Fault Diagnosis and Tolerant Operation of Multi-Channel 3L-NPC Power Converter Fed Adjustable Speed Hydro-Generating Unit

The aim of this paper is to design fault tolerant operation for multi-channel 3L-NPC back-to-back power converter system employed to adjustable speed hydro-generating unit. The post fault tolerant operation is designed based on a dc bus midpoint clamping approach for open switch faults in rotor side converter (RSC) with minimal hardware requirements (2-anti-parallel IGBTs per phase). A detailed operational and control strategy is studied with respect to its hardware limitations. One of the major drawback in designed fault tolerant operation is reduction in RSC output voltage capability due to dc link capacitor voltage limitations. Based on these operational strategies, hydraulic governor of drive is adjusted to deliver power with reduced slip variation of ±5.07%. The existence of dc component and low frequency components in fault tolerant operation causes dc link voltage fluctuation, it leads to output voltage unbalance. To compensate this dc offset value is injected with carrier based pulse-width modulation (CBPWM) to suppress dc link voltage fluctuation. The effectiveness of proposed post fault tolerant operation is examined with MATLAB/Simulink2014A environment for a 250 MW DFIM.