In recent years, it has been difficult to achieve power converter redundancy in multi-channel voltage-source inverter (VSI)-fed large rated doubly fed induction machines (DFIMs), and it has not yet been adopted in any commissioned variable-speed pumped storage power plants. Multi-channel VSIs are a fundamental part of DFIMs, and they control the real and reactive power of the machines. This paper presents the fault-tolerant operation of a large rated DFIM hydro-generating unit at open switch faults in back-to-back VSIs. A technique for diagnosing the open-switch faults in grid-side and machine-side converters is proposed. An open-switch fault is detected based on the variation in rotor currents and dc link voltage, which reduces the computational efforts during fault diagnosis. A 250-MW DFIM with five-channel three-level VSIs (each rated at 9 MVA) is simulated in the Matlab/Simulink environment to observe the drive behavior. Finally, a 2.2-kW DFIM is tested in a laboratory environment to verify the simulation results.
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