Abstract: During the start-up voltage boost inertial rotation of the new-generation condenser and transformer unit, the current with large amplitude and distorted waveform will appear at the condenser terminal, resulting in the misoperation of the start-up overcurrent protection. In this paper, the real-time calculation formula of iron-core flux linkage of boost transformer is deduced theoretically, and the accuracy of the formula is further verified by PSCAD simulation. Based on the theoretical analysis and simulation results, it can be seen that the increasing amplitude of transformer remanence and condenser terminal voltage may lead to iron-core saturation, resulting in inrush current with large amplitude and waveform distortion, causing protection operation.Considering that the rotor winding of the condenser is regarded as a large inductance in the actual project operation, when the condenser is deenergized under grid connection failure or out of operation under fault conditions, the excitation current of the condenser cannot change suddenly, the condenser terminal voltage will slowly drop to 0, and the probability of remanence flux of the transformer is small. For the protection misoperation during start-up process, the countermeasures are as follows: first,degaussing treatment after DC resistance test of the boost transformer can reduce the probability of core saturation, or reduce the saturation degree of the transformer iron-core, so as to reduce the amplitude of inrush current; secondly, considering that this protection is only put into operation during the start-up process, from the perspective of protecting the safety of the condenser unit and preferring misoperation to operation rejection, the traditional second-harmonic phase blocking start-up over-current protection method can be adopted to deal with the operation condition of transformer core saturation.