Field Measurement of Frequency and ROCOF in the Presence of Phase Steps

Abstract

A description of the importance of rate of change of frequency (ROCOF) measurements to the operation of electricity networks is given. The susceptibility of ROCOF measurements to common power system disturbances such as phase steps is described. A measurement campaign to observe ROCOF at multiple locations in an island grid dominated by renewable generation is described and some results are given. These captured ROCOF events are dominated by those associated with phase steps, which occur without significant change to underlying power system frequency. It is concluded that they constitute a "false" ROCOF event. A new algorithm is presented that attempts to remove the influence of the phase step and reduce the associated ROCOF error such that the reliability of ROCOF measurements can be improved in the presence of phase steps. The algorithm is then applied to some recorded waveform sequences from the island that contains phase steps, and the results are presented. In one example, it is shown that a false ROCOF spike in excess of 100 Hz/s was reduced to less than 5 Hz/s. Index Terms-Frequency measurement, phasor measurement units (PMUs), power system measurements, rate of change of frequency (ROCOF).

Reliable Rate-of-Change-of-Frequency Measurements: Use Cases and Test Conditions

“…This is not surprising because the concept of phase and frequency is ill-defined or even N/A under these circumstances [35] and may need redefinition [36]. Special fault-ride-through algorithms might be needed to tackle this challenging situation [16].…”

“…The ability of an ROCOF algorithm to measure the changes in the underlying system frequency to the required accuracy within the required latency time, while rejecting localized phase jumps, is the most challenging issue in delivering the useful measurement of ROCOF. Future algorithms such as [16] to reject phase jumps will most likely require added latency for decision processing.…”

“…The tests given in Table II should be used in conjunction with the specifications for the UCs given in Table I: ideally, the ROCOF worst case ripple of 0.1 Hz/s is not exceeded in each of the tests and accuracies of better than 0.05 Hz/s are achieved. This may not be possible to achieve in the presence of phase steps (tests 5 and 8) unless some form of phase step correction algorithm is used (see [16]). In addition, for lowlatency designs, tests 3 and 7 may give rise to ROCOF ripple higher than the user's desired specifications.…”

“…Several authors have worked on testing of PMUs under realistic power system conditions not covered by the present IEC/IEEE synchrophasor standard. These conditions include noise [5], [6], fault conditions and transients [3], [14]- [16], unbalance [17], voltage fluctuations [18], and other distorted signals [19]. Several of these conditions are particularly important for applications of PMUs in distribution grids [18], [20].…”

Limitations of Small-Signal Modeling of Grid-Connected VSCs under Phase Jumps and Frequency Deviations

“…However, to model the power system disturbances such as phase jumps and frequency variations, the phase of the source voltage would also change. Phase jumps are normal power system phenomena which can have many causes [3]. Moreover, in a VSC dominated grid, occurrence of frequency deviations tends to be more frequent, than in the conventional grid governed by synchronous generators [4].…”