Details

Flexible distribution systems through the application of multi back-to-back converters: Concept, implementation and experimental verification

by Graaff, Roald A.A. de, PhD

Abstract (Summary)
In recent years the planning and operation of electrical power systems has changed significantly. The unbundling of the formerly integrated generation, transmission, distribution and delivery companies and the growing penetration of DG is increasing the complexity and uncertainty in distribution network planning and operation. Due to the uncertainty, network investments that are done to anticipate load growth or the connection of expected new generators may turn out to be uneconomic. The complexity leads to a higher risk of failures. This stimulates the distribution network operator to consider flexible alternatives to traditional network reinforcements and flexible operation measures. This thesis concerns a power flow control device based on power electronics, called Intelligent Node (IN), which can provide such needs. The idea to control power flow by the application of power electronics is not new in itself. The existing applications are mainly aimed at transmission systems, with its high voltage and power levels and existence of advanced measurement and control systems. In this thesis, an overview is given of existing applications. Successful experiences in the transmission network cannot be applied directly to the distribution system due to its different network topology and operation. For example, the low-inductive character of underground cables, as opposed to the mostly inductive impedance of overhead lines, requires adaptations on the methods to control voltages and power flow in the distribution system. Also the phase-by-phase operation of load-break switches is only found in distribution networks, and the availability of measurement data for all network nodes cannot be taken for granted. The IN consists of multiple converters interconnected on their DC side, and thus the AC side voltages are decoupled. This topology has the ability to control power flow between its AC ports and can supply a radial network section with a controllable voltage. By having the ability to control the power flow it is possible to distribute redundancy over different feeders when needed. In the current practice, every feeder must be able to supply the full load of another feeder, and can therefore only be loaded up to around fifty percent of its power rating. Sharing the redundancy over more feeders allows the connection of loads and generation units beyond this limit. Since the AC voltages on the different IN ports are unrelated, the IN can connect networks with different voltage amplitudes, phase angles and/or frequencies, which makes it possible to also share redundancy in such situations. Controlling the power flow in a meshed network can also be used to optimize voltage profiles, and thus maximize the penetration level of distributed generation units in the network. Alternatively, the power flow can be optimized to reduce losses in the network. During a network disturbance, the IN can prevent spreading of this disturbance, support the disturbed network, temporarily supply part of the network as a radial network, and restore meshed operation after the disturbance. To allow the Intelligent Node (IN) to perform the described tasks, the IN converters need to be able to respond quickly to planned and unplanned events in the power system, such as load changes, short-circuits and the opening and closing of load-break switches. The ability of the converters to do so, depends, besides on their ratings, mainly on the controls that drive them. Furthermore, the protection system of the IN needs to prevent damage to the IN components due to over-currents and over-voltages. At the converter level two basic operating modes exist: power flow control and voltage control. The first operating mode is used in meshed network operation, and called PQ control mode. The converter controls its power exchange with the network by controlling its output current. In the second operating mode, called V control mode, the converter defines the amplitude, frequency and phase angle of the voltage on its AC port. The converter behaves as a voltage source with a fixed frequency and supplies or consumes the active and reactive power as required by the connected loads and generators of a radial network section. In the proposed IN concept, at least one of the converters of the IN is galvanically connected to the 'central grid', and operates in PQ control mode, in order to supply the connected sections and to control the DC bus voltage. To fully utilize the capabilities of the interconnected converters, the IN control concept also includes specific detection schemes and additional control and protections, which can change the operating mode and set-points of the converters or shut down the IN in response to power system events. When the power system is in normal operation conditions and the converters are in PQ control mode, the IN controls the power flow in the meshed network according to centrally determined P and Q set-points. During a short-circuit, and the resulting voltage dip, the IN should no longer follow these set-points, but inject reactive power to mitigate the voltage dip. To do so, a control scheme was developed, which is only active when the network voltage is outside of a certain voltage band. Although it is assumed that meshed operation is the normal situation, it might be necessary to operate some parts of the network radially for some time. In order to perform maintenance or repair work on a certain network section, it can be necessary for instance to isolate it by opening the switches on each of its sides. In such a situation, the IN can supply a resulting radial part of the network, with the applicable converter in V control mode. To do so, the applicable IN converter must stop controlling the power flow and start controlling the voltage level instead, after detecting the change in the network. A control and detection scheme was developed to implement this functionality. After the maintenance or repair work has been finished, the radial part of the grid is to be reconnected to the rest of grid. To maintain IN operation and minimize voltage discontinuities after restoring meshed operation, it is necessary that the voltage of the radial network section is synchronized with the voltage of the rest of the network. Therefore, the voltage amplitude, frequency and phase angle are periodically measured at a remote location, and transmitted to the IN with a random but limited time delay. To determine the maximum remote measurement interval, the statistics of frequency variations in the public electricity network have been gathered through measurements. The maximum interval is determined as a function of acceptable phase angle difference between the networks. After detecting that the meshed network has been restored, the applicable converter must be able to change from V control mode to PQ control mode, without disconnecting from the grid or stopping operation. The operation of circuit breakers is in many networks performed simultaneously on all three phases. In other medium voltage networks, for example in the Netherlands, however, the phase-by-phase operation of load-break switches is common, given the wide-spread application of manually operated, compact, epoxy resin insulated, single-phase switchgear. Phase-by-phase connection and disconnection of grid areas requires a different IN behavior. The control and detection schemes were developed both for three-phase and for phase-by-phase switchgear operation. Existing back-to-back applications cannot make the described transitions without supply interruption, neither for three-phase nor for phase-by-phase switchgear operation. The developed control and detection schemes are implemented in a laboratory-scale set-up. The main components of this set-up are two 400V, three-phase converters, connected on their DC sides, with the possibility to connect the AC sides to a radial network with a resistive load or to the public low voltage network. With this set-up, experiments are performed, focusing on the connection and disconnection of network areas and on voltage sag and swell mitigation. The experimental verification of the connection and disconnection control and detection schemes, as well as the voltage dip and swell mitigation implementation, shows a successful implementation of the concept.
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Bibliographical Information:

