Fault Tolerant Si-SiC based Hybrid Modular Multilevel Converter with Enhanced Efficiency

Abstract

Conventional Modular Multilevel Converter (MMC) consisting of only Half-Bridge Sub Module (HB-SM) offers scalability, modularity, flexibility, and lower losses as compared to multilevel converters. However, it lacks to provide DC fault-tolerant capability in High Voltage Direct Current (HVDC) applications. This paper proposes a Silicon-Silicon Carbide (Si-SiC) based Hybrid MMC, called as Fault Tolerant MMC (FTMMC), consisting of SiC based HB-SM and Si based Full-Bridge Sub Module (FB-SM), that provides DC fault-tolerant capability with enhanced efficiency for HVDC applications. In this paper, a control technique is used, resulting in FB-SM switching at the fundamental frequency and HB-SM switching at a significantly higher switching frequency. The FB-SM is controlled using the Nearest Level Control (NLC) modulation approach, while the HB-SM is controlled using a combination of NLC method and Phase Disposition Pulse Width Modulation (PDPWM) technique. Since the switching frequency of the HB-SM is significantly higher than the FB-SM, SiC switches are used in HB-SM to minimize the switching losses and to optimize the cost of the system. This is because SiC switches have lower switching losses and higher cost than Si switches. Furthermore, for voltage balancing of the FB-SM and the HB-SM, the Reduced Switching Frequency (RSF) technique is used, which decreases the switching frequency of each SM and therefore minimizes the overall switching loss of the FTMMC. To evaluate the effectiveness of the proposed converter topology and its control technique, simulation of the FTMMC is performed in MATLAB Simulink. The power loss of FTMMC is analytically calculated and compared with the existing solutions to prove its efficiency.