Recently as power grid voltage level rises in China and with the expansion of the grid, the problems such as transformer noise caused by geomagnetic induced current arouse concern. Owing to the frequency of geomagnetic induced current (GIC) general ranging from 1Hz to 0.001Hz, grid computing uses DC equivalent principles, namely ESP equivalent to DC power supply, system components modeled as resistors, and inductors ignored. But the frequency variation of GIC effects the transformer bias and the accuracy of the GIC calculation.With the time domain field circuit coupled method, we use a transformer model to get dynamic inductance and use the circuit model for solving current, so that to analysis the transformer bias in different frequency by the transformer magnetizing inductance and current values ??waveforms. The results show when the amplitude of the ESP is constant, if the frequency is 0.1Hz - 0.002Hz, there will be deviation if GIC is calculated in DC ,that the lower the frequency, the less the deviation; if the frequency is below 0.002Hz, GIC can be calculated in DC which the deviation can be ignored.
