Table of Contents

Chapter 1 Overview of HVDC Transmission System 1
1.1 Overview of the Development of HVDC Transmission Technology.1
1.2 Technical Characteristics of HVDC Transmission 2
1.2.1 Advantages of HVDC Transmission 3
1.2.2 Disadvantages of HVDC Transmission 4
1.3 Structure of HVDC Transmission System.6
1.3.1 Two-Terminal HVDC Transmission 7
1.3.2 Monopolar DC Transmission 9
1.3.3 Bipolar System 11
1.3.4 Back-to-Back DC System 14
1.3.5 Multi-Terminal DC System 16
Chapter 2 Thyristor-Based Conversion and Configuration of Converter Station.20
2.1 Thyristor 20
2.1.1 Thyristor Structure 20
2.1.2 Dynamic Characteristics of Thyristor 21
2.1.3 Trigger of Thyristor 23
2.1.4 Thyristors in Series 24
2.2 Thyristor Commutation Technology 26
2.2.1 Line-Commutated Converters 26
2.2.2 Analysis of Six-Pulse Rectifier 27
2.2.3 Analysis of Six-Pulse Inverter 31
2.2.4 DC Voltage of Six-Pulse Converter 33
2.2.5 DC Voltage of Twelve-Pulse Converter40
2.3 Composition and Arrangement of Converter Station 45
2.4 Equipment in Converter Station.49
2.4.1 Converter Valve 49
2.4.2 Converter Transformer 52
2.4.3 Smoothing Reactor 56
2.4.4 AC Filter 59
2.4.5 DC Filter 61
2.4.6 Reactive Power Compensation 63
2.4.7 Other Equipments 65
2.5 Main Connection of Converter Station 66
2.5.1 Wiring of Converters 66
2.5.2 Connection Between the Converter Transformer and the Converter Valves 67
2.5.3 AC Filter Access System 69
2.5.4 DC Switchyard Wiring 71
2.6 Converter Control Mode and Configuration 73
2.6.1 Converter Control Mode73
2.6.2 Converter Control Configuration 76
2.6.3 Controller Model of Converter 81
Chapter 3 Operating Characteristics and Fault Analysis of DC System 88
3.1 Steady-State Operating Characteristics of DC System 88
3.1.1 Operating External Characteristics of DC System88
3.1.2 Power Characteristics of Converter 94
3.1.3 Harmonic Characteristics of Converter.101
3.2 Reactive Power Balance in DC System 110
3.3 Operation Mode of DC Transmission System 113
3.3.1 Operation Wiring Mode 114
3.3.2 Full-Voltage Operation and Reduced-Voltage Operation 119
3.3.3 Power Forward and Reverse Transmission Mode 120
3.3.4 Bipolar Symmetry and Asymmetry Operation Mode 120
3.4 Failure Analysis of DC Transmission System 124
3.4.1 Converter Failure 124
3.4.2 Fault on DC Side of Converter Station 136
3.4.3 Fault on AC Side of Converter Station.137
3.4.4 HVDC Line Fault and Grounding Electrode Fault 146
Chapter 4 Temporary and Switching Overvoltages in HVDC Converter Station 149
4.1 Overvoltage from AC Side of the Converter Station 149
4.1.1 Ground Fault on AC Side151
4.1.2 Clearing of Ground Fault on AC Side.154
4.1.3 Switching of Converter Transformer 157
4.1.4 Operating Lines on AC Side 161
4.1.5 Operating Filters at Converter Station 164
4.1.6 Overvoltage at Load Rejection 165
4.2 Overvoltage from DC Side of the Converter Station 167
4.2.1 Commissioning or Decommissioning of Twelve Pulse Converter 167
4.2.2 Short Circuit to Ground on Valve Side of Converter Transformer 168
4.2.3 Overvoltage Generated on the Converter Valve Bridge 174
4.2.4 Ground Fault on DC Side Line.176
4.3 Losing of AC Power on the Inverter Side 180
4.4 Commutation Failure 182
4.4.1 Analysis of Commutation Failure Process 182
4.4.2 Reactive Power Fluctuation Caused by Commutation Failure 185
4.4.3 Transient Overvoltage Characteristics Caused by Commutation Failure191
4.4.4 Influence of Control System Parameters on Overvoltage Caused by Commutation Failure 193
4.5 Fault on Neutral Bus204
4.6 Open Circuit Fault on Grounding Electrode Lead 206
Chapter 5 Lightning and Steep Front Overvoltage in HVDC Converter Station 208
5.1 Lightning Simulation.208
5.1.1 Types of Lightning Stroke 208
5.1.2 Lightning Current 209
5.1.3 Lightning Stroke Density and Point 212
5.2 Lightning Overvoltage on AC Side 212
5.2.1 Lightning Overvoltage due to Shielding Failure 213
5.2.2 Lightning Overvoltage of Return Stroke 214
5.2.3 Induced Lightning Overvoltage.218
5.2.4 Lightning Intrusive Surge.219
5.3 Shielding Failure in AC and DC Switchyards 220
5.4 Lightning Overvoltage on Line of DC Side 224
5.4.1 Computation Models224
5.4.2 Lightning Overvoltage of Shielding Failure on Line of DC Side.228
5.4.3 Return Stroke Overvoltage on Line on the DC Side 231
5.5 Steep Front Surge.234
5.5.1 Short Circuit or Flashover to Earth235
5.5.2 All Turned-on of Converter Valves and Mis-operation of Bypass Pair in Part of Converters.237
5.6 Simulation Model and Research Method of Overvoltage 238
5.6.1 System Simulation Model 238
5.6.2 Research Methods for Determination of Overvoltage 242
Chapter 6 Insulation Coordination Method for

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Chapter 1 Overview of HVDC Transmission System
