Variable Frequency Transformers for Large Scale Power Systems Interconnection: Theory and Applications

By Gesong Chen, Xiaoxin Zhou and Rui Chen

This book is an all-in-one resource on the development and application of variable frequency transformers to power systems and smart grids. It introduces the main technical issues of variable frequency transformers (VFT) systematically, including its basic construction, theory equations, and simulation models. Readers will then gain an in-depth discussion of its control system, operation performance, low frequency power oscillation, and technical economics, before proceeding to practical implementation and future developments. The related concepts of energy revolution, third generation grids, and power system interconnection are discussed as well.

• The first, comprehensive introduction to variable frequency transformers (VFT)
• An in-depth look at the construction of VFT, with simulations and applications
• Demonstrates how to assess the control system and overall system performance
• Analyses future developments, energy revolution and power system interconnections

Variable Frequency Transformers for Large Scale Power Systems is a timely overview of the state of the art for VFT as it is increasingly adopted in smart grids. It is intended for engineers and researchers specializing in power system planning and operation, as well as advanced students and industry professionals of power engineering.

• Language: English
• Publisher: Wiley
• Release date: Jul 18, 2018
• ISBN: 9781119129073

Book preview

Preface to the English Version

In modern society, electricity has a profound impact on the development of human livelihood and production, as well as economic and social development. All equipment and devices, from the locomotive driving devices to mobile phones, from electric lights to factory machinery, are required to be powered by electricity. It's the responsibility of us as electrical engineers to ensure a safe and reliable power supply and make electricity available everywhere so that everyone can enjoy sustainable and affordable power supplies. To ensure power supply is not only a technical issue, but is also an economic one. In addition, it has a close relationship with resources and the environment, and has to rely on scientific and technological innovation. From Faraday's discovery of electromagnetic induction to Edison's invention of electric light, technological innovation has made the history of power industry and promoted the innovation and development of the world.

Power grid is the basic platform for ensuring power supply. With the growth in demand for electricity and advances in electric power technology, the world's power grids have evolved from initial single‐machine and single‐line systems to small town grids, urban grids, provincial grids, national grids, and continental grids. With increasing grid scale and strengthened grid functions, large‐scale grid interconnection has become the trend. Innovation is the most important driving force for the development of power grids. Thanks to the development of high voltage and ultra‐high voltage transmission technology, and the application of AC and DC transmission technology, power grids are more closely interconnected with each other and the power supply is becoming far more safe and reliable.

The variable frequency transformer (VFT) is a new type of grid interconnection device. By integrating technologies of power transformer, phase shifter, hydro generator, doubly fed generator, DC drive, and other technologies, and with functions such as smooth adjustment of transmission power, suppression of low frequency oscillation, and adjusting system frequency function, it can realize the interconnection of asynchronous power grids, supplying power to weak power systems and passive systems, and providing black‐start power. VFT is an important achievement of power technology innovation in the new century. It provides a new option for the interconnection of large power grids, has been widely applied in North America, and has achieved good results.

As an engineer who has been long engaged in research on new power technologies, I specialize in power system simulation, overvoltage and insulation coordination, flexible AC transmission technology, UHV power grids, smart grids, Global Energy Interconnection, new energy sources, and technical standards. I have undertaken major research projects such as China's first 500 kV compact line, the first thyristor controlled series compensation project, and the first UHV AC and UHV DC transmission project. VFT is the main research content of my doctoral dissertation. Learning from the VFT‐related articles published by GE and based on years of research and practice in power systems, I have profoundly studied the work principle, mathematical models, simulation methods, and control systems of VFTs and solved problems of digital simulation analysis tools, operation control strategy, and large system low‐frequency oscillation in the VFT. In addition, I have analyzed and verified the operational characteristics of the VFT in power systems and its good performance. Through in‐depth study, I firmly believe that VFT will be a promising development and have prospective application in the power system as a new interconnection technology.

