Summary form only given. The design, evaluation and implementation of a busbar current differential protection relay suitable for use with measurement type current transformers (CTs) is described in the paper. The relay operates in conjunction with a saturation detection algorithm, which effectively detects the start and end of each saturation period using a technique based on the third-difference function applied to the current signal. A blocking signal is activated immediately after the onset of saturation and is maintained active until the saturation period plus an additional delay of one cycle has expired. For internal faults that result in CT saturation, the relay issues a trip command before the blocking signal is activated. For external faults, that result in CT saturation, the blocking signal arrives first and the trip command remains inactive. Test results indicate that the relay successfully discriminates between internal and external faults, even when the remanent flux in a CT is high and the fault capabilities of the measurement CTs are severely limited. The paper concludes by describing how the relay was implemented on a prototype relay based on a digital signal processor. Compared to a conventional busbar differential protection scheme, the new relay achieves greater stability on external faults and enhanced sensitivity for internal faults.
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