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

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.