Analysis of transformer inrush under unbalanced three-phase parameters

The phase A of No.1 main transformer broke down in a certain 500kV substation and the standby has been lost, in order to maintain normal operation, phase B of No.2 main transformer has been used as a replacement, thus result in the unblanced three-phase parameters of No.1 transformer. Existing relay protection principle is based on balanced three phase parameters, to study the impacts on relay protection caused by unbalanced three-phase parameters, inrush current as a key element in relay protection be selected, a brief analysis of principle and characteristics of inrush current is given, then a simulation model for computing statistical transformer inrush current based on Monte Carlo and user-defining function of the PSCAD/EMTDC software is established, based on the real data, comparison of inrush current between balanced and unbalanced is conducted. It is pointed that short-circuit impedance unbalance has little impact on inrush current while the different of saturation characteristic has a greater influence.