Protection Principle and Technology of the VSC-Based DC Grid

By Bin Li and Jiawei He

This book discusses key techniques of protection and fault ride-through in VSC-HVDC grids, including high-speed selective protection, DC fault current limitation, converter restarting, and DCCB reclosing strategies. It investigates how high-speed transient-variable-based protection can be used to improve grids’ acting sensitivity, acting reliability, and ability to withstand high transition resistance compared with traditional protection. In addition, it discusses the applicability of the pilot protections, including the current differential protection and travelign-wave based protection, in the dc grid, as well as the improved methods. Furthermore, it proposes several DC FCL topologies, which are suitable for DC grids. Lastly, in the context of overhead line application conditions, it explores converter restarting and DCCB reclosing strategies, which not only identify the fault property, but also limit the secondary damage to the system, improving the system’s operation security and reliability. As such, the book offers a comprehensive overview of original and advanced methods and techniques for the protection of VSC-HVDC grids.

• Language: English
• Publisher: Springer
• Release date: Aug 13, 2020
• ISBN: 9789811566448

Bin Li

Today, Dr. Li’s research is focused on statistics and machine learning. He has published >75 peer reviewed research papers with >1,300 citations of his work.

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© Springer Nature Singapore Pte Ltd. 2020

B. Li, J. HeProtection Principle and Technology of the VSC-Based DC GridPower Systemshttps://doi.org/10.1007/978-981-15-6644-8_1

1. Introduction

Bin Li¹   and Jiawei He¹  

(1)

School of Electrical and Information Engineering, Tianjin University, Tianjin, China

Bin Li (Corresponding author)

Email: [e-mail]

Jiawei He

Email: [e-mail]

1.1 Development of the HVDC Transmission Technology

When the electric power was firstly applied, it was transferred based on the direct current. However, the early dc generator at the sending end and the dc motor at the receiving end are directly connected in series, so the reliability is very poor. In addition, it is difficult to transform the voltage in dc systems, and thus cannot realize the long-distance power transmission. At the end of the 19th century, the three-phase ac generator, induction motor and ac transformer were proposed successively. In view of the obvious advantages in the fields of power generation, transmission, distribution and power consumption, as well as the voltage transformation, the ac transmission and ac power system quickly occupied the dominant position in the electric power system.

In the middle and late 20th century, the dc transmission technique began to attract high attention, with the rapid development of the high-voltage and large-capacity converter technology. The dc transmission technique has outstanding application prospect in the fields of long-distance and large-capacity power transmission, power grids interconnection, submarine cable transmission, and so on, because of the advantages including low transmission loss, large transmission capacity, no frequency stability problem, et al. [1]. However, at present, the power generation and consumption in the power system are mostly based on the alternating current. In order to apply the dc transmission, the power conversion technique must be researched. Therefore, the development of the dc transmission technology is highly related to the development of the converter technology, especially the high-voltage and large-capacity converter. While the main driving force of the converter technology innovation is the revolutionary breakthrough of the power electronic switches.

In 1954, the world’s first industrial HVDC project (from Sweden to Gotland Island) was put into commercial operation. Up to 1977, there were 12 dc projects, using the mercury arc valve technique, being put into operation in the world. This period is also known as the mercury-conversion-valve period. However, the mercury arc valve has some drawbacks, such as complex manufacturing technology, expensive price, high fault probability of the reverse arc, which limit the development of the dc transmission technology. In the 1970s, the operation performance and reliability of the dc transmission system are improved significantly, with the development of the high-voltage and large-capacity thyristors. In 1970, Sweden first built a 10 MW/50 kV thyristor-based converter valve experimental project on Gotland Island. In 1972, Canada built the world’s first dc transmission project completely using the thyristor-based converters. Due to its obvious technical advantages, the thyristor-based converter quickly replaced the mercury-arc-valve based converter, and the dc transmission technology came into a fast development period. In China, the HVDC transmission technique was also researched and applied widely. The advantages of the HVDC transmission technique, on the long-distance and large-capacity power transmission, interconnection of the power systems and so on, are verified. However, the thyristor does not have the self-turned-off capability, so the thyristor-based converter has a great dependence on the ac-side system, which becomes the main constraint. This is also the reason why the HVDC transmission system is also named as the line commutated converter based HVDC (LCC-HVDC).

