Patent ID: 8824112

Claim:
A digital protection relay, comprising: a frequency measurement unit configured to measure a frequency of an input signal using a zero-crossing scheme, wherein the frequency is defined by the following equation: f=1/T where f denotes a frequency (Hz) and T denotes a period (sec); a frequency change rate calculation unit configured to calculate a frequency change rate of the frequency for each of a plurality of predetermined periods of the input signal to acquire first data, and accumulate and average the measured frequency for the plurality of predetermined periods and then calculate a frequency change rate of the averaged frequency to acquire one or more second data; a deviation calculation unit configured to calculate a deviation of consecutive frequency change rates of the first or second data for a predetermined interval of time; and an operation controller configured to compare the deviation of the frequency change rates with a reference value, and select either one of the first data and second data based on the comparison result to control the operation of the relay.