Filter Design Masks for C37.118.1a-Compliant Frequency-Tracking and Fixed-Filter M-Class Phasor Measurement Units

The new amendment to the phasor measurement unit (PMU) standard C37.118.1a makes several significant changes compared with the standard C37.118.1 (2011). This paper highlights some of the most important changes, with a particular emphasis applied to how those changes relate to the way that an M-class PMU filter needs to be designed. In particular, there is a delicate tradeoff between passband flatness (the bandwidth test) and stopband rejection in the out-of-band (OOB) test. For a PMU algorithm using frequency-tracking and adaptive filters, it is shown that passband flatness can be relaxed to about 2.5 dB, but that the stopband needs to begin up to 14.8% closer to 0 Hz than for a fixed-filter PMU. This is partly due to the exact procedures of the C37.118.1a OOB testing and partly due to the adaptive nature of a frequency-tracking PMU filter section. Both the above lead to modified filter masks being required for frequency-tracking devices compared with the mask required for fixed-filter devices. The M-class PMU with a reporting rate of 25 Hz is the most difficult to design, for reasons given in this paper. The validity of the masks is shown using filter Bode plots and simulated C37.118.1a test results of a fixed-filter and frequency-tracking device, which have been designed to meet the masks defined in this paper.