A Fault Tolerant Control Scheme of Grid Connected VSI for Balanced Power Injection

A retrofit solution is proposed in this article for providing fault-tolerant capability in the existing three-leg grid synchronized converter operating at unity Displacement Factor, during open circuit fault of one of its phases. During such a fault, the scheme controls the current in the two operative phases of the converter for equal magnitude with a mutual phase shift of 60 degrees. This 60 degrees phase-shifted two-phase current is distributed as balanced 3-phase grid current at unity Displacement Factor, using an existing Delta-Y isolation transformer or a lower power rated zig-zag autotransformer. Existing fault-tolerant schemes are applicable only if the device or its driver fails, with the addition of hardware in the form of the redundant leg for the total failure of one phase of the converter. In retrofit application, the proposed scheme modifies the control logic and does not require modification in controller hardware circuit or addition of a leg in the converter power circuit. Hence, it can be conveniently implemented to add fault-tolerant capability to an existing three-leg voltage source converter in either inversion or rectification mode of operation. In addition to semiconductor failure, the proposed scheme is applicable during an open-circuit fault in other parts of the circuit viz., line reactor, connecting wire, transformer, etc., which is not possible using the existing fault tolerant techniques. Simulation and experimental results are provided to validate the proposed concept.