Stability Analysis of High Voltage Hybrid AC/DC Power Systems

211 Pages·2017·3.14 MB·English

by  Atsede Gualu Endegnanew

Doctoral theses at NTNU, 2017:256
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ISBN 978-82-326-2583-3 (printed ver.)
ISBN 978-82-326-2584-0 (electronic ver.)
ISSN 1503-8181
Doctoral theses at NTNU, 2017:2 NTNUersity of Science and TechnologyThesis for the Degree ofPhilosophiae Doctormation Technology and ElectricalEngineeringnt of Electric Power Engineering
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Atsede Gualu Endegnanew
Stability Analysis of High Voltage
Hybrid AC/DC Power Systems
Thesis for the Degree of Philosophiae Doctor
Trondheim, September 2017
Norwegian University of Science and Technology
Faculty of Information Technology and Electrical Engineering
Department of Electric Power Engineering
NTNU
Norwegian University of Science and Technology
Thesis for the Degree of Philosophiae Doctor
Faculty of Information Technology and Electrical Engineering
Department of Electric Power Engineering
© Atsede Gualu Endegnanew
ISBN 978-82-326-2583-3 (printed ver.)
ISBN 978-82-326-2584-0 (electronic ver.)
ISSN 1503-8181
IMT-report 2017:256
Doctoral theses at NTNU, 2017:256
Printed by NTNU Grafisk senter
Abstract
Interest in large-scale integration of power from renewable energy sources (RES) has
grown in the last decade as a result of energy policies adopted by governments in an effort
to reduce CO and greenhouse gas emissions. Both large scale, and distributed solar and
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wind energy have proliferated the power system and will continue to do so in the future.
Thus, large and complex transmission systems are needed for robust, flexible and secure
operation of the future power system. Multi-terminal HVDC (MTDC) grids are expected
to play an important role in an efficient socio-economic operation of the electric power
system by acting as a means for integration of RES, exchange of balancing power, cross-
border power market trading, grid reinforcement, etc.
As the introduction of MTDC grids will eventually result in a hybrid ac/dc power system,
it is necessary to carry out a global analysis that considers the entire hybrid ac/dc power
system, which includes both dc and all synchronous areas of the power systems. The main
objective of this PhD is to study the stability of hybrid ac/dc power systems, with a
particular focus on the ac grids. The work investigates how the dynamic characteristics of
ac grids will be affected by the introduction of the MTDC grids and/or by control methods
implemented in MTDC converter controllers.
Modal analysis, in particular eigenvalues, mode shapes, and participation factors, was
used to identify and analyze interactions (dynamic coupling) between different
subsystems in a hybrid ac/dc power system. Mode shapes were used to identify
electromechanical interactions between generators located in different synchronous areas
(asynchronous grids). The inter-grid electromechanical interactions are generally weak
but are influenced by dc grid control strategy, controller tuning and damped frequency of
electromechanical modes. The source of the interactions is dynamic coupling between ac
and dc grids. When several terminals share the duty of dc voltage regulation, as in the case
of dc voltage droop control operation mode, the dynamics of the ac grids behind those
terminals are coupled to a common dc grid dynamics. This leads to indirect coupling of
dynamics of different ac grids through dc grid dynamics. A qualitative analysis of state
matrix of a single 2-level converter with and without connection to a detailed ac grid model
was used to supplement the findings of the quantitative modal analysis. It was shown that
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there is a two-way dynamic coupling between ac and dc grids when a converter is
operated in constant dc voltage or dc voltage droop control modes, i.e. ac grid dynamics
is coupled with dc grid dynamics and dc grid dynamics is coupled with ac grid dynamics.
However, there is only a one-way coupling between an ac grid and a dc grid if the
converter is operated in constant power control mode. In such cases, the ac grid dynamics
is coupled with the dc grid dynamics, but the dc grid dynamics is not coupled with the ac
grid dynamics.
