Comparison Between Two-Level and Modular Multilevel Converters in High Voltage Direct Current Systems Under Fault Conditions

Abstract

The occurrence of faults in multi-terminal high voltage direct current (MTDC) systems is a critical situation for both control and protection systems. For instance, MTDC systems work under large overcurrents during DC faults due to the discharging of capacitors on the DC side and the feeding of AC fault current to the DC system via the converters’ anti-parallel freewheeling diodes, challenging their operation, control, and protection. Therefore, the behavior analysis of MTDC systems based on modular multi-level converter (MMC) when submitted to a critical situation is essential to develop fast and accurate protection systems and control strategies. The main goal of this paper is to present a comparison between a six-level MMC (6L-MMC) and a two-level voltage source converter (2L-VSC) for MTDC systems under DC and AC faults to highlight the main challenges for the control and protection systems, such as DC fault blocking capability, integration of AC and DC systems during different operating conditions, fault propagation, and DC voltage control.