The Case for Redefinition of Frequency and ROCOF to Account for AC Power System Phase Steps

Abstract

All conventional techniques for measuring frequency result in large deviations to the perceived or calculated frequency when the AC waveform undergoes a phase step. The deviation magnitude and duration are dependent on the phase step magnitude, and the applied windowing/filtering. Such phase steps do occur in the power system, and the erroneous frequency calculation can result in inappropriate reactions by some rapidly-responding control and protection systems. If the frequency measurand is further differentiated to ROCOF (Rate of Change of Frequency), the excursion magnitudes can become far larger than any normally expected values of ROCOF. This paper discusses the meaning of the terms frequency and ROCOF, and presents a modified concept of frequency and ROCOF. This is done by allowing rapid phase steps to be disaggregated from frequency in the AC waveform model equation. This allows new measurands “underlying frequency”, and “underlying ROCOF” to be defined, as a pair of linked parameters, independent from a separate dynamic phase parameter. These new measurands have the potential to offer much more useful and stable information to be sent to fast-acting control and protection systems, than the existing measurands of AC frequency and ROCOF, particularly during fault events and large switching or disconnection events.