear Quadratic Gaussian 20

21 MTDC N4SID OH P PDCI PDC PI PMU POD PQ PRBS PSS PST PV PWM Q SISO SMSW SSSC ST1A STATCOM SVC TCBR TCPAR TCR TCSC TGR TPST TSC TSO UCTE UPFC VDC Multi-Terminal Direct Current Numerical Algorithm for Subspace State Space System Identification Ontario Hydro Active Power Pacific HVDC Intertie Phasor Data Concentrator Proportional Integral (Control) Phasor Measurement Unit Power Oscillation Damping Active Power Reactive Power Pseudo-Random Binary Sequence Power System Stabilizer Phase Shifting Transformer (Active) Power Voltage Pulse Width Modulation Reactive Power Single-Input Single-Output Single-Mode Square Wave Static Synchronous Series Compensator Static Exciter Type Static Synchronous Compensator Static Var Compensator Thyristor Controlled Breaking Resistor Thyristor Controlled Power Angle Regulator Thyristor Controlled Reactor Thyristor Controlled Series Capacitor Transient Gain Reduction Thyristor Phase Shifter Transformer Thyristor Switching Capacitor Transmission System Operator European Network of Transmission System Operators for Electricity Unified Power Flow Controller Direct Current Voltage 21

22 VSC WAMS WECC WSCC Voltage Source Converter Wide Area Measurement System Western Electricity Coordinating Council Western Systems Coordinating Council 22

23 Abstract The University of Manchester Faculty of Engineering and Physical Sciences PhD Thesis Supervisory Control Scheme for FACTS and HVDC Based Damping of Inter-Area Power Oscillations in Hybrid AC-DC Power Systems Melios Hadjikypris July, 2015 Modern interconnected power systems are becoming highly complex and sophisticated, while increasing energy penetrations through congested inter-tie lines causing the operating point approaching stability margins. This as a result, exposes the overall system to potential low frequency power oscillation phenomena following disturbances. This in turn can lead to cascading events and blackouts. Recent approaches to counteract this phenomenon are based on utilization of wide area monitoring systems (WAMS) and power electronics based devices, such as flexible AC transmission systems (FACTS) and HVDC links for advanced power oscillation damping provision. The rise of hybrid AC-DC power systems is therefore sought as a viable solution in overcoming this challenge and securing wide-area stability. If multiple FACTS devices and HVDC links are integrated in a scheme with no supervising control actions considered amongst them, the overall system response might not be optimal. Each device might attempt to individually damp power oscillations ignoring the control status of the rest. This introduces an increasing chance of destabilizing interactions taking place between them, leading to underutilized performance, increased costs and system wide-area stability deterioration. This research investigates the development of a novel supervisory control scheme that optimally coordinates a parallel operation of multiple FACTS devices and an HVDC link distributed across a power system. The control system is based on Linear Quadratic Gaussian (LQG) modern optimal control theory. The proposed new control scheme provides coordinating control signals to WAMS based FACTS devices and HVDC link, to optimally and coherently counteract inter-area modes of low frequency power oscillations inherent in the system. The thesis makes a thorough review of the existing and well-established improved stability practises a power system benefits from through the implementation of a single FACTS device or HVDC link, and compares the case and hence raises the issue when all active components are integrated simultaneously and uncoordinatedly. System identification approaches are also in the core of this research, serving as means of reaching a linear state space model representative of the non-linear power system, which is a pre-requisite for LQG control design methodology. 23

24 Declaration No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or institute of learning. 24

25 Copyright Statement I. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the Copyright ) and he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. II. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. III. The ownership of certain Copyright, patents, designs, trade marks and other intellectual property (the Intellectual Property ) and any reproductions of copyright works in the thesis, for example graphs and tables( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. IV. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy 1, in any relevant Thesis restriction declarations deposited in the University Library, The University Library s regulations 2 and in The University s policy on Presentation of Theses. 1 See 2 See 25

