High Frequency Current in the Power System and Its Influence on Transfer Accuracy of Electronic Current Transformer

摘要

Electronic Current Transformers are widely used in smart substations in China. As one type of them, the Rogowski coil based current transformer with a digital integrator could generate abnormal waveforms transferring high frequency components of current that appear in the transient process of power system, which may cause misoperation of the relay protection system. This paper studies the transfer characteristics of all elements in a Rogowski coil based current transformer, including a Rogowski coil, a low pass filter, a sampling element and an integrator. High frequency current signals are turned into low frequency in sampling due to frequency aliasing. Although there is a low pass filter in front of the sampling element, it is unable to prevent the process of frequency aliasing effectively because high frequency signals are amplified in the Rogowski coil. After being transferred in the sampling element, high frequency signals become low frequency ones, which will be enlarged again in the integrator and then mixed with the original signal components. Therefore, the transformer works abnormally in the high frequency region, which causes signal distortion. Actual experiments have been performed to verify the above analyses. In order to solve the abnormal waveform problem, two improvement approaches are proposed in this paper, namely decrease in the cut-off frequency of the low-pass filter and increase in the sampling frequency of the sampling element. The former would reduce the pass band range of the filter and largely dampen the amplitude of the high frequency signals, thereby preventing the aliasing signals from affecting original low frequency signals. The latter increases the frequency boundary to a higher amount and frequency aliasing only applies to signals beyond the boundary. Since content of the high frequency signals and their amplitude would decrease with the increase of frequency, the effect of the frequency aliasing would be weak. Simulation work demonstrates the effect of the proposals.