Reliability Comparison and Evaluation of MMC Based HVDC Systems

Modular Multilevel Converters (MMC) based High Voltage Direct Current (HVDC) systems utilize a large number of submodules (SMs). Failure of any SM may lead to shut-down of the entire system or seriously affect the converter performance. The fault-tolerant design approach can dominate faults and improve the system performance and reliability. The N+1 redundancy design is a well-known technique to improve the MMC reliability. However, this approach requires additional SMs, and accordingly higher cost. The reliability of an MMC can be improved using the MMC scale-up control method, which a large number of SMs can be predicated by a smaller cumulative set of SMs. In this paper, the reliability analysis is performed to compare the reliability performance of the MMC based conventional and scale-up control methods with and without redundant designs. The MMC control designs are presented and assessed using Markov reliability model for ±320 kV and 1000 MVA MMC system. The comparative evaluation shows that the reliability of an MMC with the scale-up approach is higher than that of the N+1 redundancy design.

I. Introduction

Increasing demand for High-Voltage Direct Current (HVDC) transmission has brought attention to Voltage Source based Converters (VSC). With the development of controllable and self-commutated semiconductor devices such as IGBT, IEGT, GTO, IGCT, and GCT, VSCs have become superior for transmitting bulk power over long distances [1]. Compared with existing VSC topologies (two-level, Neutral Point Clamped (NPC), and multilevel floating capacitor), Modular Multilevel Converter (MMC), [2], is considered as a breakthrough technology with its scalability, modularity, good power quality, and redundancy features.