26 Acknowledgements This is a good opportunity to express my honest gratitude to my supervisor Professor Vladimir Terzija for the full professionalism and commitment to excellence he has shown throughout this project, but most importantly his unconditional encouragement and enthusiasm he widely passed to me. It is an honour of meeting personalities like him, dominated by high quality manners and principles. Beyond any doubt I am exceptionally appreciative of the invaluable and endless support and love I received from my family all these years. My family is my source of energy, encouragement and ambition, always been there for me especially during hard times. They undeniably take the lead for any success I accomplished in my life and I am very grateful for that. I would like to assure them that my love and thoughts will accompanying them at all times. I am also thankful for some good relationships developed over the years at The University of Manchester. I would like to send my honest compliments to these people for giving me the opportunity of sharing some nice moments with them, and ensuring them they will be hard to be forgotten. Finally, I owe my credits to the Engineering and Physical Sciences Research Council (EPSRC) for the financial support of this project. 26

27 27 To the people who always reserve a place in my heart, my family - Andreas, Marina, Kyriakos and Christofakis

28 The Author Melios Hadjikypris started his academic career by following undergraduate studies in Electrical and Electronic Engineering at The University of Manchester ( ), obtaining a Bachelor of Science (BSc) degree with honours. He then proceeded in the field of control systems engineering, obtaining a Master of Science (MSc) degree with merit in Advanced Control and Systems Engineering within the same institute ( ). Since October 2011, Mr Hadjikypris has undertaken research studies in the department of Electrical Energy and Power Systems within The University of Manchester, persuading his Doctor of Philosophy (PhD) degree in Power System Dynamics and Control considering FACTS devices and HVDC technology under the supervision of Professor Vladimir Terzija. 28

29 Chapter 1 Introduction 1 Introduction Inter-area oscillation phenomena have always been a matter of concern for Transmission System Operators (TSOs) and power system engineers as very often they form the root for system s poor performance. This is a result of the persistent low-frequency electromechanical oscillations taking place in system, which very often lead to undesired phenomena such as cascading events and catastrophic blackouts. A considerable research has been developed identifying feasible and effective ways to adequately compensate inter-area electromechanical oscillations through supplementary damping provision targeting critical inter-area oscillatory modes of concern. Solutions involved the traditional applications of Power System Stabilizers (PSSs), which primarily developed for addressing local mode oscillations. Despite that, when appropriately tuned could also obtain considerable damping on inter-area mode oscillations to some extent. This method demonstrated restricted potentials in providing sufficient damping on inter-area modes, as local input signals utilized by PSSs had limited observability information of the inter-area modes. Recent approaches involved implementation of power electronics based technology, such as Flexible AC Transmission Systems (FACTS) and High Voltage DC (HVDC) links once appropriately combined with Wide Area Monitoring Systems (WAMS). The use of this technology seemed to be a viable solution in establishing effective damping of inter-area electromechanical oscillation phenomena. The proposed methodology is based on implementation of a single FACTS device or HVDC link reinforced with a supplementary Power Oscillation Damping (POD) controller. The POD controller provides a stabilizing input signal to FACTS device or HVDC link, assisting towards the damping action on inter-area oscillations. The application of FACTS devices and HVDC links once appropriately equipped with POD controllers, demonstrated an effective way of improving the damping of critical inter-area modes and therefore contributing towards the security of system s wide-area stability. 29

30 Chapter 1 Introduction A potential risk arises however, when multiple FACTS devices and HVDC links are integrated under the same scheme with no supervising/coordinating control actions considered between them. In that case, the overall system response might deviate from the optimal as each power electronic device might attempt to individually damp power oscillations neglecting the control status of the rest. This introduces an increasing chance of ineffective/destabilizing interactions taking place between them. Therefore, a non-optimal (under-utilized) performance of the power electronic equipment is established, resulting in increasing costs and system wide-area stability deterioration leaving open possibilities for catastrophic blackouts. As a result, the necessity is emerged for the development of a novel multi-variable supervisory control scheme that will optimally coordinate a parallel operation of a number FACTS devices and HVDC link distributed across a large scale power system. The proposed control scheme will supply coordinating/supervising control signals to WAMS based FACTS devices and HVDC link to coherently counteract inter-area modes of low frequency power oscillations under a wide range of system operating conditions and disturbances. The scope of this research is to investigate the options for the development of such a novel control scheme guaranteeing optimal operation amongst FACTS devices and HVDC link in a system, and therefore assisting synchronous damping action against critical inter-area oscillatory modes. This chapter briefly reviews some fundamental concepts related to power system stability, oscillations, and blackouts. Furthermore, it introduces the concepts surrounding WAMS systems, FACTS devices and HVDC links. The chapter concludes by emphasizing the aims and objectives of this research, its major contributions, and a short summary of the subsequent chapters. 1.1 Power System Stability Power system stability may be broadly defined as that property of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance [1]. 30

