Table of Contents

Title Page

Copyright

About the Authors

Preface

Chapter 1: Principles of Transformer Differential Protection and Existing Problem Analysis

1.1 Introduction

1.2 Fundamentals of Transformer Differential Protection

1.3 Some Problems with Power Transformer Main Protection

1.4 Analysis of Electromagnetic Transients and Adaptability of Second Harmonic Restraint Based Differential Protection of a UHV Power Transformer

1.5 Study on Comparisons among Some Waveform Symmetry Principle Based Transformer Differential Protection

1.6 Summary

References

Chapter 2: Malfunction Mechanism Analysis due to Nonlinearity of Transformer Core

2.1 Introduction

2.2 The Ultra-Saturation Phenomenon of Loaded Transformer Energizing and its Impacts on Differential Protection

2.3 Studies on the Unusual Mal-Operation of Transformer Differential Protection during the Nonlinear Load Switch-In

2.4 Analysis of a Sort of Unusual Mal-operation of Transformer Differential Protection due to Removal of External Fault

2.5 Analysis and Countermeasure of Abnormal Operation Behaviours of the Differential Protection of the Converter Transformer

2.6 Summary

References

Chapter 3: Novel Analysis Tools on Operating Characteristics of Transformer Differential Protection

3.1 Introduction

3.2 Studies on the Operation Behaviour of Differential Protection during a Loaded Transformer Energizing

3.3 Comparative Investigation on Current Differential Criteria between One Using Phase Current and One Using Phase–Phase Current Difference for the Transformer using Y-Delta Connection

3.4 Comparative Analysis on Current Percentage Differential Protections Using a Novel Reliability Evaluation Criterion

3.5 Comparative Studies on Percentage Differential Criteria Using Phase Current and Superimposed Phase Current

3.6 A Novel Analysis Methodology of Differential Protection Operation Behaviour

3.7 Summary

References

Chapter 4: Novel Magnetizing Inrush Identification Schemes

4.1 Introduction

4.2 Studies for Identification of the Inrush Based on Improved Correlation Algorithm

4.3 A Novel Method for Discrimination of Internal Faults and Inrush Currents by Using Waveform Singularity Factor

4.4 A New Principle of Discrimination between Inrush Current and Internal Fault Current of Transformer Based on Self-Correlation Function

4.5 Identifying Inrush Current Using Sinusoidal Proximity Factor

4.6 A Wavelet Transform Based Scheme for Power Transformer Inrush Identification

4.7 A Novel Adaptive Scheme of Discrimination between Internal Faults and Inrush Currents of Transformer Using Mathematical Morphology

4.8 Identifying Transformer Inrush Current Based on Normalized Grille Curve

4.9 A Novel Algorithm for Discrimination between Inrush Currents and Internal Faults Based on Equivalent Instantaneous Leakage Inductance

4.10 A Two-Terminal Network-Based Method for Discrimination between Internal Faults and Inrush Currents

4.11 Summary

References

Chapter 5: Comprehensive Countermeasures for Improving the Performance of Transformer Differential Protection

5.1 Introduction

5.2 A Method to Eliminate the Magnetizing Inrush Current of Energized Transformers

5.3 Identification of the Cross-Country Fault of a Power Transformer for Fast Unblocking of Differential Protection

5.4 Adaptive Scheme in the Transformer Main Protection

5.5 A Series Multiresolution Morphological Gradient Based Criterion to Identify CT Saturation

5.6 A New Adaptive Method to Identify CT Saturation Using a Grille Fractal

5.7 Summary

References

Index

End User License Agreement

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Guide

Cover

Table of Contents

Preface

Begin Reading

List of Illustrations

Figure 1.1

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List of Tables

Table 1.1

Table 1.2

Table 1.3

Table 1.4

Table 1.5

Table 1.6

Table 1.7

Table 2.1

Table 2.2

Table 2.3

Table 2.4

Table 4.1

Table 4.2

Table 4.3

Table 4.4

Table 4.5

Table 4.6

Table 4.7

Table 4.8

Table 4.9

Table 4.10

Table 4.11

Table 4.12

Table 4.13

Table 4.14

Table 5.1

Table 5.2

Table 5.3

Table 5.4

Table 5.5

ELECTROMAGNETIC TRANSIENT ANALYSIS AND NOVEL PROTECTIVE RELAYING TECHNIQUES FOR POWER TRANSFORMERS

Xiangning Lin

State Key Laboratory of Electromagnetic Engineering
School of Electrical and Electronic Engineering
Huazhong University of Science and Technology

Jing Ma

State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources
School of Electrical and Electronic Engineering
North China Electric Power University

Qing Tian

Chief Dispatching Center
Southern China Power grid

Hanli Weng

College of Electrical Engineering & New Energy,
China Three Gorges University.

