How frequency measurements can impact security of frequency elements in digital relays

Fast penetration of distributed generation into distribution networks and smart grid initiatives are jeopardizing security of frequency relays and particularly involved in UFLS schemes. In spite of IEEE C37.117 is giving a guidance of securing UFLS schemes, slower voltage decays are more likely to happen now and in the future making undervoltage supervision is less reliable than before. This paper presented field cases where UFLS relays operated incorrectly and gave some insights of frequency measurements in digital relays. It also presented some examples of how digital relays can 383 secure frequency measurements for such critical elements as underfrequency, overfrequency or volts/hertz relay elements. Frequency relays are usually tested in the lab using test sets providing smooth frequency ramp. In the real life, frequency relays are exposed to system conditions, which can force them to make a wrong measurement and/or decision Frequency relay could be evaluated before use with either real life system waveforms during system disturbances or simulated using appropriate software programs. It is quite often that utility is qualifying transformer or line relays using simulation software, but this is not common for frequency elements in relays.