Low-Voltage Ride Through Strategy for MMC With Y₀/Y₀ Arrangement Transformer Under Single-Line-to-Ground Fault

Abstract

In the offshore wind farm high-voltage direct-current (HVDC) system, the power delivery capability of the onshore modular multilevel converter (MMC) decreases severely under grid fault, which makes the DC-bus voltage increase rapidly and threatens the safe operation of the system. This paper proposes a low-voltage ride through (LVRT) strategy for MMC with Y0/Y0 arrangement transformer under single-lineto-ground (SLG) fault. The influence of different transformer arrangements to the MMC under SLG fault is analyzed. On this basis, a power delivery capability enhanced method is proposed for MMC with Y0/Y0 arrangement transformer to take advantage of its control ability on zero sequence current. In addition, an optimized LVRT strategy based on resonant controller is proposed, which has simple control structure and can ride through the SLG fault without DC chopper. The offshore wind farm MMC-HVDC simulation system is established in PSCAD/EMTDC and simulation studies are conducted to validate the effectiveness of the proposed LVRT strategy. INDEX TERMSModular multilevel converter (MMC), grid fault, high-voltage direct-current (HVDC), low-voltage ride through (LVRT)

AC Fault Ride-Through Strategy of a Transformerless Soft Normally Open Point

“…Reference [12] analyzes the fault characteristics of the bridge arm fault, and proposes a protection strategy based on closing the sub-module (SM) bypass switch to ensure the reliable action of the AC breaker. Reference [13] studies the relevant characteristics of single-phase grounding faults in AC systems and single-pole grounding faults in DC SNOP systems with a single transformer. Reference [14] analyzes the AC and DC fault characteristics of the transformerless SNOP, and derives the analytical equations of the fault current of the post-fault converter under different working modes.…”