The UPQC has the capability of improving power
quality at the point of installation on power distribution
systems or industrial power systems. The UPQC,
therefore, is expected to be one of the most powerful
solutions to large capacity loads sensitive to supply
voltage flicker/imbalance. With ideal compensation, the
voltage at PCC is the fundamental positive sequence
sinusoidal voltage of the power source side. The currents
of the source are sinusoidal current and the phase angles
of them are the same as the fundamental voltage in phase
respectively. In another words, with the function of the
UPQC, the load is equal to a resistance. As the UPQC is
a combination of series and shunt active filters, two
active filters have different functions. The series active
filter suppresses and isolates voltage-based distortions.
The shunt active filter cancels current-based distortions.
At the same time, it compensates reactive current of the
load and improves power factor. There are many control
methods to determine the reference value of the voltage
and the current, the most famous is the instantaneous
active and reactive power theory (the pq theory) that
Akagi proposed in and now the most popular is the dq0