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A routine switching event in the Xcel Energy transmission system resulted in sustained oscillations between wind generators and the system connected through a series compensated line. This event led to the development of a new microprocessor based sub-harmonic protection relay that could detect such conditions and take preventative measures. This paper discusses a new technique to detect sub-harmonics in the range of 5-25 Hz using current and voltage level detectors, sub-harmonic calculations of individual sub-harmonics and total sub-harmonic distortion factor (TSHD) to generate an alarm and/or trip to isolate the faulty system. Various test cases are presented to illustrate the effectiveness of the new technique.
Narendra et al. (Sat,) studied this question.