Decentralized control techniques were used to study interactions between power
oscillation damping (POD) controllers on multiple terminals of an MTDC that
interconnects several asynchronous ac grids. Interaction between the selected control
loops was assessed using dynamic relative gain array and performance relative gain array
techniques in the frequency domain. In addition, modal and time domain analyses carried
out for the study case supported the findings from the frequency domain analysis. For the
study case analyzed, it was found that due to control loop interactions the performance of
one of the controllers was augmented, while the performance of the other controller
deteriorated. The analyses clearly showed that control loop interaction should be
considered while tuning PODs on converters even if they are connected to different grids.
Finally, a coordinated control strategy for terminal converters of a dc grid was proposed
to address the issue of frequency disturbance in other ac grids when one grid receives
frequency support from an offshore wind farm. It was shown that by coordinating
converter controllers at the terminals of an offshore wind farm and one ac grid, it is
possible to maximize frequency support contribution of the offshore wind farm and avoid
disturbance in other ac grids connected to the MTDC. However, the proposed method
works when only one ac grid is receiving frequency support and the remaining ac grids are
connected MTDC terminals, which are operating in dc droop or constant power control
mode. If more than one ac grids are to receive frequency support through MTDC grid, then
negative interactions occur when the proposed controller is used. Therefore, in such cases,
distributed dc voltage and frequency droop control is the best control option. However, it
should be noted that with distributed dc voltage and frequency droop control method, the
frequency support comes not only from the wind farm but also from other ac grids behind
an MTDC terminal operating in dc voltage droop control mode.
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Acknowledgement
This thesis summarizes the research conducted during my PhD study, which was carried
out at the Department of Electric Power Engineering at the Norwegian University of
Science and Technology. The PhD was partly financed by the “DC grids for integration of
large scale wind power-OffshoreDC” project that was funded by Nordic Energy Research
and several industry partners.
First and foremost, I would like to thank my supervisor Professor Kjetil Uhlen for giving
me the opportunity to work on this topic, for his positive encouragement, unwavering
support and open door policy throughout the PhD period. I would also like to thank my
co-supervisor Salvatore D’Arco for finding time in his busy schedule to discuss and
comment on the progress of the work. In addition, I am grateful to Jef Beerten who has
guided and advised me at an earlier stage of the research. The work has greatly benefited
from the collaboration works with Jef Beerten, Lester Kalemba and Gilbert Bergna-Diaz.
Thank you all.
I would like to extend my gratitude to Ula, Trond, Sverre and Abel who have kindly read
the draft of the thesis and provided valuable comments. Many thanks goes to my
colleagues at the Electric Power Engineering department and at SINTEF Energy Research
for creating an easy, fun and comfortable working environment. Special thanks is due to
Abel for being a commendable office-mate, Andrzej and Ula for all their support, for being
good friends and neighbors, Trond for being my mentor, friend, colleague and family,
Sverre for his patience and understanding, and my Ethiopian friends in Trondheim for all
the good times and for always being there for me.
Finally, I would like to express my heartfelt thanks to my parents and siblings for their
endless love and invaluable support and the Almighty God for everything I am and have.
