Investigation of the inrush phenomenon: a quasi-stationary approach in the harmonic domain

A novel approach for the investigation of the inrush phenomenon in single and three-phase transformers is presented. It consists of the calculation of a sequence of quasi-steady-state images using harmonic domain computation. Thus, lengthy-time domain simulations with the related round-off and truncation errors are avoided. The advantage of the method is its robustness compared to a time-domain simulation with sufficiently small time steps to represent the wave shapes of currents and voltages accurately over a relatively long time span. An important result of the analysis is the fact that the attenuation of inrush currents is not affected by the eddy current and hysteresis losses in the iron. Therefore, no discretization of the laminations is needed, which considerably simplifies the computations without any loss of accuracy. The attenuation is solely determined by the resistance of the external circuit and the total inductance of the circuit, including also that of the iron core. It is shown how remanence and off-peak switching combine to establish the initial bias which determines the magnitude of the inrush current