Multi-terminal Direct-Current Grids - Nilanjan Chaudhuri - ebook

Multi-terminal Direct-Current Grids ebook

Nilanjan Chaudhuri

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A generic DC grid model that is compatible with the standard AC system stability model is presented and used to analyse the interaction between the DC grid and the host AC systems. A multi-terminal DC (MTDC) grid interconnecting multiple AC systems and offshore energy sources (e.g. wind farms) across the nations and continents would allow effective sharing of intermittent renewable resources and open market operation for secure and cost-effective supply of electricity. However, such DC grids are unprecedented with no operational experience. Despite lots of discussions and specific visions for setting up such MTDC grids particularly in Europe, none has yet been realized in practice due to two major technical barriers: * Lack of proper understanding about the interaction between a MTDC grid and the surrounding AC systems. * Commercial unavailability of efficient DC side fault current interruption technology for conventional voltage sourced converter systems This book addresses the first issue in details by presenting a comprehensive modeling, analysis and control design framework. Possible methodologies for autonomous power sharing and exchange of frequency support across a MTDC grid and their impact on overall stability is covered. An overview of the state-of-the-art, challenges and on-going research and development initiatives for DC side fault current interruption is also presented.

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Table of Contents

Cover

Title Page

Copyright

Dedication

Foreword

Preface

Acronyms

Symbols

Chapter 1: Fundamentals

1.1 Introduction

1.2 Rationale Behind MTDC Grids

1.3 Network Architectures of MTDC Grids

1.4 Enabling Technologies and Components of MTDC Grids

1.5 Control Modes in MTDC Grid

1.6 Challenges for MTDC Grids

1.7 Configurations of MTDC Converter Stations

1.8 Research Initiatives on MTDC Grids

1.9 Focus and Scope of the Monograph

Chapter 2: The Voltage-Sourced Converter (VSC)

2.1 Introduction

2.2 Ideal Voltage-Sourced Converter

2.3 Practical Voltage-Sourced Converter

2.4 Control

2.5 Simulation

2.6 Symbols of the VSC

Chapter 3: Modeling, Analysis, and Simulation of AC–MTDC Grids

3.1 Introduction

3.2 MTDC Grid Model

3.3 AC Grid Model

3.4 AC–MTDC Load flow Analysis

3.5 AC–MTDC Grid Model for Nonlinear Dynamic Simulation

3.6 Small-signal Stability Analysis of AC–MTDC Grid

3.7 Transient Stability Analysis of AC–MTDC Grid

3.8 Case Studies

3.9 Case Study 1: The North Sea Benchmark System

3.10 Case Study 2: MTDC Grid Connected to Equivalent AC Systems

3.11 Case Study 3: MTDC Grid Connected to Multi-machine AC System

Chapter 4: Autonomous Power Sharing

4.1 Introduction

4.2 Steady-state Operating Characteristics

4.3 Concept of Power Sharing

4.4 Power Sharing in MTDC Grid

4.5 AC–MTDC Grid Load flow Solution

4.6 Post-contingency Operation

4.7 Linear Model

4.8 Case Study

Chapter 5: Frequency Support

5.1 Introduction

5.2 Fundamentals of Frequency Control

5.3 Inertial and Primary Frequency Support from Wind Farms

5.4 Wind Farms in Secondary Frequency Control (AGC)

5.5 Modified Droop Control for Frequency Support

5.6 AC–MTDC Load Flow Solution

5.7 Post-Contingency Operation

5.8 Case Study

Chapter 6: Protection of MTDC Grids

6.1 Introduction

6.2 Converter Station Protection

6.3 DC Cable Fault Response

6.4 Fault-blocking Converters

6.5 DC Circuit Breakers

6.6 Protection Strategies

References

Index

End User License Agreement

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Guide

Cover

Table of Contents

Preface

Chapter 1: Fundamentals

List of Illustrations

Figure 1.1

Figure 1.2

Figure 1.3

Figure 1.4

Figure 1.5

Figure 1.6

Figure 1.7

Figure 1.8

Figure 1.9

Figure 2.1

Figure 2.2

Figure 2.3

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List of Tables

Table 1.1

Table 1.2

Table 3.1

Table 3.3

Table 4.1

Table 5.1

Multi-Terminal Direct-Current Grids

Modeling, Analysis, and Control

 

 

 

Nilanjan Ray Chaudhuri

 

 

Balarko Chaudhuri

 

 

Rajat Majumder

 

 

Amirnaser Yazdani

 

 

 

 

 

 

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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MATLAB®, and Simulink® are trademarks of The MathWorks, Inc. and are used with permission. The MathWorks does not warrant the accuracy of the text or exercises in this book. This book's use or discussion of MATLAB®, and Simulink® software or related products does not constitute endorsement or sponsorship by The MathWorks of a particular pedagogical approach or particular use of the MATLAB®, and Simulink® software.