This edition first published 2015

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Library of Congress Cataloging-in-Publication Data

Lin, Xiangning.

Electromagnetic transient analysis and novel protective relaying techniques for power transformers / Xiangning Lin, Jing Ma, Qing Tian, Hanli Weng.

pages cm

Includes bibliographical references and index.

ISBN 978-1-118-65382-1 (hardback)

1. Electric relays. 2. Electric transformers–Protection. 3. Transients (Electricity) I. Title.

TK2861.E4233 2014

621.31′4–dc23

2014021807

About the Authors

Professor Xiangning Lin has been working in this area since 1996. His research is mainly concentrated in the areas of power system protection/operation/control/modelling/simulation/analysis and smart grids. He has carried out very systematic research and practiced on power transformer electromagnetic transient simulation and protective relaying, especially approaches on operating characteristic analysis and studies on the novel principle of the transformer differential protection, for more than 18 years. He was the first to discover the ultra-saturation phenomenon of the power transformer and then designed appropriate operating characteristics analysis planes to make clear the advantages and disadvantages of the existing differential protection of power transformers. On the basis of this, he invented a variety of novel protection algorithms for the main protection of the power transformer. A series of papers were published in authoritative journals such as the IEEE Transactions on Power Systems and IEEE Transactions on Power Delivery. The work has been widely acknowledged and cited by international peers. Part of his research results have been used in many practical engineering projects. He is also a pioneer to the introduction of modern signal processing techniques to the design of the protection criteria for power transformers.

In recent years, Professor Lin has undertaken many major projects in China. For example, he guided a project of the National Natural Science Foundation of China to study the abnormal operation behaviour analysis and appropriate countermeasures of power transformers. Then he set up an advanced simulation and protection laboratory for the main equipment of power systems and pioneered the design and implementation of the corresponding protection techniques. He was also responsible for several projects from governments and enterprises on the study of the power transformer protection and monitoring. In addition, Professor Lin is a major member of the National Basic Research Program of China (973 Program) on the study of the interaction between large-scale electric power equipment characteristics and power system operation. He cooperated with the China Electric Power Research Institute to guide the study on the main protection for wind farms, including different types of power transformer. He has been teaching courses on Power system protective relaying and Power system analysis for many years. Much of the material covered in this book has been taught to students and other professionals.

Professor Jing Ma has been working in this area since 2003. His research is mainly concentrated in the areas of power system protection/control, modelling/simulation/analysis and smart grids. He has carried out very systematic research and practiced on power system protection, especially approaches concerning power transformer protection, for more than 10 years. He was the first to apply the two-terminal network algorithm to the areas of power system protection. A series of papers were published in authoritative journals such as the IEEE Transactions on Power Delivery. The work has been widely acknowledged and cited by international peers. He also proposed an approach based on the grille fractal to solve the Transient Analysis saturation problem, and the related paper has been published in the IEEE Transactions on Power Delivery. His research results have been used in many practical engineering projects.

In recent years, Dr Ma has undertaken many major projects in China. For instance, he participated in a key project of the National Natural Science Foundation of China to study the wide-area protection. He was also responsible for a project of the National Science Foundation project on the study of the power system smart protection and control. He is a major member of the National Basic Research Program of China (873 Program) on the study of the utilization of large-scale renewable energy with high security and efficiency, and was responsible for the design of the Power System Protection and Control Simulation Platform. Professor Ma has been teaching courses of Power System Protection Theory and Power System Automation for many years. He is also a key member of National Prime Course – Power System Protection Theory. Much of the material covered in this book has been taught to students and other professionals.

