Possessing such advantages as continuously adjustable reactive power and high operating speed

the thyristor controlled transformer-type controllable shunt reactor(TCT-CSR)is preferred to reactive power control in extra/ultra-high voltage transmission systems. This paper focuses on the impact of power regulation of TCT-CSR on its protection system. Based on the structure and operating principles of TCT-CSR

a simplified mathematical model for its operation in both the steady state and transient state during power regulation is proposed. The transient characteristics during power regulation and its influence on zero-sequence overcurrent protection of control windings are analyzed. The fundamental zero-sequence current peak value and the transient state duration after power regulation are determined according to simulation results. Based on this

an approach to increasing the resistance of control windings is proposed. The value of the resistor and its influence on the performance of zero-sequence overcurrent protection during power regulation are discussed in detail. Finally

the appropriate value of the resistor of control windings is recommended as a reference for substantial design following a comparison of simulation results in different scenarios. This work is supported by National Basic Research Program of China(973 Program)(No. 2012CB2015200).