31 Chapter 1 Introduction Classification of Stability Power system stability can be classified into the following categories [1]: Rotor Angle Stability is the ability of interconnected synchronous machines of a power system to remain in synchronism. The stability problem involves the study of the electromechanical oscillations inherent in power systems. Voltage Stability is the ability of a power system to maintain steady acceptable voltages at all buses in the system under normal operating conditions and after being subjected to a disturbance. The main factor causing voltage instability is the inability of the power system to meet the demand for reactive power. Mid-Term and Long-Term Stability relates to the slower dynamics exhibited in the power system following severe upsets in voltage and frequency. The characteristic time constants of these phenomena range from a matter of seconds to several minutes, reflecting the responses of generator controls and protections as well as prime mover energy supply systems and load-voltage regulators. Particular interest in this thesis is on rotor angle stability, especially in inter-area electromechanical oscillation phenomena inherent in inter-connected power systems, which very often form the prime reason for wide-spread cascading events and power systems collapse. When a power system experiences a disturbance, it can be shown [2] that the electrical torque changes at each machine can be decomposed into two elements, the synchronizing torque and damping torque as shown in (1.1): Te = Ts δ + TD ω (1.1) where Ts δ is the synchronizing torque component which is in phase with the rotor angle perturbation δ and T D ω is the damping torque component which is in phase with the speed deviation ω. 31

32 Chapter 1 Introduction As can be observed, power system stability is dependent on both components of the electrical torque change for every synchronous machine. In the case of inadequate synchronizing torque instability of the power system will occur through an aperiodic drift in rotor angle. On the other hand, insufficient damping torque can lead to oscillatory instability suffered by the system. Rotor angle stability is commonly classified into small-signal (or small-disturbance) stability and transient (or large-disturbance) stability [1] Small Signal Stability Small signal stability is the ability of the power system to maintain synchronous operation under small disturbances. The disturbances could be small variations in load and generation and occur continually. These disturbances are considered to be small enough to permit a linear model representation of power system s dynamic behaviour, which can be used for modal analysis purposes. Possible instability that may arise from small-signal disturbance could be manifest in either: Steady increase in rotor angle due to lack of sufficient synchronizing torque Rotor oscillations of increasing amplitude due to lack of sufficient damping torque Transient Stability Transient stability is the ability of the power system to maintain synchronous operation when subjected to a severe disturbance. Such disturbance could be the loss of generation or a large load, as well as a fault or outage on the transmission system. The resulting system response under transient (large-signal) disturbances, involves large deviations of generator rotor angles, power flows, frequency, bus voltages, and is influenced by the non-linear power-angle relationship. In large power systems, transient stability may not always occur as first-swing instability; in fact it could be the result of the superposition of several modes of oscillation causing large excursions of rotor angle beyond the first swing. 32

