As the key equipment in the UHV AC substation, the main transformer must be switched on and off before the power is officially transmitted. The overvoltage and inrush current during the switching process need to be paid attention to. This paper conducts the simulation calculation of the electromagnetic transient of the main transformer of the UHV AC substation, and studies the influence of the closing resistance and dispersion of mechanical properties of the circuit breaker on the electromagnetic transient process.

In this paper, ATP-EMTP is used to model the main transformer and related equipment in the station, and the electromagnetic transient situation during the switching process of the main transformer is simulated, for obtaining the overvoltage and inrush current waveforms. Furthermore, by changing the value of the closing resistance and adjusting the coordination relationship of the three-phase’s action time of the circuit breaker to simulate the dispersion of the mechanical characteristics of the circuit breaker, research on the influence of the closing resistance and the dispersion of mechanical properties of the circuit breaker on the electromagnetic transient of the main transformer is also be carried out.

It can be seen from the calculation results that charging the main transformer from the high-voltage side (without closing resistance) may generate larger overvoltage multiples and excitation inrush currents, which can reach 1.71 and 1892A, respectively. Considering factors such as overvoltage, inrush current, and energy consumption of  closing resistance , it is recommended that the closing resistance of the 500kV side circuit breaker be selected as 1500Ω. Moreover, the increase of the dispersion of mechanical properties of the circuit breaker will significantly increase the values of the possible overvoltage.

The simulation in this paper can effectively calculate the electromagnetic transient process of the UHV AC main transformer’s switching, solve the possible maximum overvoltage and inrush current values, and study the effect of circuit breaker’s mechanical properties on the main transformer, guide the value selection of closing resistance, and provide technical basis for power commissioning of main transformer.