Dr Qing Tian is a senior engineer with the Chief dispatching centre, Southern Power grid, Guangzhou, China. He has been working in this area since 1998. His main research fields include power system protection, operation and control, together with the operation and maintenance technologies in HVDC and UHV AC transmission projects. He has published some well cited papers in authoritative international and Chinese journals. In particular, he has many experiences in solving practical engineering problems concerning UHVAC and UHVDC protections.

Dr Hanli Weng is a senior engineer with College of Electrical Engineering & New Energy, China Three Gorges University. She has been working in this area since 2004. Her main research fields include protective relaying of power transformer. She has published some well cited papers in the authoritative international and Chinese journals. In particular, she has many experiences in solving practical engineering problems concerning main equipment protection of large-scale hydro power station.

Preface

As the heart of the power system, the power transformer is crucial for the safety and stability of the power system, and the reliability of the protection arranged for the power transformer becomes a critical factor in guaranteeing the security of the system. Nevertheless, according to existing fault reports in the power industry, it is accidental event for the differential protection to mal-operate under some operating conditions. With the growing complexity of the power system structure and its components, the differential protection mal-operation events revealed have become an area of intense investigation in order to eliminate potential uncertainty and danger. Moreover, the electric circuit and the magnetic circuit, coupling in conjunction with each other, make the above phenomena even more intricate, as transformer switching events may cause electromagnetic transients. These phenomena remain an open issue and comprehensive studies are needed. However, while it is clearly essential to find out the origin of the abnormal operational behaviour in the power transformer, basic theory about electromagnetic transients in the power transformer is currently lacking. This book is published to address this problem directly.

The content of this book is arranged as follows: Chapter 1 defines the fundamental principle of the power transformer differential protection and some problems in this background. Second harmonic restraint based differential protection of Ultra High Voltage (UHV) power transformers is also investigated in this chapter. Chapter 2 attempts to study the unusual mal-operation of the differential protection of the transformer caused by ultra-saturation phenomena. In Chapter 3, appropriate theoretical bases for the existing protection method are discussed, preliminary comparative studies between phase current based and superimposed current based differential criteria are conducted and the results are compared. The main focus of Chapter 4 is on inrush identification by means of several novel schemes. Chapter 5 deals with the problems revealed in Chapter 4, with new methods put forward to eliminate the magnetizing inrush. Simulation verifications for the methods are also proposed.

The book is intended for graduate students in electric power engineering, for researchers in correlative fields or for anyone who wishes to keep an eye on the power transformer and the power system. We also gratefully acknowledge the technical assistance of State Key Laboratory of Electromagnetic Engineering, School of Electrical and Electronic Engineering, Huazhong University of Science and Technology. The work was also partly supported by the National Natural Science Foundation of China (project numbers 50177011, 50407010, and 50777024). The authors are continuing their research in this field and would welcome contact with new ideas or if there is any confusion generated.

Xiangning Lin
Wuhan, China
2014

Chapter 1
Principles of Transformer Differential Protection and Existing Problem Analysis

1.1 Introduction

With the development of the electric power industry, large capacity power transformers are more and more widely applied in power systems. As the heart of the whole power system, the performance of the transformer directly affects the continuous and stable operation of the whole power system. In particular, once a modern transformer of large capacity, high voltage, high cost and complicated structure is destroyed by a fault, a series of problems will emerge, such as wide-ranging impact, difficult and lengthy maintenance, and great economic loss. Statistics show that during the years 2001–2005, the average correct operating rate of transformers 220 kV and above is only up to 79.97%, far below the correct operating rate of line protection (more than 99%).

Differential protection is one of the foremost protection schemes used in the power transformer. The theoretical foundation of differential protection is Kirchhoff's current law (KCL), which is applied successfully in the protection of transmission lines and generators. However, there are many problems when it is necessary to identify transformer internal faults under various complicated operation conditions [1]. From the perspective of an electric circuit, the transformer's primary and secondary windings cannot be treated as the same node, with the voltage on each side being unequal. Besides, the two sides are not physically linked. In terms of basic principle, transformer differential protection is based on the balance of the steady magnetic circuit. However, this balance will be destroyed during the transient process and can only be rebuilt after the transient process is finished. Therefore, many unfavourable factors need to be taken into account in the implementation of transformer differential protection:

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