Xiangning Lin
Current transformer, differential protection, mathematical morphology, multi-resolution morphological gradient, saturation
A new saturation-blocking scheme of current transformer imple- mented with mathematical morphology algorithm for differential protections is presented. According to the studies of the current transformer used for the differential protection, in the event of high through fault accompanied by the current transformer saturation, the time instant that the false differential current emerges is always later than the change of the through currents. On the contrary, this time interval will not exist if it is an internal fault and one current transformer saturates. Therefore, the external fault can be identified from the internal fault by this time interval. It should be noted that this time interval may be too short when detecting very fast current transformer saturation. In this case, the current transformer blocking scheme using this interval may fail. The multi-resolution morphological gradient highlights this time interval by means of outstanding abilities of singularity detection and noise suppression. The time difference between the occurrence of fault and the emer- gence of differential current can be extracted in real time in terms of high accuracy. Therefore, we can set the time difference by a reasonable threshold for detecting saturation of current transformer because of very high through current during external faults. The results of simulation tests based on real-time dynamic simulater (RTDS) show that the method can avoid improper operations of the differential protection due to high through current. At the same time, it is able to guarantee the operating speed of the protection.
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