A Hybrid MMC Topology with dc Fault Ride-Through Capability for MTDC Transmission System

Hindawi Publishing Corporation Mathematical Problems in Engineering Volume 2015, Article ID 512471, 11 pages http://dx.doi.org/10.1155/2015/512471 Research Article A Hybrid MMC Topology with dc Fault Ride-Through Capability for MTDC Transmission System Xinhan Meng,1 Ke-Jun Li,1 Zhuodi Wang,1 Wenning Yan,1 and Jianguo Zhao2 1 School of Electrical Engineering, Shandong University, Jinan 250061, China State Grid of China Technology College, Jinan 250002, China 2 Correspondence should be addressed to Ke-Jun Li; Received 1 April 2015; Revised 18 July 2015; Accepted 21 July 2015 Academic Editor: Ruben Specogna Copyright © 2015 Xinhan Meng 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. This paper proposes a hybrid modular multilevel converter (MMC) topology based on mismatched-cascade mechanism. The blocking conditions of different submodule (SM) structures under dc fault are analyzed and a series double submodule is presented. With series-double submodules and mismatched-cascade submodules, the proposed hybrid MMC can ride-through the dc side short-circuit fault and provide an output voltage with the feature of low harmonic content. This hybrid MMC topology can be used in the VSC based multiterminal dc (VSC-MTDC) transmission system. The dc fault ride-through properties of the new structure and the total harmonic distortion (THD) are analyzed compared with the previous full-bridge and clamp-double architectures. An appropriate fault blocking procedure is presented, and a typical four-terminal dc transmission simulation system is given in the power system simulation software. Finally, simulation of steady-state and dc bipolar short-circuit fault verifies that the MTDC system based on this new hybrid MMC topology is stabilized and can block the dc fault and return the nonfault parts to normal. 1. Introduction With the development of power electronic technology, the voltage source converter (VSC) based on full-controllable electric semiconductor device is widely applied to high voltage direct current transmission field. Compared to the traditional HVDC, VSC based high voltage dc (VSC-HVDC) transmission system has the advantages such as flexible power control, reactive power compensation, supplying power to passive network, and forming multiterminal dc network [1– 5]. According to the different structures of voltage source converter, VSC-HVDC can be divided into two kinds: the low level traditional VSC-HVDC and modular multilevel converter based high voltage dc (MMC-HVDC) transmission system. MMC-HVDC is superior to the low level VSCHVDC in the following aspects: it has lower switching frequency, lower switching loss, and higher scalability; it does not need to switch the serial IGBTs at the same time and can be applied in high voltage occasion. Consequently, it has been rapidly developed for the last few years [6–8]. The present research of VSC-HVDC is mostly focused on half-bridge MMC (HBMMC) and its control strategies. However, half-bridge MMC cannot clear the fault current when dc fault occurs because of the freewheeling diode [9, 10]. At the same time, high power dc current breaker for HVDC applications is not sufficiently mature and costeffective [11]. So, when dc fault occurs, the common method is to turn off the whole dc system with ac circuit breakers [12]. This approach costs lots of time and reduces the reliability of VSC-HVDC system. To avoid dc fault, cable with low failure rate is used as transmission lines, but this increases the engineering cost and is easily restricted by the working environment. Thus, the present VSC-HVDC technology could hardly be used in long distance or multiterminal dc transmission system [13, 14]. To overcome the shortcomings of traditional MMC, different topologies have been addressed by many scholars. References [15, 16] propose an MMC-HVDC system based on full-bridge MMC (FBMMC) topology. FBMMC can block the fault current when dc fault occurs. However, as too many IGBTs are needed, under the same dc voltage and power level, FBMMC’s engineering investment and operation cost is high, which limits its application in engineering practice. 2 Mathematical Problems in Engineering In order to reduce the IGBT used quantity and make the converter capable of blocking dc fault, a new kind of clampdouble MMC (CDMMC) is proposed [14, 17, 18]. When a fault occurs on the dc side, CDMMC turns off IGBTs immediately and utilizes the diode reversed-phase blocking ability to complete the fault handling process. CDMMC needs less semiconductors than FBMMC and also has the ability to block dc fault. However, due to the characteristics of parallel structure, the equivalent capacitance in a bridge arm shows two kinds of states according to the different flows of shortcircuit current. So it requires longer time to cut off the shortcircuit current, and its dc fault blocking ability is inferior compared with FBMMC [14, 19]. The contribution of this paper is to analyze the equivalent states of various MMC topologies under the dc short-circuit fault and propose an improved MMC topology to improve its performance. Based on the mismatched-cascade mechanism and the principle of dc fault blocking, a hybrid MMC topology which has dc fault ride-through capability and is very suitable for MTDC system is presented. According to the “handshaking method” of MTDC system [20, 21], the process of clearing dc fault and recovering nonfault lines is explained in detail. Finally, a typical four-terminal dc system is introduced and a simulation model is built to verify the system characteristics under the bipolar short-circuit fault which is the most serious dc fault. This paper is organized as follows: after introduction, the dc fault blocking analysis, which includes analyzing the fault blocking principle and the current paths of different submodules under blocking states, is explained in detail in Section 2. In Section 3, a new topology of hybrid MMC based on the mismatched-cascade mechanism is introduced. In Section 4, a four-terminal dc simulation model is built to explain the application of the new hybrid MMC in MTDC transmission system, including the process to remove the fault lines and recover the nonfault lines under the dc fault. This new hybrid MMC used in MTDC system is tested with the steady-state and dc bipolar short-circuit fault simulations in Section 5. The conclusion of this paper is made in Section 6. 2. Fault Blocking Ability Analysis 2.1. Fault Blocking Principle. An MMC topology consists of two arms per each phase where each arm is comprised of 𝑛 series-connected submodules and a series-connected inductor. These submodules of each bridge arm can be replaced with an ideal voltage source. After the treatment of presenting network parameter by per-unit value normalization, the equ (...truncated)