Fault Tolerant Operation of Parallel-Connected 3L-Neutral-Point Clamped Back-to-Back Converters Serving to Large Hydro-Generating Units

In recent years, doubly fed induction machine (DFIM) is increasingly being preferred in variable speed pumped storage power plants due to its merits like partial-rated excitation system (cost factor), unity power factor, real and reactive power controls. In large-rated DFIM (250 MW), parallel-connected three-level neutral-point clamped (3L-NPC) back-to-back converter systems are employed as rotor excitation circuit to handle slip power requirements (-10% to +8.33% in Tehri PSPP, India). The power semiconductor device failure in the rotor excitation circuit may cause serious concerns in grid stability and system reliability. To ensure the continuous operation of a generating unit, this paper aims to design a redundant parallel leg configuration with six antiparallel IGBTs for a 3L-NPC back-to-back converter structure to acquire fault tolerant operation under power semiconductor device failure. Furthermore, an efficient active current sharing control strategy with fault tolerant operation is implemented for each converter to obtain better performance of the drive system. In fault tolerant operation, fault identification is computed based on a normalized phase currents average method with less computational effort and timely identification of open switch faults in both grid side converter and rotor side converter. The circuit reconfiguration under postfault operation is performed by antiparallel IGBTs with the help of fault tolerant control scheme. The effectiveness of a designed fault tolerant control scheme is validated through MATLAB/Simulink environment for a 250-MW DFIM system with 25 MW (5-channel) power converters. To ensure the trustworthiness of the simulation, an experimental demonstration is performed through a 2.2-kW DFIM laboratory prototype with a 2-channel 3L-NPC back-to-back converter structure.