Virtual Group Control Algorithm for Modular Multilevel Converter

INTERNATIONAL JOURNAL of ENGINEERING SCIENCE AND APPLICATION Kim et al., Vol.3, No.1, 2019 Virtual Group Control Algorithm for Modular Multilevel Converter Chan-Ki Kim *, Soo-Yeon Sim **‡, Chang-Hwan Park ***, Jang-Mok Kim**** * Power Transmission Lab R&D Center of Korea Electric Power Co., KEPRI, DaeJeon, Korea ** Power Conversion Lab. Pusan National University, Busan, Korea *** Power Conversion Lab. Pusan National University, Busan, Korea **** Power Conversion Lab. Pusan National University, Busan, Korea (, , , ) ‡ Corresponding Author; Second Author, Postal address, Tel: +82 10 9228 4875, Fax: +82 42 865 5879, Received: 09.01.2019 Accepted:28.03.2019 Abstract- This paper deals with the group control method of MMC (Modular Multilevel Converter) HVDC system. The proposed scheme increases the number of SMs (Submodules) of MMC HVDC system to more than 500 (2GW class) without changing the sampling time. This method is called Cluster Stream Buffer (CSB) method, and the total SMs are divided into several clusters, and a cluster is composed of 32 SMs. It is easy for the Cluster Stream Buffer method to expand HVDC system to several GW Class and it is easy to keeping the system performances. Consequently, this method was validated using MATAB/Simulink program. Keywords MMC HVDC(Modular Multilevel Converter HVDC), CSB(Cluster Stream Buffer;Level Selection Method), NLC(Nearsst Level Control), SM(Submodule). 1. Introduction Recently, the installation of HVDC system is increasing due to the energy transit policy and the expansion of renewable energy around the world. Until now, HVDC system have been mainly composed of LCC (Line Commuted Converters) HVDC, but VSC (Voltage Source Converters) HVDC is mainly installed in the last 10 years, and 150[GW] is planned by 2030. Since LCC HVDC system is highly dependent on AC system, it causes overvoltage, harmonic instability, and the resonance between AC system and HVDC system, while VSC HVDC system is independent of the AC system. VSC HVDC system have several problems, that is it is difficult to make high power conversion equipment due to the limitation of the device, and the switching loss is too high because the PWM method is used. To solve these problems, a phase shift transformer is used as shown in Fig. 1, and MMC topologies are developed as shown in Fig. 2. Fig. 1. High power converter topology with phase shift transformer Fig. 2 shows the classification of the MMC topologies (Fig. 2 (a)) and switching algorithm (Fig. 2 (b)). In the VSC HVDC system, MMC and NLC (Nearest Level Control) are used as basic standards because it is excellent in all respects. The MMC topology proposed by R. Marquardt has the many advantages, high power capacity possibility and low harmonics and so on [1], [2]. INTERNATIONAL JOURNAL of ENGINEERING SCIENCE AND APPLICATION Kim et al., Vol.3, No.1, 2019 (a) Technical classification for high power converter (b) Switching topology of modular multilevel converter Fig. 2. Classification of high power converter and switching topology of MMC In addition, since the MMC method is that IGBT is connected in series, it requires an electrical bypass switch (thyristor) and mechanical bypass switch (PMA type (Permanent Magnetic Actuator), pyromatic type). The NLC (Nearest Level Control) method has been proposed to reduce switching loss within 1[%]. It is the simple and suitable for the NLC method to implement high of levels in the MMC HVDC system [3], [4]. As shown in Fig.3., ABB uses the method grouping submodules in hardware, this method controls a group module as SMs which is connected in series to compose a group module. This method is caused high-frequency harmonics and the unbalance between the IGBT devices, it is required 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 snubber circuit to reduce the and . 