Discrimination of Three-Phase Three-Limb Transformer Inrush Current Based on Characteristics of Instantaneous Excitation Inductances

Existing methods utilizing excitation inductances to discriminate inrush current and internal fault of transformer are all derived from the singe-phase transformer circuit model, yet there are no discussions considering the widely used three-phase three-limb transformers. In view of this question, the mathematical model of three-phase three-limb transformer based on the characteristics of its magnetic equivalent circuit is established, through which the excitation inductances of the three-phase three-limb transformer can be calculated. The calculated excitation inductances have definite physical meanings and can reflect the saturation state of the transformer core under inrush condition. Analysis of the circuit model demonstrates that the delta circulating current is not the excitation current of three-phase three-limb transformer, so the proposed method can be used to the transformers with delta connection directly. The proposed method has been verified by electromagnetic transients program including direct current (EMTDC) simulations, simulation results show that the calculated excitation inductances have different characteristics under inrush and internal fault conditions, and can be applied to identify the inrush current of three-phase three-limb transformer.