Fault Characteristics and Control Strategies of Multiterminal High Voltage Direct Current Transmission Based on Modular Multilevel Converter

Hindawi Publishing Corporation Mathematical Problems in Engineering Volume 2015, Article ID 502372, 11 pages http://dx.doi.org/10.1155/2015/502372 Research Article Fault Characteristics and Control Strategies of Multiterminal High Voltage Direct Current Transmission Based on Modular Multilevel Converter Fei Chang,1 Zhongping Yang,1 Yi Wang,2 Fei Lin,1 and Shihui Liu1 1 School of Electrical Engineering, Beijing Jiaotong University, Beijing 100044, China School of Electrical Engineering, Tsinghua University, Beijing 100084, China 2 Correspondence should be addressed to Fei Chang; Received 22 April 2015; Revised 26 May 2015; Accepted 27 May 2015 Academic Editor: Xiaosong Hu Copyright © 2015 Fei Chang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The modular multilevel converter (MMC) is an emerging voltage source converter topology suitable for multiterminal high voltage direct current transmission based on modular multilevel converter (MMC-MTDC). This paper presents fault characteristics of MMC-MTDC including submodule fault, DC line fault, and fault ride-through of wind farm integration. Meanwhile, the corresponding protection strategies are proposed. The correctness and effectiveness of the control strategies are verified by establishing a three-terminal MMC-MTDC system under the PSCAD/EMTDC electromagnetic transient simulation environment. 1. Introduction The rapid development of power electronic technology has promoted the development of sustainable transportation and power systems [1–5]. The modular multilevel converter (MMC) was first introduced in 2001 [6] and has drawn great attention due to its excellent output waveform and high efficiency [7, 8]. As a new topology of voltage sourced converter based high voltage direct current transmission (VSCHVDC), MMC-HVDC has prodigious potential in transmission and distribution applications, such as wind farm connection [9–13], multiterminal operation [14], and a passive network power supply [15]. Multiterminal HVDC transmission based on MMC (MMC-MTDC) is defined as the flexible HVDC transmission system which has three or more voltage source converters (VSCs) under the same DC grid [16]. Its prominent feature lies in providing multiple power supplies, power receiving in multiple places. As a more flexible and efficient power transmission mode, MMC-MTDC shows great potential in renewable energy connection, urban DC distribution network, and so on. In the world, there are only two MMC-MTDC projects and they are all in China [17]. One of which is Nanao threeterminal MMC-MTDC project constructed in Dec. 2013 which is the world’s first MMC-MTDC project; the other one is Zhoushan five-terminal MMC-MTDC project constructed in Jul. 2014 which is the world’s largest number of terminals in MMC-MTDC projects. At present, the research of MMC-MTDC is focused on DC voltage stability [17], which can be divided into two categories, including control with communication or no communication. The control with no communication is basically adopted in the actual project which includes DC voltage slope control and DC voltage deviation control. However, the related research on fault protection is also rarely reported [18], in which, a multipoint DC voltage control strategy based on DC voltage margin method is proposed. Furthermore, the impact of different DC faults of the system is analyzed and the corresponding control and protection strategies are given. This paper has been further research on fault characteristics and control strategies of MMC-MTDC, including submodule fault, DC line fault, and fault ride-through of wind farm integration. 