In the 1990s, a new type of full-controlled semiconductor device, namely, the insulated gate bipolar transistor (IGBT), began to be used in the dc transmission area [2, 3]. With the development of the high-voltage IGBT, it is possible to use the full-controlled switches to form the voltage source converter (VSC) for dc transmission and distribution. In 1997, the HVDC project based on the voltage source converter—Hershey experimental project was put into operation. The International Council on Large Electric systems (CIGRE) and the Institute of Electrical and Electronics Engineers (IEEE) named this new HVDC technology as the VSC-based HVDC (VSC-HVDC) technology [4]. In China, this kind of HVDC is named as the flexible HVDC. The on and off states of the full-controlled power electronic switch can be controlled completely, thus can overcome the essential drawbacks of the LCC-HVDC system (mainly referring to the commutation failure).

The control of the two-level VSC and three-level VSC is very simple, but they have the drawbacks including high harmonic content, large switching loss and so on. Meanwhile, the voltage tolerance and current tolerance of the IGBT are not very high, thus being difficult to be applied in the high-voltage and large-capacity power transmission area. At the beginning of the 21st century, the modular multilevel converter (MMC) topology was proposed, which significantly improved the operation efficiency of the VSC-HVDC transmission system, and promoted the development and application of the VSC-HVDC transmission technology. In 2010, the MMC-based HVDC transmission project—Trans Bay Cable project, was put into operation in America [5]. Since then, the theoretical research and engineering application based on the MMC technique have been developed rapidly. In 2011, the Shanghai Nanhui ±30 kV MMC-based dc project in China was put into operation. After that, a number of VSC-HVDC transmission projects have been successfully put into operation in China. At the same time, the development and application of modular multilevel structure in the field of dc transformer will further promote the development of the VSC-based dc transmission technology.

1.2 Outline of the HVDC Transmission Technology

1.2.1 Technical Superiority of the HVDC Transmission System

The HVDC transmission system mainly consists of the rectifier station, inverter station and dc transmission line. The topology of the 12-pulse LCC-HVDC transmission system is shown as Fig. 1.1. At the power transmission side, the ac power is rectified to the dc power, and the station realizing the power rectification is named as the rectifier station. At the power receiving side, the dc power is converted to ac power for the ac load, and the station realizing the power inversion is named as the inverter station.

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Fig. 1.1

The typical LCC-HVDC system topology

In the LCC-HVDC system, the thyristors are used to compose the converter. As we know, the thyristor has the unidirectional conducting characteristic, which means the current can only flow from the thyristor anode to the cathode. Therefore, the current in the LCC-HVDC system can only flow from the rectifier side to the inverter side.

In the dc system, the dc voltage at the rectifier side should be a little larger than the inverter side, to guarantee the power transmission on the dc line, i.e.,

$$ I_{{_{\text{d}} }} { = }\frac{{U_{\text{dR}} - U_{\text{dI}} }}{{R_{L} }} $$

(1.1)

where the subscript R represents the rectifier side, and the subscript I represents the inverter side. UdR is the dc voltage at the rectifier side, and UdI is the dc voltage at the inverter side. In addition, RL is the equivalent resistance of the dc line.

Obviously, the transmission powers of the ac transmission system and dc transmission system can be respectively expressed as

$$ P_{\text{ac}} = \sqrt 3 V_{\text{N}} I_{\text{ac}} \cos \varphi = 3V_{{\varphi ,{\text{N}}}} I_{\text{ac}} \cos \varphi $$

(1.2)

$$ P_{\text{dc}} = V_{\text{dc}} I_{\text{dc}} = 2V_{\text{d}} I_{\text{dc}} $$

(1.3)

where Pac and Pdc are the transmission powers of the ac transmission system and dc transmission system respectively. VN is the rated line voltage of the ac transmission system, Vφ,N the rated phase voltage, Iac the rated current, and cosφ is the power factor. Vdc is the dc pole-to-pole voltage of the dc transmission system, Vd the pole-to-ground voltage, and Idc is the dc line current.

Supposing that the cross-sectional area and the insulation level of the ac transmission line and the dc transmission line are the same, it can be recognized that: (1) The current RMS on the ac line is the same as that on the dc line, namely, Idc = Iac. (2) The voltage tolerance of the ac line is the same as that of the dc line, namely, Vd = √2 · Vφ,N. Therefore, it can be obtained that

$$ \frac{{P_{\text{dc}} }}{{P_{\text{ac}} }} = \frac{{2V_{\text{d}} I_{\text{dc}} }}{{3V_{{\varphi , {\text{N}}}} I_{\text{ac}} { \cos }\varphi }} = \frac{2\sqrt 2 }{{3{ \cos }\varphi }} $$

(1.4)

In the ac transmission system, the power factor cosφ is generally close to 1. When cosφ = 2√2/3 ≈ 0.9428, Pdc/Pac = 1. This means the transmission power of the dc line with two wires (Pdc) is the same as that of the ac line with three wires (Pac).

