PMU’s measurements are a mathematical fitting problem, where the measurements are being fit to a sinusoidal curve. Thus, when the waveform is non-sinusoidal, the PMU is unable to fit it exactly. The less sinusoidal the waveform is, such as grid behavior during a voltage sag or fault, the worse the phasor representation becomes.
PMU’s measurements are a fitting problem where data is adjusted to match a sine wave; when the waveform deviates from sinusoidal behavior—such as during voltage sag or fault conditions on a grid—the PMU cannot accurately fit it. The more non-sinusoidal the waveform becomes, like during a voltage sag or fault event, the poorer the phase angle representation.