Protection schemes for turn-to-turn faults within control windings of TCT-CSR and impacts of power regulation

The commonly used protection scheme for turn-to-turn faults detection of shunt reactors lacks sensitivity when applied to thyristor controlled transformer-type controllable shunt reactor (TCT-CSR), especially when TCT-CSR is working almost at its rated power and the short-circuited turn ratio is small. Based on the analysis and comparison of the characteristics under internal faults and external faults, this paper proposes zero-sequence overcurrent protection with zero-sequence voltage blocked to detect turn-to-turn faults within control windings. The impacts of power regulation on the proposed protection scheme is investigated in depth. The results indicate that the proposed protection scheme may mal-operate during power regulation. As a solution, the approach of increasing the resistance of control windings is proposed in this paper. The value of resistance and its impacts on the performance of the proposed protection scheme during power regulation are discussed in detail. Finally, the optimal value of the resistance of control windings is recommended as the reference for the future design of TCT-CSR after the comparison of simulation results under different scenarios.