  As the complexity increases in modern power systems, power quality analysis
considering interharmonics has become a challenging and important task. This
paper proposes a novel decomposition and estimation method for instantaneous
power quality indices (PQIs) monitoring in single-phase and three-phase systems
with interharmonics and transient disturbances. To separate the interharmonic
components, a set of new scaling filter and wavelet filter with narrow
transition bands are designed for the undecimated wavelet packet transform
(UWPT). Further, a two-stage decomposition method for multi-tone voltage and
current signals is proposed. The Hilbert transform (HT) is applied to calculate
the instantaneous amplitude and phase of each frequency component, which
accordingly allows the monitoring of different PQI parameters. Numerical tests
are conducted to check the performance of the proposed method. The test results
show that compared to other conventional approaches, instantaneous PQIs
estimated by the proposed method present significant advances for tracking
transitory changes in power systems, and could be considered as a helpful tool
for high-accuracy PQ detections.
