Fault Tolerant Schemes for Multi-Channel 3L-NPC Bi-directional PWM Power Converter Fed Asynchronous Hydro-Generating Unit

Abstract

The main aim of this paper is to design fault tolerance schemes for multi-channel three-level neutral point clamped (3L-NPC) bi-directional PWM power converters under open switch faults to increase the availability of asynchronous hydro-generating unit. Open switch faults in grid side converter (GSC) of doubly fed induction machine (DFIM) causes dc link voltage fluctuation, unbalanced ac input current, high current harmonic distortion, higher voltage and current stress across healthier components. Furthermore, voltage fluctuation across dc link capacitor generates unbalance output voltage in rotor side converter (RSC) and it influences zero sequence circulating currents in parallel connected rotor excitation system. Therefore, this paper implements two fault tolerant schemes such as: (i) for inner open switch faults (Sx2 and Sx3), faulty converter control is disabled and its power delivery is reduced, and (ii) outer open switch faults (Sx1 and Sx4), reactive power is circulated among DFIM and rotor excitation circuit. To demonstrate the feasibility of proposed fault tolerant schemes, a 250 MW DFIM with 5-channel 3L-NPC back-to-back power converter is simulated in MATLAB/Simulink and its results are verified.