Given a PD instance, we can divide the power cycle with
360o into 360 small phase windows with a size of 1o. For
each phase window, we calculate three features: the number of
pulses, maximum amplitude, and average amplitude. Finally,
we extract statistical moments for each of these features over
all the phase windows, including mean, variance, skewness,
and kurtosis. Moreover, PD pulses generally occur in both
positive and negative halves of voltage cycle. That means we
totally have 3 × 4 × 2 = 24 features for each data instance.