1.1 Overview of the Development of HVDC Transmission Technology High voltage direct current (HVDC) is a way of power transmission developed in the 1950s[1]. Since the 1960s, the development of thyristor rectifier has opened up a new approach for the manufacture of commutation devices, bringing new prospects to HVDC. From the 1980s, HVDC transmission has gained its jump development in the world’s power grid with the development of high-power siliconcontrolled thyristor and microprocessor control technology. As a new transmission technology, HVDC has many advantages comparing to high voltage alternating current (HVAC) transmission, including asynchronous interconnection between the different AC systems with different or the same nominal frequencies, smaller power consumption of the line, less harm to the environment as well as higher self-protection ability with line faults. Also, HVDC is especially suitable for interconnection between large-area power grids, particularly for large-capacity and long-distance transmission with high-voltage.
In 1954, the world’s first HVDC transmission based on mercury arc valves was put into commercial operation in Sweden. But then, mercury arc valves were replaced by silicon-controlled thyristor with the development of high-power electronics technology, after the application of silicon-controlled thyristor in a back-to-back HVDC project constructed Eel River Basin in Canada. The structure and characteristics of HVDC converters have remained unaltered for more than half a century. In 1979, the first HVDC transmission system based on microprocessor control technology was put into operation, promoting the development of HVDC transmission toward higher voltage and larger capacity. In 1984, Itaipu hydropower station in Brazil was built with HVDC transmission with the highest voltage level up to ±600kV at that time. Up to now, there are more than 100 DC transmission projects in the world, in which there are 14 DC projects in the United States with a total capacity (including 8 back-toback projects) of 10.8GW and the transmission distance is 5803km, 10 projects in Canada with a total capacity of 8.1GW and the transmission distance of 2814km.
In China, HVDC transmission started late. The first ultra-HVDC transmission project (Gezhouba to Shanghai) officially started in 1986, but developed rapidly. In the 1990s, China began to build Guizhou Tianshengqiao to Guangdong (referred to as Tian-Guang) and Three Gorges to Changzhou (referred to as San-Chang) ±500kV DC transmission project. Based on the construction of Tian-Guang, San-Chang as well as the Three Gorges to Guangdong (referred to as San-Guang) and Guizhou to Guangdong (referred to as Gui-Guang) ±500kV DC transmission projects started in 2001, China has made great progress in the construction of HVDC transmission, including system design, equipment manufacturing, engineering construction and commissioning. In recent years, HVDC transmission has developed further rapidly in China. Up to now, there are 35 HVDC transmission projects in or under construction, in which there are 15 ultra-high voltage direct current (UHVDC) transmission projects and 3 back-to-back projects, which will provide an important transmission route for China’s national networking, west-to-east transmission, and north-south mutual supply in the future[2]. In January 2016, China began to build UHVDC project with a 1100kV rated voltage, a 3300km length and a 12GW rated power, setting world records for highest voltage, longest distance and largest transmission capacity.
1.2 Technical Characteristics of HVDC Transmission
The original motivation for the development of DC technology was to improve efficiency by decreasing the power loss along transmission lines, which requires the energy conversion from AC to DC. However, this required the use of HVDC and depended on the development of conversion switches capable of withstanding high voltage and large current. Therefore, the most technology difference between DC and AC transmission is embodied in the converter or converter valve for energy conversion. The technical development of the converter valves for HVDC transmission experienced the process from mercury arc valves to silicon-controlled thyristor, from electronic controlled to light-controlled thyristor, and in recent years the development of fully-controlled power semiconductors with improved characteristics such as insulated gate bipolar transistor (IGBT), integrated gate commutated thyristor (IGCT), which has provided the basis for flexible AC transmission system (FACTS) technology and the flexible HVDC transmission. Substantial progress in the ratings and reliability of thyristor valves has changed the attitude towards HVDC transmission and increased the competitiveness of HVDC schemes, which promotes the further development in commutation technology, including a variety of converter