A cross connected cell based modular multilevel converter for HVDC system with DC short-circuit fault ride-through capability

Abstract

The non-permanent dc short-circuit fault is a crucial issue in overhead high voltage dc transmission (HVDC) lines. In this paper, a cross connected (CC) cell also known as hexagonal switch cell (HSC) based modular multilevel converter (MMC) with dc short-circuit fault ride through feature is addressed for an HVDC system. During normal operation, the CC cell-MMC is capable of producing an identical number of levels while using fewer power devices than full-bridge (FB) and clamp double sub-module (CDSM)-based MMCs. Simi larly to the FB-MMC and CDSM-MMC, the provided CC cell-MMC not only blocks the dc fault current but can also function as a static synchronous com pensator (STATCOM) during a DC-Bus fault. The Proposed control technique uses MMC as a STATCOM to inject reactive power during a pole-to-pole DC-Bus fault to keep the grid stable. The control approach comprises injecting zero-sequence voltage into each phase to balance the mean voltage of capacitors and arm currents. The fundamental zero-sequence voltage is obtained by a simple stationary reference frame-based controller. The control strategy's efficacy is evaluated in MATLAB and confirmed in the Opal-RT environment.