From the aspect of the construction investment, the metal wire and insulating material required for unit length of dc line is one third less than that of the ac line. Moreover, in the case of overhead line application scene, the required corridor of the dc line can also be narrower, due to the smaller load of dc line tower.

From the aspect of the operation cost, the transmission loss of the dc line is one third less than that of the ac line, because the wires number of dc line is one third less. In addition, the ac wire has larger power loss than that of the dc wire, due to the ac current skin effect. Moreover, the distributed capacitor current of the ac line is also much larger than that of the dc line, which leads to larger power loss.

In addition, compared with the ac transmission, the dc transmission has some other advantages, including higher operation stability, interconnection of the asynchronous grids, and so on.

1.2.2 The Typical HVDC Transmission Types

1.2.2.1 The LCC-HVDC

Generally, the HVDC system based on thyristor-based converter is named as the traditional HVDC system, which also named as the line commutated converter HVDC (LCC-HVDC) system. The LCC-HVDC system has the advantages including long transmission distance, large transmission capacity, low transmission power loss, interconnection of the asynchronous grids, and so on. At present, the LCC-HVDC technology is very mature, and lots of practical projects have been put into operation around the world.

However, the LCC-HVDC system also has some technical disadvantages. For example, the normal operation of the thyristor needs the ac power grid to provide commutation voltage. So the commutation failure will occur when it is connected with a weak ac power grid. In addition, the thyristor-based converter requires large reactive power, which means a large number of filters and capacitors should be installed at the ac side. Obviously, it increases the investment of the converter station, and may lead the ac bus voltage to increase. Moreover, during the power flow reverse, the dc voltage polarity should be reversed. Under this condition, the charging and discharging problem of the transmission line cannot be neglected.

1.2.2.2 The VSC-HVDC

With the rapid development of the power semiconductor devices, such as the large-capacity IGBT, the pulse width modulation (PWM) and multi-level control technologies begin to be used in the HVDC transmission area, promoting the application of the VSC-HVDC transmission system. The main difference between the VSC-HVDC and the LCC-HVDC is the converter station. At present, the typical VSC topologies mainly include the two-level VSC and the MMC, as shown in Fig. 1.2.

../images/490413_1_En_1_Chapter/490413_1_En_1_Fig2_HTML.png

Fig. 1.2

The typical VSC topologies

The VSC uses the full-controlled IGBT switch, which avoids the commutation failure problem of the LCC-HVDC system. In addition, the VSC can control the active power and reactive power respectively, and can also operate in the STATCOM mode, to provide the reactive power for ac system. During the power flow reverse, the VSC-based dc system only needs to change the current direction, without changing the voltage polarity. Moreover, the filters and reactive compensation equipments can be reduced significantly in the VSC station, because the waveform quality of the VSC is much better than the LCC, and the VSC itself can control the reactive power.

The VSC-HVDC also has some drawbacks. For example, the operation power loss of the VSC is larger than the LCC. The fault damage in the VSC-HVDC system is much more serious than that in the LCC-HVDC system, and the VSC itself does not have the dc fault handling capability. The technical characteristics of the LCC-HVDC system and the VSC-HVDC system are listed in Table 1.1.

Table 1.1

The technical characteristics of the LCC-HVDC and VSC-HVDC

The MMC topology is shown as Fig. 1.2b. It uses the step wave to approach the sine wave during voltage modulation. When the voltage level number is large enough, the converter output ac voltage can be very close to the sine wave, thus reducing the harmonic component effectively. Compared with the two-level VSC or the three-level VSC, the switching loss of the IGBT can be significantly reduced when the step wave modulation is used. The modular design of the MMC avoids the IGBT series-connection problems, thus can be used in the higher voltage system. In addition, the capacitors in the MMC are distributedly installed in each sub-module, and there will be arm reactors in the converter, which can limit the dc fault current to a certain extent. But the MMC requires more IGBTs, resulting in higher investment. The topology and control are also more complex, which leads to new problems, such as the sub-module capacitor voltages balance control and the circulation control.

1.3 Development of the VSC-HVDC Grid

1.3.1 The Forms of the VSC-Based DC System

1.3.1.1 Multi-terminal HVDC Transmission System

In general, most of the HVDC projects are point-to-point, to realize the long-distance and large-capacity power transmission. In recent years, the multi-terminal HVDC transmission system has developed rapidly. The multi-terminal HVDC transmission system generally has more than three converter stations, which connect with each other in series, in parallel or in hybrid mode. This kind of transmission system can realize the multi-source power supply and multi-terminal power receiving.