Advisor:W.L. Kling

School:Technische Universiteit Eindhoven

School Location:Netherlands

Source Type:Doctoral Dissertation

Keywords:Power distribution, Power electronics, Load flow control, Power distribution control, Power quality, Voltage control

ISBN:9789038622200

Date of Publication:05/26/2010

Document Text (Pages 1-10)


Page 2

Flexible distribution systems through the

application of multi back-to-back

converters: Concept, implementation and

experimental verification

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de
Technische Universiteit Eindhoven, op gezag van de
rector magnificus, prof.dr.ir. C.J. van Duijn, voor een

commissie aangewezen door het College voor
promoties in het openbaar te verdedigen
op woensdag 26 mei 2010 om 14.00 uur

door

Roald Antonius Adrianus de Graaff

geboren te Waalwijk


Page 3

Dit proefschrift is goedgekeurd door de promotor:

prof.ir. W.L. Kling

Copromotor:
dr. J.L. Duarte

Copyright © 2010 R.A.A. de Graaff
All rights reserved. No part of this publication may be reproduced
or transmitted in any form or by any means, electronic, mechanical,
including photocopy, recording, or any information storage
and retrieval system, without the prior written permission of the
copyright owner.

The work leading to this thesis was supported by KEMA and the
IOP-EMVT program of SenterNovem.

CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EIND-
HOVEN
Graaff, Roald A.A. de
Flexible distribution systems through the application of multi
back-to-back converters: Concept, implementation and experimental
verification / by Roald Antonius Adrianus de Graaff. -
Eindhoven: Technische Universiteit Eindhoven, 2010.
Proefschrift. - ISBN 978-90-386-2220-0
NUR 959
Trefw.: Elektriciteitsdistributie / Vermogenselektronica / Vermogenssturing
/ Besturing elektriciteitsdistributie / Spanningskwaliteit
/ Spanningsregeling
Subject headings: Power distribution / Power electronics / Load
flow control / Power distribution control / Power quality / Voltage
control


Page 4

To Susana
To my parents


Page 5

Promotor:
prof.ir. W.L. Kling, TU/e

Copromotor:
dr. J.L. Duarte, TU/e

Core committee:
prof.dr.ir. R.W. De Doncker, RWTH Aachen University
prof.dr. E. Lomonova, TU/e
prof.dr. J.A. Peças Lopes, University of Porto

Other members:
prof.dr.ir. J.H. Blom (reserve), TU/e
dr.ir. F. van Overbeeke, EMforce
prof.dr. A.G. Tijhuis (chairman), TU/e
ir. P.T.M. Vaessen, KEMA