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 One of the main issues in the MMC HVDC system is to increase the capacity to 1~2[GW] with the currently developed IGBT (Max. 3.3[kV], 1,500[A]), as the DC voltage is 500[kV] and 500 IGBTs of SM are connected in series. It is required high performance of the controller hardware and high speed of the sampling time and high speed of the communication to turn on/off IGBTs in real time. In this paper, CSB (Cluster Stream Buffer) method is proposed, which is consist of 32 levels as a cluster and can overlap the clusters in series [5]. The CSB method can be increased over 1 [GW] without changing the sampling time. First, the basic characteristics of MMC is introduced and review the NLC method. On this base, the algorithm of the CSB method using fixed sampling time is dealt with and the feasibility on the proposed method is simulated by MATLAB /Simulink program. 2. Basic Structure and Operation Principle of MMC 2.1. Basic structure Fig. 3. Series connected module group control method (ABB’s method) 46 Fig. 4 (a) shows the basic structure of the MMC system. A three-phase MMC system is divided into six arms, and each arm contains N SMs and one inductor 𝐿𝐿0 . The upper and lower arm with in a single leg of MMC system comprise a phase unit. A half bridge SM circuit is shown on the right side of Fig. 4. The SM voltage, 𝑣𝑣𝑆𝑆𝑆𝑆 , is determined by the switching states of the upper and lower IGBTs. SM voltage becomes the capacitor voltage, 𝑣𝑣𝑐𝑐 . Whereas, if the upper and lower IGBTs are turnoff and turn-on respectively, the SM voltage become zero. This means a SM has two normal operation states: inserted (𝑣𝑣𝑆𝑆𝑆𝑆 = 𝑣𝑣𝑐𝑐 ) or bypassed (𝑣𝑣𝑆𝑆𝑆𝑆 = 0). INTERNATIONAL JOURNAL of ENGINEERING SCIENCE AND APPLICATION Kim et al., Vol.3, No.1, 2019 And is given by: 𝑖𝑖𝑗𝑗𝑗𝑗 = 𝑖𝑖𝑗𝑗𝑗𝑗 +𝑖𝑖𝑗𝑗𝑗𝑗 (3) 2 According to [6], the MMC HVDC system can be expressed by the following equations: 𝑅𝑅 𝐿𝐿 + 𝑅𝑅0 𝑖𝑖𝑗𝑗𝑗𝑗 = 𝑈𝑈𝑑𝑑𝑑𝑑 𝑢𝑢𝑗𝑗𝑗𝑗 = 𝑒𝑒𝑗𝑗 − 0 𝑖𝑖𝑗𝑗𝑗𝑗 − 0 ∙ 𝐿𝐿0 𝑑𝑑𝑖𝑖𝑗𝑗𝑗𝑗 𝑑𝑑𝑑𝑑 2 2 2 𝑑𝑑𝑑𝑑𝑗𝑗𝑗𝑗 (4) 𝑑𝑑𝑑𝑑 − 𝑢𝑢𝑗𝑗𝑗𝑗 +𝑢𝑢𝑗𝑗𝑗𝑗 2 (5) where 𝑒𝑒𝑗𝑗 in (4) is the inner EMF in phase j and is expressed as: 𝑒𝑒𝑗𝑗 = (a) Basic structure of MMC 𝑢𝑢𝑗𝑗𝑗𝑗 −𝑢𝑢𝑗𝑗𝑗𝑗 (6) 2 From the relation in (4), considering the ac grid network voltage, 𝑢𝑢𝑗𝑗𝑗𝑗 , the line current, 𝑖𝑖𝑗𝑗𝑗𝑗 , can be regulated by manipulating the control variable 𝑒𝑒𝑗𝑗 . The inner dynamic performance of the MMC which characterize the relation between inner unbalance voltage by and leg current is given by (5) and it is redefined as: 𝑢𝑢𝑗𝑗𝑗𝑗 = 𝐿𝐿0 A. Operational Principle An equivalent single phase MMC system is shown in Fig. 4 (b). In Fig. 4 (b), the arm inductor and resistors are represented by L0 and R 0 , respectively. The total dc bus voltage is 𝑈𝑈𝑑𝑑𝑑𝑑 , the converter voltage of phase j is represented by 𝑢𝑢𝑗𝑗𝑗𝑗 (𝑗𝑗 = 𝑎𝑎, 𝑏𝑏, 𝑐𝑐) . The line current of the each phase is denoted by 𝑖𝑖𝑗𝑗𝑗𝑗 (𝑗𝑗 = 𝑎𝑎, 𝑏𝑏, 𝑐𝑐). The arm voltages generated by series of the SMs express as 𝑢𝑢𝑗𝑗𝑗𝑗 and 𝑢𝑢𝑗𝑗𝑗𝑗 (𝑗𝑗 = 𝑎𝑎, 𝑏𝑏, 𝑐𝑐, 𝑢𝑢: 𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢𝑢, 𝑙𝑙: 𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙). The arm currents 𝑖𝑖𝑗𝑗𝑗𝑗 and 𝑖𝑖𝑗𝑗𝑗𝑗 in Fig. 4 can be expressed by (1) and (2). 𝑖𝑖𝑗𝑗𝑗𝑗 = 𝑖𝑖𝑗𝑗𝑗𝑗 − 𝑖𝑖𝑗𝑗𝑗𝑗 (1) 𝑖𝑖𝑗𝑗𝑗𝑗 (2) 2 1 + 𝑅𝑅0 𝑖𝑖𝑗𝑗𝑗𝑗 = [𝑈𝑈𝑑𝑑𝑑𝑑 − �𝑢𝑢𝑗𝑗𝑗𝑗 + 𝑢𝑢𝑗𝑗𝑗𝑗 �] 2 (7) The u (...truncated)