33 Chapter 1 Introduction 1.2 Power System Oscillations In a power system, each synchronous machine consists of a large rotating mass with inertia. Following a stable system disturbance, mechanical oscillations at these masses take place during the process where rotor angles and speeds of the affected machines settle to equilibrium. These mechanical oscillations are reflected as deviations in electrical output power and frequency of the machines. Insufficient damping of these oscillations is the main concern of rotor angle stability. The oscillatory modes inherent in a power system that need to be adequately addressed ensuring wide area stability can be categorized as follows [3]-[4]: Torsional Modes (f = Hz) are related with mechanical oscillations existed within the rotational components of the turbine-generator shaft system. These oscillations could lead to instability due to interactions with excitation controls, speed governors, HVDC controls, and series compensated lines. A well-known incident in the history of power systems where unstable torsional modes (sub-synchronous resonance oscillations) lead to instability with disastrous consequences took place in South California power system in 1970 [5]. Control Modes (f depends on actual control) are associated with local generator s controllers such as poorly tuned excitation systems or speed governors, in addition to FACTS devices and HVDC links related controllers. Local Modes (f = 1-2 Hz) concern one or more synchronous machines at a generating station swing together against the rest of the power system. These types of localised oscillations can be satisfactorily damped through the use of appropriately tuned PSSs installed on the affected machines of the generating unit. Inter-Area Modes (f = Hz) involve two or more closely coupled coherent groups of generators in a power system swinging against each other while being interconnected over weak tie-lines. This is a system wide area phenomenon. The damping characteristic of the inter-area mode is dictated by the tie-line strength, the nature of the loads and the power flow through the 33

34 Chapter 1 Introduction interconnection, as well as the interaction of loads with the dynamics of generators and their associated controls. The operation of the power system in the presence of lightly damped inter-area modes is much taught. In the unfortunate event where any of the above oscillatory modes becomes poorly damped or even unstable, this could lead to growing power oscillations experienced by the system followed by equipment tripping and possibly wide area cascading events and system collapse. Focus of the thesis is to investigate the effects of inter-area oscillation phenomena on the performance measures of next-generation inter-connected power systems. It is well known that lightly damped inter-area modes continuously challenge transmission system operators seek to gradually increase power transmission boundaries over long and stressed transmission corridors. Therefore the need to adequately address this challenge emerges more frequent than ever before. 1.3 Role of Inter-area Oscillations in Power System Blackouts It is well known that electromechanical oscillations have a central role in stable operation of a power system. Considering the case of inter-area modes exhibited in inter-connected power systems, inadequate damping is very often the primary factor leading to persistent growing oscillations followed by separation events and in some cases system blackouts. This has been witnessed in the past with numerous examples of power systems experiencing increasing inter-area oscillations that finally led to cascading events and systems blackouts. Some worth noting cases involve the following systems [4]-[10]: UCTE European Interconnected Grid (2006) Mexican Interconnected System (MIS) (2008) Western Electricity Coordinating Council (WECC) (1964, 1996) Colombia-Venezuela Interconnected System (1993) Detroit Edison (DE)-Ontario Hydro (OH)-Hydro Quebec (HQ) (1960s, 1985) 34

35 Chapter 1 Introduction Finland-Sweden-Norway-Denmark (1960s) Saskatchewan-Manitoba Hydro-Western Ontario (1966) Italy-Yugoslavia-Austria ( ) Mid-continent area power pool (MAPP) (1971, 1972) South East Australia (1975) Scotland-England (1978) Western Australia (1982, 1983) Taiwan (1985) Ghana-Ivory Coast (1985) Southern Brazil ( , 1984) Special interest attracts the power blackout incident occurred in Western Electricity Coordination Council (WECC) on 10 th of August In that case, high temperatures caused sagging of high capacity lines and in combination with malfunction of protection mechanisms, five 500 kv transmission lines were tripped. This, as a consequence, caused the excitation of a lightly damped inter-area mode present between the north and south of the network which became unstable. As a result, increasing power oscillations were observed on a critical inter-tie line known as the California-Oregon Inter-tie (COI). Eventually, the oscillations became large enough to cause all three lines comprising the COI corridor to trip, followed by cascading failures that finally led to system electrical islanding in four regions. The outcome of this disastrous incident was the disconnection of 28 GW of load affecting 7.5 million customers [11]. 1.4 Wide Area Measurement Systems Sophisticated control system designs are emerging in power systems more often than ever before, in an attempt to reach robust damping of critical inter-area modes inherent in a system. This strategical approach has given rise to the implementation of Wide Area Measurement Systems (WAMS) where a collection of system signals is obtained over a large span within the power system, obtaining sufficient observability of the critical inter-area modes. This technology is based on Phasor Measurement Units (PMUs) which continuously collect a series of time-stamped signals across the 35