Funded by the electrical power scientific work publication program of the State Grid Corporation of China (hereinafter referred to as SGCC), the Chinese version of Large Electric Power System Grid Connection New Technology – Theory and Application of Variable Frequency Transformers was finally published and released by the China Electric Power Press in 2013, receiving positive responses from the industry. In 2014, Wiley organized experts to study the book and decided to publish the book in English in order to promote the application of this new technology to the world. The publication and release of this book as the world's first English monograph on the VFT reflects the forward‐looking awareness and innovation spirit of Wiley. In order to enrich the research content of variable frequency transformer and respond to Wiley's requirements, the author has added 40% extra content to the book based on the new development of UHV power grid, smart grid, and Global Energy Interconnection. I would say that the English monograph is more than just a translation of the Chinese version, as it has richer content.

Through the joint efforts of all parties, the English version of Variable Frequency Transformers for Large Scale Power Systems Interconnection: Theory and Applications has finally been published and released. As the world's first monograph that systematically studies and introduces VFT, it has won the concern and support of a lot of experts and friends. Wiley, the China Electric Power Press, and Mr. R.J. Piwko from GE, Mr. Wang Qiankun from GEIDCO, Mr. He Wei from China Jibei Electric Power Company LTD. have given me a lot of support and help. I would like to take this opportunity to express my heartfelt thanks to all my peers and friends who have supported and shown concern for the publication and release of the book. I hope this book can play a positive role in spreading some basic knowledge about VFT and promoting the development of the world's power technology.

Despite repeated checks and modifications, it is inevitable that this book still has some inadequacies. We sincerely invite readers to criticize and correct them.

May 18, 2018

Gesong Chen

Preface

Electricity is an important substance in modern society. The rapid growth in electricity demand advances higher requirements for power grid development. The essence of ensuring sustainable electricity supply is scientific and technological innovation. By reviewing the world's power grid development in the past 100 years, we find it is a history of constant innovation, reform, and improvement. Research shows that the interconnection of large power grids can effectively improve the reliability of power supply, reduce spare capacity, advance economic operation of power system, and further achieve great benefits such as peak load shifting, hydro and thermal power mutual support, as well as inter‐basin complementation, which is also the trend for the world's power grid development. The interconnection of traditional power grids includes synchronous AC and asynchronous DC. VFT is a new type of asynchronous grid interconnection device. Compared with back‐to‐back DC, it has obvious advantages such as less land occupation, lower transmission loss, and no harmonics. The world's first VFT, with a capacity of 105 MVA, was developed by GE and put into operation at the Langlois Substation, Quebec, Canada in October 2003. Years of practical operation proves the excellent performance of VFTs and effectively promotes their popularity and application. By 2010, a total of seven sets of VFTs had been put into commercial operation in North America.

Due to China's extremely uneven distribution of energy resources and energy demands, it is required that the power grid should be strengthened by interconnection to effectively improve the ability for optimal allocation of energy resources over large areas to ensure a safe, economical, good quality, green, and sustainable power supply. Since 1949, China's power grids have experienced rapid development from urban to provincial, regional, and inter‐regional interconnections. By the end of 2011, with the integration of the Tibet power grid into the Northwest Power Grid through the Qinghai‐Tibet DC interconnection project, all China's power grids were interconnected (except for those in Taiwan). Currently, SGCC is speeding up the construction of UHV AC/DC hybrid power grids. Up until 2013, China formed five regional synchronous power grids: North China‐Central China, East China, Northeast China, Northwest China, and South China. Asynchronous interconnection of these power grids was achieved through HVDC and UHVDC.

The proposal and development of VFT technology is the progress of the world's power grid interconnection technology and provides a new choice for China's power grid interconnection. On the one hand, in asynchronous power grid interconnection, with its unique advantages, the VFT is more suitable for marginal interconnection between asynchronous networks. Particularly, it can help solve problems of weak or isolated power grids as well as wind‐farm integration. On the other hand, with the higher voltage level of an AC power grid, an electromagnetic loop caused by the various voltage transmission lines' parallel operation has become increasingly prominent, which leads to large‐scale power flow transfer and affects voltage stability. However, simply disconnecting low voltage AC circuits will bring about wasteful investment, power supply stability problems, decline in economic efficiency, and other problems. On the base of ensuring a power grid's safety, a VFT can achieve controllable interconnection and power exchange between adjacent power grids by utilizing existing AC power transmission lines to improve reliability and economic efficiency of the power supply.

