Energy-Based Virtual Damping Control of FB-MMCs for HVDC Grid

Abstract

Full-bridge submodule based modular multilevel converters (FB-MMCs) have attracted wide attention due to the DC fault blocking capability. However, the blocking of the FB-MMC can only suppress its DC terminal current while the fault currents may still circulate along the meshed DC network. To address this issue, an energy based virtual damping control is proposed, where the DC terminal current of the FB-MMC is regulated to follow the DC voltage in the event of a DC fault. The FB-MMC is thus controlled as a virtual damping resistor to actively absorb the residual energy in the DC network and quickly suppress the potential circulating DC fault currents. This enables fast fault isolation using DC switches and thereby fast fault recovery after fault isolation. The fault isolation time is significantly reduced from around 120 ms with the FB-MMC simply being blocked and 50ms with the conventional FB-MMC fault control method to around 15 ms. The validity of the proposed control is verified in a three-terminal meshed DC network. Index Terms-DC fault protection, full-bridge, HVDC grid, modular multilevel converter (MMC), virtual damping control. I. INTRODUCTION igh-voltage DC (HVDC) transmission systems based on modular multilevel converters (MMCs) have developed rapidly due to their significant advantages. However, the behaviors and characteristics of half-bridge (HB) submodule (SM) based MMC (HB-MMC) during DC faults are major issues to be considered in applications. In the event of a DC short circuit, high AC current flowing through the freewheeling diodes of HB-MMC from the AC to the DC side, requires additional measures and could potentially cause serious damage to the converters and associated semiconductor devices [1-3]. Due to the DC fault blocking capability, full-bridge (FB) SM based MMCs (FB-MMCs) have attracted much attention. By blocking all the semiconductor switches in the FB SM, the SM capacitor voltage is inserted into the conduction path in D. Zhou was with the