| With the power grid voltage level rising and the expansion of the grid in China, the problems caused by geomagnetically induced current(GIC) are discovered by people so much that’calculating the GIC accurately appears to be very important. Owing to the low frequency, GIC calculation refers to the DC equivalent principle, but the variation of GIC frequency effects the accuracy of GIC calculation and the transformer bias.Therefore it is very important to assess the impact of the frequency on GIC computing in grid and the transformer flux to accurately calculate the GIC in grid.This thesis proposes the use of the time domain magnetic field circuit coupled method to build a3D nonlinear transformer model for calculating at different frequencies seperately. To make the time domain magnetic field circuit coupled method convergence and quickly calculated,it is necessary to find the reasonable time step and the calculation number of the magnetic field in one cycle time. Because of the low frequency of GIC and the long simulation cycle,it is essential to take the calculation convergence and speed in consideration.The numerical example consists of two transformers and one line, assuming the two transformers identical. According to the symmetry, it is able to calculate the GIC with only one transformer modeled. At one frequency, the dynamic inductances are got through the magnetic field analysis, while the transformer primary current, namely GIC is got by circuit calculating. It is able to get the influence of frequency on GIC calculation in grid.Considering the influence of the transformer secondary wind on flux, transformer is simulated in the time domain magnetic field circuit coupled method to calculate the transformer flux. By comparing the change of flux in different frequencies, it is able to obtain the influence of frequency on transformer flux. The influence of frequency on transformer bias can be got, by the influence of frequency on GIC calculation in grid. |
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