Library of Congress Cataloging-in-Publication Data:

Chaudhuri, Nilanjan Ray, 1981-

Multi-terminal direct-current grids : modeling, analysis, and control / Nilanjan Ray Chaudhuri, Balarko Chaudhuri, Rajat Majumder, Amirnaser Yazdani.

pages cm

Includes bibliographical references and index.

ISBN 978-1-118-72910-6 (hardback)

1. Electric power distribution–Direct current. I. Chaudhuri, Balarko, 1977- II. Majumder, Rajat, 1977- III. Yazdani, Amirnaser, 1972- IV. Title.

TK3111.C44 2014

621.31′2–dc23

2014017303

To our parents and families

Foreword

In recent years, the electric power delivery system has been changing dramatically. Concern for the environment, the economy, and power system security has placed stringent demands on power transmission. We must acknowledge the inevitable depletion of fossil fuels and the growing importance of sustainable and environmental awareness. Electricity derived from renewable energy sources offers advantages in terms of economical efficiency, especially when compared with oil, gas and coal.

Existing rights-of-way for high voltage transmission lines are generally underutilized and some transmission circuits are nearing their end of life. New rights-of-way for overhead transmission lines are becoming difficult if not impossible to acquire. Voltages Sourced Converter (VSC) transmission is the way forward with its capability to operate with many terminals, allow for greater power density in its circuits, and replace aging AC transmission lines or existing AC lines that may be converted to DC. DC overhead transmission lines can be compacted to share right-of-way with roads and railway lines. VSC transmission is more favorable for use with underground and undersea cable transmission where it is expedient to do so.

In the future, issues of the combination of diversity of loads, wind and solar energy across wide geographical areas will become crucial. DC grids are critical to solving these major concerns.

This book initiates the technical background that applies to multi-terminal DC transmission as well as the evolution through to the VSC converter configurations and the anticipated and inevitable DC grids. It covers basic theory setting the stage for on-going developments. At this present time, this book is likely to be a useful resource and of great benefit to the student of DC transmission, both in graduate studies and in the workplace.

Dennis Woodford

Electranix Corporation

Winnipeg, Canada

Preface

Electric power transmission primarily relies on the alternating current (AC) technology with the direct current (DC) alternative being used only in very specific (e.g., very long distance overhead lines and subsea or underground cables for moderate distances) applications. The majority of the presently operational and planned high voltage direct current (HVDC) links are point-to-point with two points of connection with the AC networks. However, it is envisaged that DC electric networks embedding more than two terminal converter stations, which are commonly referred to as multi-terminal direct current (MTDC) grids, would offer the possibility of meshed subsea interconnections between countries and continents to share the diversity in renewable energy portfolios for better supply reliability. It is not clear what the exact topology of such DC grids would be and where they are going to be installed. However, the vision is that there would be an MTDC grid around the North Sea to tap into the rich offshore wind resources in the region and also strengthen the interconnection among the UK, Scandinavia, and continental Europe. In the long-run, the plan is to build a pan-European subsea grid to interconnect the major generation and load centers and eventually, interconnect this grid with the solar power generation sites in sub-Saharan Africa. Although some of these visions are quite ambitious and futuristic, the need for a subsea DC grid is now almost established.

There is a strong case for a subsea MTDC grid to share the diverse renewable energy resources across different geographical regions (e.g., UK, Scandinavia, and continental Europe; parts of North America) and also interconnect offshore energy (e.g., WFs) sources for better supply reliability and operational flexibility. However, at present, there is no such MTDC grid in operation except the first phase of the Zhoushan project commissioned in China in 2013. Despite the need, the uptake is delayed due to technological barriers like unavailability of fast protection system, DC circuit breakers and efficient voltage source converters (VSCs) with DC-side fault-clearing capability. Alongside protection and DC fault current interruption, another major cause of concern for the system operators is the unknown interaction between the DC grid and the host AC systems. An MTDC grid based on VSC technology is unprecedented with no operational experience. Hence, there is a lack of understanding of the unknown interaction between the MTDC grid and the host AC systems. For instance, what kind of services (e.g., frequency support and power oscillation damping) can an MTDC grid provide for the host AC systems. What are the implications of such service provisions on the overallstability of the AC–DC grid system. In order to analyze these system level issues, a systematic modeling framework for stability studies of combined AC-MTDC grid system is essential. These have received little attention until recent times, when a number of research papers were published on DC grid modeling, control, power quality, and so on, by and large in the context of offshore WFs. However, a single and comprehensive source of reference which includes the modeling, analysis, and control of MTDC grids has not yet been published.

This monograph is the first of its kind on the topic “multi-terminal direct current (MTDC) grids.” Although several research papers have been published recently on this important topic area, a comprehensive text is yet to be compiled as a single source of reference. This monographs aims to fill this gap by presenting the following:

A generic modeling framework for combined AC-MTDC grids which can be used by system planners and operators for stability analysis.