Page 6

Contents

List of Figures

List of Tables

Abstract

Samenvatting
v

ix

xi

xv

1 Introduction 1
1.1 Changes in electrical power generation . . . . . . . . . . . 2
1.2 Changes in the organization of power systems . . . . . . . 4
1.3 Consequences for the distribution network . . . . . . . . . 5
1.4 Ongoing research . . . . . . . . . . . . . . . . . . . . . . . 6

1.4.1 Communication and automation . . . . . . . . . . 8
1.4.2 Load control . . . . . . . . . . . . . . . . . . . . . 8
1.4.3 Generation control . . . . . . . . . . . . . . . . . . 9
1.4.4 Storage . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4.5 Power electronics . . . . . . . . . . . . . . . . . . . 10
1.4.6 Active distribution networks . . . . . . . . . . . . 11

1.5 Research objective . . . . . . . . . . . . . . . . . . . . . . 11
1.6 Research questions . . . . . . . . . . . . . . . . . . . . . . 12
1.7 Research approach . . . . . . . . . . . . . . . . . . . . . . 12
1.8 IOP-EMVT programme . . . . . . . . . . . . . . . . . . . 13
1.9 Outline of the thesis . . . . . . . . . . . . . . . . . . . . . 15
1.10 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2 Distribution systems 17
2.1 The network operator’s role . . . . . . . . . . . . . . . . . 17
2.2 Network topology and redundancy . . . . . . . . . . . . . 19

2.2.1 Network topology . . . . . . . . . . . . . . . . . . 19
2.2.2 Redundancy . . . . . . . . . . . . . . . . . . . . . . 20

2.3 Power quality aspects . . . . . . . . . . . . . . . . . . . . 21
2.3.1 Steady state voltage amplitude . . . . . . . . . . . 21

i


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ii Contents

2.3.2 Flicker . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3.3 Voltage dips . . . . . . . . . . . . . . . . . . . . . . 24
2.3.4 Phase angle jumps . . . . . . . . . . . . . . . . . . 25
2.3.5 Power frequency . . . . . . . . . . . . . . . . . . . 26
2.4 Voltage control . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3 FACTS in distribution systems 29
3.1 Principles of power flow control . . . . . . . . . . . . . . . 29

3.1.1 Power flow in overhead line . . . . . . . . . . . . . 30
3.1.2 Power flow in underground cable . . . . . . . . . . 35

3.2 FACTS technologies . . . . . . . . . . . . . . . . . . . . . 36
3.2.1 Solid-state switching devices . . . . . . . . . . . . 36
3.2.2 Converter topologies and switching strategies . . . 37
3.2.3 Mechanical switches . . . . . . . . . . . . . . . . . 39

3.3 FACTS and D-FACTS applications . . . . . . . . . . . . . 40
3.3.1 Shunt FACTS and D-FACTS devices . . . . . . . . 41
3.3.2 Series FACTS and D-FACTS devices . . . . . . . . 45
3.3.3 Mixed form FACTS and D-FACTS devices . . . . 46

3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 52

4 Functional concept of the Intelligent Node 55
4.1 Facilitating increased loading . . . . . . . . . . . . . . . . 56

4.1.1 Controlled sharing of redundancy . . . . . . . . . . 56
4.1.2 Controlled power exchange between grid areas . . 60

4.2 Controlling voltage profiles . . . . . . . . . . . . . . . . . 61

4.2.1 Example application . . . . . . . . . . . . . . . . . 63
4.3 Voltage dip mitigation . . . . . . . . . . . . . . . . . . . . 70

4.3.1 Example application . . . . . . . . . . . . . . . . . 71
4.4 Possible Intelligent Node topologies . . . . . . . . . . . . . 74

4.4.1 Power electronics controlled auto transformers . . 74
4.4.2 Power electronics controlled series impedances . . 75
4.4.3 Power electronics converters . . . . . . . . . . . . . 76

4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5 Intelligent Node control and protection 79
5.1 Basic converter controls . . . . . . . . . . . . . . . . . . . 79

5.1.1 Controller discretization . . . . . . . . . . . . . . . 81
5.1.2 AC current control . . . . . . . . . . . . . . . . . . 82
5.1.3 AC voltage control . . . . . . . . . . . . . . . . . . 83
5.1.4 Active and reactive power control . . . . . . . . . . 85
5.1.5 DC bus voltage control . . . . . . . . . . . . . . . 85

5.2 IN response to unplanned power system events . . . . . . 86
5.2.1 Voltage dip mitigation by injecting reactive power 88