At present, some multi-terminal HVDC transmission system projects have been put into operation, such as the Italy—Corsica—Sardinia three-terminal DC system, Canada Quebec—New England five-terminal DC system, Japan Shinano three-terminal back-to-back DC system and Canada Nelson River four-terminal DC system, etc. In China, the 1000 MW, ±200 kV, Zhoushan five-terminal flexible DC transmission project, as shown in Fig. 1.3, was put into operation in 2014 [6].

../images/490413_1_En_1_Chapter/490413_1_En_1_Fig3_HTML.png

Fig. 1.3

Zhoushan five-terminal flexible DC transmission project

1.3.1.2 DC Grid

With the development of the multi-terminal HVDC system, the converter station terminals number increases significantly, and the interconnection between different terminals through the dc lines becomes much closer, thus coming into a new dc system form, which is named as the dc grid. Compared with the multi-terminal dc system, the dc grid has lower investment, more operation states, and higher operation reliability.

In Europe, the Super Grid concept was proposed, which is based on the dc grid. It connects the large-scale renewable energy power in remote areas and transfers it to the power consumption centers. In America, the dc transmission backbone-grid is proposed, to realize the large-scale interconnection of power grids. In China, the ±500 kV Zhangbei DC grid project, as shown in Fig. 1.4, is under construction [7]. In this project, three sending converter stations are built in Zhangbei, Kangbao and Fengning respectively. The capacity of Zhangbei station is 3000 MW, while those of Kangbao station and Fengning station are both 1500 MW. The 3000 MW receiving station is built in Beijing. In addition, the dc circuit breaker (DCCB) will be installed at each line terminal, cooperating with the high-speed selective dc protection. Obviously, the dc grid has become the development trend of the HVDC technology, and has attracted great attention around the world, both in the theoretical research and practical application.

../images/490413_1_En_1_Chapter/490413_1_En_1_Fig4_HTML.png

Fig. 1.4

The Zhangbei DC grid project

1.3.1.3 The Development of the VSC-HVDC System in China

In recent years, the theoretical research and engineering application of the VSC-HVDC technique have fast developed in China, as shown in Fig. 1.5. In 2011, the first VSC-HVDC transmission project in China was put into operation in Nanhui, Shanghai. This is a ±30 kV, 18 MW, two-terminal system, which effectively integrates the Nanhui wind farm into the Shanghai Power Grid. In 2013, the ±160 kV, 200 MW three-terminal VSC-HVDC transmission system was put into operation in Nan’ao Guangdong. It is the first multi-terminal VSC-HVDC system in the world, which realizes the integration and transmission of multiple wind farms. In 2014, the ±200 kV, 1000 MW five-terminal VSC-HVDC system was put into operation in Zhoushan, Zhejiang, which has the largest number of converter terminals (until 2014). From 2016 to 2019, several VSC-HVDC projects were put into operation respectively. The voltage and capacity were increased significantly. At present, the ±800 kV, 8000 MW Wudongde hybrid three-terminal system, and the ±500 kV, 9000 MW Zhangbei DC grid are under construction, which indicate that the voltage level and transmission capacity of the VSC-HVDC system become larger, meanwhile, the system topology and the operation mode become more complex.

../images/490413_1_En_1_Chapter/490413_1_En_1_Fig5_HTML.png

Fig. 1.5

The development of the VSC-HVDC transmission systems in China

1.3.2 The Challenge of the DC Fault Protection in DC Grid

It should be pointed out that, there are still some technical difficulties, which limit the rapid development of the dc grid. And it is universally acknowledged that, the protection and handling of the dc fault are the key techniques for operation security and power supply reliability of the dc grid.

(1)

The damage of dc fault is great: In dc grid, a dc fault at any position may lead to a large-scale power outage. In most of the existing VSC-based dc grids, the dc cable is used to transmit the power, to reduce the fault probability. However, with the increasing of the voltage level and system capacity, it is obvious that the overhead line will be widely applied. How to deal with the dc fault becomes one of the key problems in the dc grid, because the fault probability of the overhead line is much higher than that of the dc cable.

(2)

The challenge of the protection in dc grid: The fault propagation speed in VSC-based dc grid is very high, thus requiring the dc protection can detect the fault and distinguish the fault line quickly in several milliseconds. Obviously, the traditional protections in ac system and LCC-HVDC system cannot satisfy the requirement of the VSC-based dc grid. For example, the distance protection and overcurrent protection in ac system cannot identify the fault line with selectivity. The traveling-wave protection in LCC-HVDC system does not have the capability against high transition resistance. For using in the VSC-based dc grid, this problem must be considered, because of the higher requirement on the power supply reliability.

