A Modular Multilevel Converter Integrated With DC Circuit Breaker

Abstract

The direct-current (dc) fault-clearance capability of the modular multilevel converter (MMC) is a key issue for overhead line applications of voltage source converter-based high-voltage dc (VSC-HVDC) transmission systems. An MMC integrated with a dc circuit breaker (IDCB-MMC) is proposed to provide a cost-effective DC fault clearance and isolation solution. A mechanical switch (MS) and a load commutation switch are connected in series as the main branch of the dc circuit breaker. DC fault currents can be forced to transfer to converter- and line-side energy absorbing branches when submodule bypass protection and dc circuit breaker interrupting operations are coordinated. Therefore, the bulky and costly solid-state branch that uses numerous semiconductor devices and surge arresters can be eliminated. The dc fault clearance and isolation capability of the IDCB-MMC were verified through the simulation results for one pole of a 1000 MW/±320 kV VSC-HVDC system. Compared with an MMC that uses half-bridge SMs, the IDCB-MMC only requires few additional capacitors and switching devices and a simple MS, thereby resulting in several advantages, such as low cost, footprint requirements, and additional conduction losses.