Accurate measurement of wide-range power system frequency changes for generator protection

Many turbine generators may be operated for several minutes (e.g. gas-turbine driven generators, top-loaded hydro-turbine generators during startup) or hours (cross-compound generators during warmup) at frequencies well below their rating. It is well known that the performance of power protection schemes at these reduced frequencies is far from being perfect. Some of the protective functions must be even incapacitated during startup, whereas relays or subsystems used for low-frequency protection must be removed from service for normal operation, i.e. after synchronization. However, these imperfections can be eliminated if respective frequency measuring algorithms are used, which activates the individual protection functions. This paper presents two groups of algorithms for the digital measurement of power system frequency in the range from 5 Hz to 80 Hz. The first group is based on a variable sampling frequency which is self-adaptive to the measured fundamental harmonic of the input signal, whereas the second one works with a constant sampling frequency of 600 Hz. The measuring principles as well as test results are presented.