核电厂在主变空载合闸过程中产生励磁涌流，可能导致相邻运行变压器产生和应涌流，厂用电动机三相电流不平衡度增大，甚至引起三相不平衡保护误动作。针对以上问题，推导了变压器空载合闸励磁涌流的表达式，将剩磁作为可调参量，发现对合闸主变进行与合闸时产生磁通方向相反的方向充磁，令剩磁与偏磁相互抵消，从而抑制励磁涌流的产生。通过对和应涌流表达式分析得出系统阻抗值决定了涌流对三相不平衡度的影响程度。随后利用PSCAD搭建了含有6台发电机组的核电厂一次回路仿真模型，仿真了空载合闸过程中的励磁涌流、和应涌流以及相邻变压器所带中压电动机的不平衡状态，分析电动机负荷水平以及系统联系阻抗对电动机三相不平衡状态的影响，并证明预充磁可显著降低三相不平衡度，防止异步电动机保护误动作，为主变空载合闸导致的异步电机不平衡保护误动作提供了新的解决方案。

Excitation inrush current is generated during no-load switching of the main transformer in a nuclear power plant. This current may cause sympathetic inrush in adjacent operating transformers and increase the three-phase current imbalance in auxiliary motors. It may also lead to misoperation of three-phase unbalanced protection. To address these issues, an expression for the inrush current during no-load closing of transformers was derived, with remanence introduced as an adjustable parameter. The findings show that pre-magnetizing the main transformer in the opposite direction to the closing magnetic flux can cancel out the residual magnetism and bias magnetism. Thus, the inrush current was effectively suppressed. Based on the analysis of the sympathetic inrush current expressions, it was concluded that the system impedance value greatly influences the degree of three-phase unbalance caused by the inrush. A simulation model of the primary circuit of a nuclear power plant with six generating units was constructed using PSCAD. The magnetizing inrush current, the sympathetic inrush current, and the unbalanced state of the medium voltage motor associated with the adjacent transformer during the no-load closing process. The effects of motor load levels and system contact impedance on the motor's three-phase unbalanced were analyzed. The results demonstrate that pre-magnetization can effectively reduce the three-phase unbalance and prevent misoperation of the asynchronous motor protection. This approach offers a new solution for protecting asynchronous motors affected by no-load closing of the main transformer.