Reactive Power Circulation Based Fault Tolerance Schemes for Multi-Megawatt 3L-NPC Paralleled Converters in Variable Speed Hydro Applications

Parallel connected multi-MW three-level neutral point clamped power converters (3L-NPC) are widely preferred in large-rated variable speed pumped storage unit as it improves operating efficiency and reliability. During open circuit fault conditions/gate drive malfunction, the entire unit must be shut down to ensure the safety of other healthier components. This article proposes reactive power circulation-based fault tolerance control strategies for grid side converter (GSC) during inner and outer switch fault conditions. In case of an open-circuit fault in outer switches, the normal sinusoidal currents are achieved in GSC by circulating reactive power among the healthy to faulty power converters. In case of an open-circuit fault in inner switches, the reactive power is circulated between healthy converters and a doubly fed induction machine (DFIM). In addition, neutral-point (NP) voltage oscillations are suppressed by injecting a zero-sequence signal into the reference voltage. The feasibility study and performance analysis of the proposed fault tolerance control strategies are carried out in the MATLAB/Simulink 2014a environment for the 250 MW DFIM unit. Furthermore, experimental validation is carried out in 3HP DFIM scale-down laboratory prototype with three parallel connected converters.