(3)

The challenge of the dc isolation: The lack of natural-zero-crossing point of the dc fault current makes it difficult to be isolated. How to quickly isolate the fault line is the key technology for fault ride-through of the dc grid. At present, the dc fault isolating methods mainly include the converter self-eliminating technique and the DCCB technique. The typical half-bridge MMC cannot eliminate the dc fault current by itself. Therefore, some improved topologies have been proposed, which can eliminate the dc fault current by inserting the reverse capacitors voltage. This kind of isolating method is generally named as the converter self-eliminating technique. In dc grid, it is most ideal to isolate the fault line with selectivity by the DCCB. So the DCCB technology becomes one of the key techniques in the VSC-based dc grid.

(4)

The fault ride-through of the dc grid: After the dc fault, the fault line will be distinguished by the dc protection and then cut off by the DCCB. During the dc fault handling period, the whole system suffers the damage of the dc fault, including the healthy network and fault part. The fault ride-through of the dc grid mainly refers to that the healthy network in the dc grid can operate continuously during the fault handling period. However, the dc fault propagation speed is very high. For the fault ride-through of the healthy network, it even may require the dc fault line can be cut off in hundreds of microseconds. Obviously, the existing dc protection and dc fault isolation speed still cannot match the dc fault propagation speed.

The dc fault current limiting method can limit the dc fault current effectively, thus reducing the requirement on the operating speeds of the dc protection and dc fault isolation, and guaranteeing the fault ride-through of the dc grid. Therefore, the dc fault current limiting technique becomes the key technique for the fault ride-through of the dc grid.

In summary, the dc fault protection and fault handling are the key technologies in the VSC-based dc grid. In this book, the core techniques of the dc protection and fault handling are researched, mainly including the dc fault analysis, dc protection, dc fault current limitation, dc fault isolation, and dc fault recovery.

References

1.

Kalair, A., Abas, N., & Khan, N. (2016). Comparative study of HVAC and HVDC transmission systems. Renewable and Sustainable Energy Reviews,59, 1653–1675.Crossref

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Ooi, B. T., & Wang, X. (1991). Boost-type PWM HVDC transmission system. IEEE Transactions on Power Delivery,6(4), 1557–1563.Crossref

3.

Ooi, B. T., & Wang, X. (2002). Voltage angle lock loop control of the boost type PWM converter for HVDC application. IEEE Transactions on Power Electronics,5(2), 229–235.Crossref

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Asplund, G., Eriksson, K., Jiang, H., et al. (1998). DC transmission based on voltage source converters. In Proceedings of 37th Sessions, Paris.

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Teeuwsen, S. P. (2011). Modeling the trans bay cable project as voltage-sourced converter with modular multilevel converter design. In IEEE Power and Energy Society General Meeting, Detroit.

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Tang, G., He, Z., Pang, H., et al. (2015, June). Basic topology and key devices of the five-terminal DC grid. CSEE Journal of Power and Energy Systems,1(2), 22–35.

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Tang, G., Pang, H., He, Z., & Wei, X. (2018). Research on key technology and equipment for Zhangbei 500 kV DC grid. In Proceedings of the IPEC, Niigata, Japan (pp. 2343–2351).

© Springer Nature Singapore Pte Ltd. 2020

B. Li, J. HeProtection Principle and Technology of the VSC-Based DC GridPower Systemshttps://doi.org/10.1007/978-981-15-6644-8_2

2. Working Principle and Basic Control Strategy of the VSC-HVDC Grid

Bin Li¹   and Jiawei He¹  

(1)

School of Electrical and Information Engineering, Tianjin University, Tianjin, China

Bin Li (Corresponding author)

Email: [e-mail]

Jiawei He

Email: [e-mail]

In the VSC-HVDC grid, the converter is the core equipment for energy conversion and control between ac side and dc side. The voltage source converter (VSC) based on the full-controlled power electronic switches has varied topologies and control strategies. According to the used modulation principles, the VSC type mainly includes the PWM based VSC (two-level VSC and three-level VSC) and the modular multilevel converter (MMC).

2.1 Working Principle of the Two-Level VSC

2.1.1 Basic Topology of the Two-Level VSC

The three-phase two-level VSC realizes the AC/DC conversion based on the PWM principle [1]. And the basic topology of the two-level VSC is shown as Fig. 2.1.

../images/490413_1_En_2_Chapter/490413_1_En_2_Fig1_HTML.png