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

Abstract

This version is available at https://strathprints.strath.ac.uk/61522/ Strathprints is designed to allow users to access the research output of the University of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may not engage in further distribution of the material for any profitmaking activities or any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge.Any correspondence concerning this service should be sent to the 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. Keywords-Frequency estimation, Power System Control, Power System Faults, Power system protection, Power system reliability, Rate of Change of Frequency I. PHASE STEPS IN THE POWER NETWORKSOn 16 August 2016 a major fire broke out in the Cajon Pass in southern California. It impacted several high-voltage transmission lines in the area, and caused the loss of nearly 1200 MW of generation. Some of the generation loss was triggered by a phase jump that caused the "calculated frequency" to cross a level that required the generation (photovoltaic) to relay out. Fig. 1, adapted from [1] shows the three phase voltage waveform, with two times at which abrupt phase shifts took place. The first of these, just before t = 0 in the figure, occurred as a result of a phase-to-phase fault on the transmission circuit.The voltages of the two phases become one at the instant of the fault, and rap...

Reduced Leakage Synchrophasor Estimation: Hilbert Transform Plus Interpolated DFT

“…Given the limited order, the Fast HT Filter is characterized by a step response with a reduced settling time, but a non-negligible ringing effect: the cosine argument exhibits a damped oscillation that can be interpreted as either a phase or frequency modulation. In fact, the synchrophasor estimation algorithm is unable to discriminate between these contributions, due to the ambivalent definition of synchrophasor phase angle depending on both fundamental frequency and phase [40], [41]. Therefore, the same damped oscillation is noticeable in TVE and FE trends in Fig.…”

“…On one side, we adopt two different window lengths, i.e. [40,60] ms, corresponding to two and three cycles of the nominal system frequency, respectively. On the other side, we vary the sampling rate F s within [10,50] kHz, that represent typical values in current commercial PMUs.…”

PMU-Based ROCOF Measurements: Uncertainty Limits and Metrological Significance in Power System Applications

“…However, this finite-spectrum model cannot account for rapid variations of signal parameters with a satisfying level of accuracy [11]. In this case, a parametric ROCOF estimation (as provided by window-based techniques) becomes model-dependent and cannot be considered as a unique property of the signal [12].…”

Under Frequency Load Shedding based on PMU Estimates of Frequency and ROCOF

“…From a measurement perspective, PMUs rely on a synchrophasor signal model that does not perfectly fit with time-varying conditions, particularly if transient events occur [8], [9]. In this sense, the reliability and accuracy of PMU-based estimates has to be carefully considered, before determining the most suitable threshold level [10], [11]. From a control perspective, ROCOF variability does not follow pre-defined statistical distribution, but depends on the characteristics of the considered power network and on its state before the contingency.…”