Page 8

Contents iii

5.2.2 IN protection concept . . . . . . . . . . . . . . . . 94
5.3 IN role in planned power system events . . . . . . . . . . 96

5.3.1 Energization and de-energization . . . . . . . . . . 96
5.3.2 Disconnecting grid areas . . . . . . . . . . . . . . . 97
5.3.3 Connecting grid areas . . . . . . . . . . . . . . . . 103
5.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 114

6 Laboratory-scale demonstration 115
6.1 Experimental set-up . . . . . . . . . . . . . . . . . . . . . 115

6.1.1 Converter control implementation . . . . . . . . . 117
6.1.2 Modeling of experimental set-up . . . . . . . . . . 118

6.2 Basic converter step responses . . . . . . . . . . . . . . . . 119
6.2.1 Changing power reference values . . . . . . . . . . 120
6.2.2 Changing voltage reference value . . . . . . . . . . 121
6.2.3 Changing load . . . . . . . . . . . . . . . . . . . . 121

6.3 Transition from radial to meshed operation . . . . . . . . 124
6.3.1 Synchronization . . . . . . . . . . . . . . . . . . . 124
6.3.2 Three-phase load-break switch closing . . . . . . . 124
6.3.3 Phase-by-phase load-break switch closing . . . . . 127
6.3.4 Ensuring load-break switch closing detection . . . 131

6.4 Transition from meshed to radial operation . . . . . . . . 132
6.4.1 Three-phase load-break switch opening . . . . . . . 133
6.4.2 Phase-by-phase load-break switch opening . . . . . 134

6.5 Voltage dip and swell mitigation . . . . . . . . . . . . . . 136
6.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . 137

7 Conclusions, thesis contribution and recommendations 139
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 139
7.2 Thesis contribution . . . . . . . . . . . . . . . . . . . . . . 143
7.3 Recommendations . . . . . . . . . . . . . . . . . . . . . . 144

References 147

Abbreviations, symbols and notations 159

A DC current of AC/DC converter 163
A.1 DC link current in single phase voltage source converter . 163
A.2 DC link current in three-phase voltage source converter . 164

B Simulations and experimental results practical set-up 167

Acknowledgements 185

Curriculum vitae 187


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Page 10

List of Figures

1.1 Small and medium scale renewable energy sources. . . . . . . 3
1.2 Google Timelines results for selected search terms. . . . . . . 7
1.3 Structure of the IOP-EMVT Intelligent Power Systems project. 14

2.1 Types of medium and low voltage grids. . . . . . . . . . . . . 19
2.2 Each feeder can supply the load of another feeder. . . . . . . 20
2.3 Sequence of events during a short-circuit. . . . . . . . . . . . 22
2.4 Curve for Pst = 1 for rectangular voltage changes. . . . . . . 24
2.5 Typical voltage coordination radial MV/LV network. . . . . . 27

3.1 Single-line and phasor diagram for overhead line. . . . . . . . 30
3.2 Power flow control using a series voltage source. . . . . . . . . 32
3.3 Power flow control using a series impedance. . . . . . . . . . 33
3.4 Power flow control using a parallel device. . . . . . . . . . . . 34
3.5 Power flow in cable with series voltage source. . . . . . . . . . 35
3.6 Ratings of solid-state switching devices. . . . . . . . . . . . . 37
3.7 Basic diagrams current and voltage source converters. . . . . 38
3.8 Topology and output voltage source converters. . . . . . . . . 39
3.9 PWM switching technique applied to a single-switch topology. 39
3.10 Mechanical switches applied in FACTS. . . . . . . . . . . . . 40
3.11 Basic (D-)FACTS connection methods. . . . . . . . . . . . . . 41
3.12 Single-line diagram and operating characteristic SVC. . . . . 42
3.13 Single-line diagram and operating characteristic STATCOM. 44
3.14 Single-line diagram and operating characteristic TSSC/TCSC. 47
3.15 Single-line diagrams SSSC and D-SSSC. . . . . . . . . . . . . 47
3.16 Single-line diagrams UPFC and IPFC. . . . . . . . . . . . . . 48
3.17 Transfer switch. . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.18 Single-line diagram and operating characteristic B2B device. 50

4.1 Sharing of redundancy. . . . . . . . . . . . . . . . . . . . . . . 57
4.2 Sequence of events during short-circuit in a network with a

3-port IN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

v

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