Research into VFT technology is quite necessary and challenging. In April 2004, GE's expert Dr. Hamid Elahi came to the China Electric Power Research Institute (CEPRI) for a technical exchange and displayed the company's newly invented VFT technology for the first time. In order to learn this new technology, I encouraged and guided doctoral student Gesong Chen to take the VFT as a research direction in preparing his dissertation and conduct systematic research on it. In the following 6 years, with the support and help of relevant experts and peers, he overcame practical difficulties such as lack of data and no ready‐made simulation tools. Through in‐depth theoretical research and simulation, he gradually mastered the basic theory of VFT; derived VFT calculation models covering steady state, electromechanical transients, and electromagnetic transients of electric power systems; acquired the ability to use mature power system software packages such as PSASP, EMTPE, and PSCAD/EMTDC to carry out all‐digital simulations of VFTs; set up VFT control systems applicable to different applications; and conducted plenty of simulation research by using simplified power systems, typical multi‐machine systems, and actual large power grid systems, based on which, the dissertation entitled Research on Modeling, Simulation and System Control of the Variable Frequency Transformer was completed in 2010, and also constitutes the basic content of this book.

At present, China is speeding up the construction of information‐based, automated, and interactive smart power grids with UHV as the backbone network and characterized by the coordinated development of power grids at all voltage levels. Various new technologies, devices, and materials will be more widely and effectively used in smart grids. As a new power grid interconnection technology, the VFT shows promising development and application prospects in China. The systematic introduction of the technical features, research methods, and simulation tools of the VFT is of great significance to promoting its development and application and advancing power grid technical levels in China. For this purpose, based on previous research as well as the basic trends in current energy reform, electric power development, and modern grid technology innovations, we have further enriched and improved VFT theories and application research, reinforced its theorization and readability, and revealed the general law of modern power grid development, thereby producing this book for your reference.

As China's first professional book focusing on systematic research of VFTs, this book has integrated research results accomplished over 8 years and received the support from experts and friends in many fields. In the research process, Mr. R.J. Piwko, Mr. Einar Larsen, and Dr. Hamid Elahi from GE; Professor Yuan Rongxiang from Wuhan University; Engineer Chen Ying from the Sichuan Electric Power Company Dispatching Center; Dr. Song Ruihua from CEPRI; and Dr. Wu Xuan, director of the research office, SGCC; as well as family members of the authors offered great support and help. The research results in the reference documents cited in this book also laid a good foundation for our VFT research and the drafting of this book.

In 2012, this book won the financial support of the electric power scientific works publication program of the SGCC. Through the joint efforts of all parties, Large Electric Power System Grid Interconnection New Technology – Theory and Application of Variable Frequency Transformers was finally published and released. I would like to take this opportunity to extend our heartfelt thanks to all peers and friends for their support and hope this book will play a positive role in spreading the basic knowledge about variable frequency transformers and promoting China's electrical power technology development.

Despite repeated checks and modifications, it is inevitable that this book still has some inadequacies. We sincerely invite readers to criticize and correct them.

June 18, 2013

Xiaoxin Zhou

Preface

Electricity is an important substance in modern society. The rapid growth in electricity demand advances higher requirements for power grid development. The essence of ensuring sustainable electricity supply is scientific and technological innovation. By reviewing the world's power grid development in the past 100 years, we find it is a history of constant innovation, reform, and improvement. Research shows that the interconnection of large power grids can effectively improve the reliability of power supply, reduce spare capacity, advance economic operation of power system, and further achieve great benefits such as peak load shifting, hydro and thermal power mutual support, as well as inter‐basin complementation, which is also the trend for the world's power grid development. The interconnection of traditional power grids includes synchronous AC and asynchronous DC. VFT is a new type of asynchronous grid interconnection device. Compared with back‐to‐back DC, it has obvious advantages such as less land occupation, lower transmission loss, and no harmonics. The world's first VFT, with a capacity of 105 MVA, was developed by GE and put into operation at the Langlois Substation, Quebec, Canada in October 2003. Years of practical operation proves the excellent performance of VFTs and effectively promotes their popularity and application. By 2010, a total of seven sets of VFTs had been put into commercial operation in North