The Research And Implementation Of Distribution Network’s Protection On Magnetizing Inrush Current

With the development and promotion of The Smart Grid, people need more power, but have much more requests for the power quality and safety. The more important the transformer in power distribution network is, the more prominent the need for its effective protection is. The transformer faces the problem of magnetizing inrush current when being energized. The inrush can lead the protection prone to malfunction or misstrip. So it needs to be solved. Researchers study to discriminate inrush current from internal fault current. At present, no matter applying the classic identification methods or with the modern subjects, every method has its advantage, but has applicable requirements and limitations as well.The paper firstly models the transformer. Besides the steaty-state, the main work is fitting the transformer’s magnetizing curve, including the curve for general single value and the hysteresis loops. With the model, we can analyze the influences on inrush current and different fault currents, from both internal structure and external environment. Contrasting the characteristics for those currents, linkages and so on, a scheme was proposed that percentage differential protection with even-harmonic current content recognition.For the scheme, the digital signal processing tools, Fourier algorithm, has improved on two aspects. One is the compensation for decaying dc component. Another is for traking the frequency, when there is asynchronous sampling. Contrasting the results of the two kinds of current by means of Fourier algorithm, it can verify that the project works.The scheme’s design includes the compensation for phase difference and ratio for the sample datas, the logic judgement for percentage differential protection, and aided functions: quick-break protection, over-excitation alarm, dc blocking and even-harmonic content recognization.For the heavy faults, quick-break protection works when the current exceeds the threshold by means of half-cycle Fourier algorithm. Other functions are by means of full-cycle Fourier algorithm. For the situation of over-voltage or under-frequency, over-excitation blocking alarms when the 5th-harmonic current exceeds the threshold. Dc blocking is made for speeding up recognizing, as the inrush current usually on one side of time. There has two ways to apply the even-harmonic content recognizing. One is applying in blocking module. Another is for percentage differential module to raise the threshold.Programing code for the scheme, debug it on the hardware terminal platform: T60MC- V2.1. The function of the percentage differential protection with magnetizing inrush current protection, has come ture, and the logic judgment time is within 2 ms while removing the sampling time.