Based on the engineering application

the excitation system of an extra-high voltage (EHV) magnetic controllable shunt reactor (MCSR) is described and the characteristics of control winding turn-to-turn fault of a MCSR is analyzed. According to numerical simulations and experimental tests

a problem in the application of overvoltage protection against control winding turn-to-turn fault is discovered

that is

such a protection operation is delayed or even a malfunction. This is because the fault features DC buses during a turn-to-turn fault intermittent voltage spikes rather than standard sinusoidal overvoltage and the fault feature may even not exist in some fault cases. The current distribution model for the excitation system during a turn-to-turn fault is built and it is concluded that the appearance of fault features on DC buses depends on the zero-crossing of the total control current. A new protective scheme that uses the fundamental component in the total control current as criterion is proposed and simulation results have verified the effectiveness of the new scheme.