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Table of Contents
Abstract ....................................................................................................... i(cid:3)
Acknowledgement ...................................................................................... iii(cid:3)
List of Tables .............................................................................................. xi(cid:3)
List of Figures .......................................................................................... xiii(cid:3)
List of symbols ......................................................................................... xvii(cid:3)
Abbreviation ............................................................................................. xix(cid:3)
1.(cid:3) Introduction .......................................................................................... 1(cid:3)
(cid:3) Background and Motivation ................................................................................ 1(cid:3)
(cid:3) Objectives and Main Contributions ................................................................... 2(cid:3)
(cid:3) Scope ................................................................................................................... 4(cid:3)
(cid:3) List of publications ............................................................................................. 6(cid:3)
(cid:3) Outline of the thesis ............................................................................................ 8(cid:3)
2.(cid:3) High Voltage Direct Current Transmission Systems .............................. 9(cid:3)
(cid:3) Why HVDC? ...................................................................................................... 10(cid:3)
(cid:3) Drivers and challenges ...................................................................................... 11(cid:3)
(cid:3) Technologies ...................................................................................................... 13(cid:3)
2.3.1.(cid:3) LCC .......................................................................................................... 13(cid:3)
2.3.2.(cid:3) VSC ......................................................................................................... 14(cid:3)
(cid:3) Topology ............................................................................................................ 15(cid:3)
(cid:3) HVDC transmission system control .................................................................. 16(cid:3)
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2.5.1.(cid:3) Hierarchical MTDC control structure .................................................... 17(cid:3)
2.5.2.(cid:3) Ancillary services .................................................................................... 18(cid:3)
(cid:3) Chapter summary ............................................................................................. 20(cid:3)
3.(cid:3) Modelling of hybrid ac/dc system ......................................................... 21(cid:3)
(cid:3) AC grid ............................................................................................................... 21(cid:3)
3.1.1.(cid:3) Synchronous generators .......................................................................... 21(cid:3)
3.1.2.(cid:3) Synchronous machine Controllers ........................................................ 24(cid:3)
(cid:3) DC grid .............................................................................................................. 29(cid:3)
3.2.1.(cid:3) Voltage Source Converters ..................................................................... 29(cid:3)
3.2.2.(cid:3) DC cables ............................................................................................... 49(cid:3)
(cid:3) Complete Hybrid AC/DC Power System ........................................................... 51(cid:3)
3.3.1.(cid:3) Reference frame transformation ............................................................ 51(cid:3)
3.3.2.(cid:3) AC Transmission Network Equations ................................................... 52(cid:3)
3.3.3.(cid:3) Interfacing the different DAE ............................................................... 54(cid:3)
3.3.4.(cid:3) Linear and Non-Linear Model Simulation ........................................... 55(cid:3)
(cid:3) Chapter summary ............................................................................................. 63(cid:3)
4.(cid:3) Methods for interaction analysis in hybrid ac/dc power systems ........ 65(cid:3)
(cid:3) Modal analysis .................................................................................................. 66(cid:3)
4.1.1.(cid:3) Concept of a state and state equations ................................................... 66(cid:3)
4.1.2.(cid:3) Eigenvectors ........................................................................................... 69(cid:3)
4.1.3.(cid:3) Observability and Controllability for decoupled systems ..................... 70(cid:3)
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4.1.4.(cid:3) Free motion (natural) response .............................................................. 71(cid:3)
4.1.5.(cid:3) Mode shapes .......................................................................................... 72(cid:3)
4.1.6.(cid:3) Participation factor analysis .................................................................. 73(cid:3)
(cid:3) Control loop interaction analyses ...................................................................... 75(cid:3)
4.2.1.(cid:3) Multiple-Input Multiple-Output power system ..................................... 75(cid:3)
4.2.2.(cid:3) Relative Gain Array ................................................................................ 77(cid:3)
4.2.3.(cid:3) Performance relative gain array ............................................................ 79(cid:3)
(cid:3) Chapter summary ............................................................................................. 80(cid:3)
5.(cid:3) Interaction analyses – Electro-mechanical dynamics ........................... 81(cid:3)
(cid:3) Effect of DC grid interconnection ..................................................................... 83(cid:3)
(cid:3) Mode shapes as a measure of dynamic interaction ......................................... 86(cid:3)
(cid:3) AC-AC Interaction Identification ..................................................................... 90(cid:3)
(cid:3) Sensitivity to converter controller strategy and parameter settings ............... 94(cid:3)
5.4.1.(cid:3) Master slave ........................................................................................... 94(cid:3)
5.4.2.(cid:3) Frequency support ................................................................................ 96(cid:3)
5.4.3.(cid:3) Controller Tuning .................................................................................. 97(cid:3)
5.4.4.(cid:3) Resonant conditions .............................................................................. 98(cid:3)
(cid:3) Participation factor based interaction analysis .............................................. 102(cid:3)
5.5.1.(cid:3) AC-Converter interaction modes ......................................................... 106(cid:3)
5.5.2.(cid:3) Converter interaction modes ............................................................... 108(cid:3)
5.5.3.(cid:3) AC-AC interaction modes .................................................................... 109(cid:3)
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