2. MMC-MTDC System MMC-MTDC system is composed of three or more MMC converter stations and DC power transmission interconnection lines, as shown in Figure 1. Wherein, the structure of 2 Mathematical Problems in Engineering SM1 .. . SMn AC3 MMC3 SM1 1 .. . AC1 SM1 .. . 2 SMn SMn DC overhead line AC2 MMC2 MMC1 Figure 1: Structure of MMC-MTDC system. + SM1 SM1 SM1 SM2 SM2 SM2 .. . .. . .. . .. . SMn + .. . .. . SMn SMn Ls Ls Ls Ls Ls Ls Submodule ua ub uc .. . Udc SM1 SM1 SM1 SM2 SM2 SM2 .. . SMn .. . .. . SMn .. . 2.2. Mathematical Model of MMC. Considering the circumstances of bridge reactance, the simplified equivalent circuit of MMC is illustrated in (1), where 𝑢𝑠𝑎 , 𝑢𝑠𝑏 , and 𝑢𝑠𝑐 are the fundamental components of the three-phase voltage in AC side, respectively. 𝑖𝑠𝑎 , 𝑖𝑠𝑏 , and 𝑖𝑠𝑐 are the fundamental components of the three-phase current in AC side, separately. 𝐿 is the sum of bridges’ inductance which is in single-phase as well as leakage inductance of the converter transformer. 𝑅 is the equivalent resistance which consists of bridge reactor and converter transformer. 𝑢𝑎 , 𝑢𝑏 , and 𝑢𝑐 are the fundamental components of the three-phase voltage in converter side, respectively [19]: .. . SMn − Figure 2: Structure of MMC converter station. MMC converter station is shown in Figure 2. The system has the advantages of providing multiple power supplies, power receiving in multiple places, and linking several AC systems or separating one AC system into several independent grids. 2.1. Topology of MMC. The main circuit topology of a threephase MMC is shown in Figure 2; the basic circuit unit of MMC is known as submodule (SM). Each bridge arm is constructed by a certain number of submodules and an arm reactance 𝐿 in series. The MMC topology can change the output voltage and power level of converter in a flexible way, only by changing the number of submodules. As a consequence, the MMC topology has less switching losses and harmonic distortion. In addition, the MMC topology has positive and negative DC bus, which is especially suitable for HVDC applications. 𝐿 𝑑𝑖𝑠𝑎 + 𝑖𝑠𝑎 𝑅 = 𝑢𝑠𝑎 − 𝑢𝑎 , 𝑑𝑡 𝐿 𝑑𝑖𝑠𝑏 + 𝑖𝑠𝑏 𝑅 = 𝑢𝑠𝑏 − 𝑢𝑏 , 𝑑𝑡 𝐿 𝑑𝑖𝑠𝑐 + 𝑖𝑠𝑐 𝑅 = 𝑢𝑠𝑐 − 𝑢𝑐 . 𝑑𝑡 (1) 3. Submodule Fault Normally, the submodule fault occurs mainly due to overvoltage, overcurrent or excessive 𝑑V/𝑑𝑡, 𝑑𝑖/𝑑𝑡, or the control fault due to false triggering pulses. The system operation should not be influenced by one or several fault submodules, so the submodule needs fault redundancy protection to make the converter have the ability of fault tolerance and improve the reliability of the system. 3.1. Fault Characteristics. Taking phase 𝑎, for example, the upper and lower arms energy of MMC 𝑊𝑝𝑎 and 𝑊𝑛𝑎 can be expressed as [20] 1 2 𝑊𝑝𝑎 = 𝐶𝑁𝑢𝑐𝑝𝑎 2 =∫ 𝑇 0 𝑈dc 1 (1 − 𝑚 cos 𝜔𝑡) 𝑖𝑝𝑎 𝑑𝑡 + 𝐶𝑁𝑈𝐶2 , 2 2 1 2 𝑊𝑛𝑎 = 𝐶𝑁𝑢𝑐𝑛𝑎 2 𝑇 𝑈dc 1 (1 + 𝑚 cos 𝜔𝑡) 𝑖𝑛𝑎 𝑑𝑡 + 𝐶𝑁𝑈𝐶2 , 2 2 0 =∫ (2) Mathematical Problems in Engineering 3 1.2 25 1 20 0.8 idc1 (kA) Eda (kV) 30 15 10 5 Eda2n 0.4 0.2 0 0 −5 1.5 0.6 −0.2 2 2.5 3 −0.4 1.5 2 2.5 t (s) (a) 3 t (s) (b) Figure 3: Fault characteristics of submodule. (a) Capacitor volta (...truncated)