Digital Filters for Phasor Measurement Units: Design Criteria, Advantages and Limitations

The synchrophasor estimation model suggested in Annex D of the IEEE/IEC Standard 60255-118-1:2018 is based on the direct frequency down-conversion of 50-Hz or 60-Hz AC signals followed by two low-pass filters in parallel: one for the inphase component and the other for the quadrature component, respectively. However, the filter specifications reported in the Standard rely on the rather optimistic and questionable assumption that dynamic fluctuations and steady-state disturbances are mutually exclusive. This paper instead, after a critical review of such specifications, provides more conservative criteria for filter design and reports the results obtained with two minimum-order equiripple FIR filters conceived for Class P and Class M applications, respectively. Several simulation results show that, even if the proposed filter-based estimators exhibit worse accuracy than another state-of-the-art dynamic estimator in the individual testing conditions reported in the IEEE/IEC Standard, TVE compliance is met in any case and with a lower computational burden. Moreover, and more importantly, the estimation accuracy achievable by using suitable filters can be kept under tight control even under the combined effect of multiple steady-state and dynamic fluctuations.