Research on optimization strategy of differential protection for inrush current in converter transformer considering magnetic flux restraint

The converter transformer is a critical component of the HVDC transmission system and its operational stability is essential for the overall safety of the power grid. Inrush current, a non-fault large current phenomenon, can cause the differential protection device to malfunction, thus compromising the normal operation of the transformer. To enhance the accuracy and reliability of differential protection for converter transformers, this paper proposes an optimization strategy for inrush current differential protection based on the Magnetic Flux Restraint principle. First, the generation mechanism of inrush current and the operating principles of the differential protection system are analyzed, with mutual inductance identified as a key parameter for distinguishing between excitation current and fault current. Subsequently, simulations are conducted to study the behavior of the second harmonic, mutual inductance, and magnetic flux under both inrush current and fault conditions. Based on the simulation results, an optimization method for the original differential protection system, utilizing variations in mutual inductance parameters, is established. The feasibility of the proposed approach is validated through simulation studies. The method for optimizing differential protection in converter transformers is presented, which is considered crucial for enhancing the security and stability of power system operations.