Validation of the averaged model of an AC-MTDC grid in MATLAB®/SIMULINK® against switched model in EMTDC/PSCAD.

Modal analysis to study the interactions between the MTDC grid and surrounding AC system as observed in time-domain simulation.

Design of control to enable the MTDC grid to provide AC system support (e.g., reduced loss-of-infeed and frequency support).

State-of-the-art research challenges in protection of MTDC grids.

The main body of this monograph begins with an overview of the voltage-sourced converter (VSC) systems that are the basic building blocks of MTDC grids. A generic modeling framework for MTDC grid is developed, enabling easy integration of the MTDC grid model with a multi-machine AC system model for stability studies. One particular concern is how an MTDC grid would react to loss/outage of one or more converter stations and the resulting power imbalance. Sharing the burden of such a loss/power mismatch has to be appropriate in order to minimize the impact on the converters and the neighboring AC systems. The concept of droop control for autonomous power sharing in MTDC grids is covered. With increasing penetration of asynchronous generation (wind farms) and transmission (HVDC), reduction in system inertia and its effect on frequency control is a growing concern. Hence, provision for frequency support among the surrounding AC systems acrossthe MTDC grid is critical. Modification of the autonomous power sharing (droop) control to exercise such frequency support across the MTDC grids is described. Towards the end of the monograph, a chapter is dedicated to a review of the protection issues of MTDC grids including the present state of research in high power DC circuit breakers and fault blocking VSC systems. Also, the challenges in MTDC grid protection are highlighted along with an overview of some of the protection strategies that could be used in future.

This monograph is targeted at the academicians with research and/or teaching interest in DC electric power transmission, graduate students focusing on DC power transmission, manufacturers of DC power transmission systems, developers of offshore transmission networks and utilities/system operators who have or are planning to have VSC DC links connected to their system or contemplating being part of a DC grid in future.

N. Ray Chaudhuri

Niskayuna, NY, USA

April, 2014

B. Chaudhuri

London, UK

April, 2014

R. Majumder

Raleigh, NC, USA

April, 2014

A. Yazdani

Toronto, ON, Canada

April, 2014

Acronyms

A2MC

Alternate-arm multi-level converter

AC

Alternating current

AGC

Automatic generation control

BW

Bandwidth

CSC

Current source converter

DAEs

Differential algebraic equations

DC

Direct current

DFIG

Doubly fed induction generator

EWIP

East-West Interconnector Project

FC

Full converter

FDWT

Fast Dyadic Wavelet transform

HIL

Hardware in loop

HVDC

High voltage direct current

IGBT

Insulated gate bipolar transistor

IGCT

Integrated gate commutated thyristor

JWG

Joint working group

LCC

Line commutated converter

MMC

Modular multi-level Converter

MTDC

Multi terminal direct current

NETS

National electricity transmission system

NPC

Neutral point clamped

NR

Newton Raphson

ODEs

Ordinary differential equations

OPF

Optimal power flow

PCC

Point of common coupling

PI

Proportional integral

PLL

Phase-locked loop

PWM

Pulse width modulation

SACOI

Sardinia Corsica Italy

SG

Synchronous generator

SHE

Selective harmonic elimination

SVCs

Static VAr compensators

SVM

Space vector modulation

TMR

Triple modular redundancy

VCO

Voltage controlled oscillator

VSC

Voltage source converter

WF

Wind farm

WGs

Working groups

WTGs

Wind turbine generators

Symbols

Shunt capacitance at each end of pi section of DC cable

Converter DC-side filter capacitor

Field voltage of synchronous generator

Transient emf due to flux-linkage in

q

-axis damper coil of synchronous generator

Transient emf due to field flux-linkage of synchronous generator

Available headroom in the converter rating

Compensator used in the PLL

Current flowing into DC side of converter

X

-axis component of current phasor

Y

-axis component of current phasor

Total DC current flowing into the converter and corresponding DC link capacitor from DC grid side

d

-axis component of stator current of synchronous generator

q

-axis component of stator current of synchronous generator

Compensator for inner current control loop of converter

Converter AC voltage compensator

Converter DC link voltage compensator

L

Aggregated inductance of converter transformer and phase reactor

Series inductance of each pi-section of DC cable

Real power, reactive power, and voltage magnitude in generator or load bus

Real power injection into the PCC of the converter station

Reactive power injection into the PCC of the converter station

Converter real power rating

R

Aggregated resistance of converter transformer and phase reactor

Series resistance of each pi-section of DC cable

Effective series resistance of DC link capacitor of converter

DC-side fault resistance

DC grid grounding resistance

Armature resistance of synchronous generator

Droop coefficient of governor

d

-axis open-circuit transient and sub-transient time constants of synchronous generator

q

-axis open-circuit transient and sub-transient time constants of synchronous generator

DC bus voltage of converter

Common DC voltage feedback signal

Magnitude of AC system voltage phasor at the PCC of converter station

X

-axis component of AC system voltage phasor at the PCC of converter station

Y

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