APPARATUS PROVIDING ON-LINE INDICATION OF FREQUENCY OF AN AC ELECTRIC POWER SYSTEM

摘要

Apparatus for determining the frequency of an ac power signal, includessensing circuitry to sense the ac signal. In a digital implementation of theinvention, adouble average of the samples is generated by a processor as the sum of themost recentsample, twice the next most recent sample plus the third most recent sample.The leastmean square of a running window of the most recent double average signals isgeneratedto avoid discontinuities at the zero crossings and to reduce the effects ofnoise. Thefrequency is determined from this least mean square value of the window ofdoubleaverage signals. As implemented in a protective relay, a trip signal isgenerated if thefrequency is out of limits.