u ¼ u0. Herein, it is worth noting that in order to keep a concise
context of the paper, only IFL(t) was calculated for flicker assessment.
Pst was not calculated because it is only the statistics of IFL
over 10 min.
3.3. Total harmonic distortion (THD)
Simply speaking, the THD is a criterion used to describe how
much the time waveform of the electrical signal is distorted by
undesired frequencies. In the IEC standard, the THD is assessed via
the spectral analysis of current signals [12]. It needs to detect the
fundamental frequency and its higher-orders first from the signal
and then calculate the THD by taking into account the harmonics
up to order 50. Apparently, this is not a straightforward method and
moreover, it is unable to take into account the influences of noise,
fractal frequencies, and those frequencies higher than order 50. To
overcome these disadvantages, a new THD assessment method is
dedicatedly developed by using a band-pass filter and a high-pass
filter in combined. The former is for extracting the fundamental
frequency and the latter for extracting those frequencies higher
than the fundamental frequency. Both are the windowed linearphase
finite impulse response (FIR) digital filters [34]. As Fig. 4
shows, the central frequency of the band-pass filter is 50 Hz, and
the corner frequency of the high-pass filter is 60 Hz. Apply both
filters to the current signals to obtain Ifundamental(t) and Iharmonics(t).
Then, the THD of the signal can be estimated by