UPQC is a custom power device which
is responsible for the reduction of PQ disturbances in both supply and load
side. The schematic diagram of UPQC is shown in Fig.2. UPQC comprises of two
Voltage Source Inverters (VSI) series and shunt which are connected back to
back with each other along with a common dc link. The shunt inverter is controlled
in the current control mode such that it injects a current which is equal to
the set value of the reference current as governed by the UPQC control
algorithm and also to maintain the dc bus voltage at a set reference value.

 

Fig.2. Schematic
diagram of Unified Power Quality Conditioner

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In order to mitigate the current harmonics introduced by the nonlinear load,
the current injected by shunt VSI is given by the following equation:

Ish
(?t) = is*(?t) – iL
(?t)                                                                  (1)

Where Ish
(?t), is*(?t) and iL
(?t) represents the shunt VSI current, reference supply current, and load
current respectively. The series inverter of UPQC is employed in the voltage
control mode it injects voltage in series with the line to realize a sinusoidal,
distortion free voltage at the load terminals. The voltage injected by series
VSI is given by the following equation:

Vsr (?t) =
VL*(?t) – Vs (?t)                                                             (2)

Where Vsr
(?t), VL*(?t)
and Vs
(?t) represents the series VSI voltage, reference load voltage, and actual
supply voltage respectively. UPQC is responsible for mitigating both current
and voltage related issues and also has the subsequent facilities:

•      It
eradicates the current harmonics, thus improving the current quality for
nonlinear loads.

•      UPQC
maintains load end voltage at the rated value even in the existence of supply
side disturbances.

•      UPQC
supports VAR requirement of the load, so that the supply voltage and current
are in phase. Thus no additional power factor correction equipment is
essential.

•      The
voltage injected by UPQC to keep the load voltage at the desired value is taken
from the same dc link, thus no extra dc link voltage support is involved.