3.4. Methodology

of UPQC Operating Conditions 24

Base on the above analysis the different modes of operation are

discussed conditions.

Case I: Reactive power

flow of diagrams

During the normal operating condition when UPQC is not connected

to the network as shown in the Figure (a). In this condition the reactive power

required to the load is supplied by the source only. When the UPQC is connected

in the network and the shunt APF is put into the operation, the reactive power

demanded by the load is provided by the shunt APF alone; such that no reactive

power support is put on the mains. So as long as the shunt APF is ON, it is

handling all the reactive power even during voltage sag, voltage swell and

current harmonic compensation condition from the load side. The series APF is

not taking any active part in supplying the load reactive power. The reactive

power flow during the entire operation of UPQC is shown in the Figure3.3 (b).

In this case no active power transfer takes place via UPQC, named as Zero Active Power

Consumption Mode.

a)

No UPQC

b) With Shunt APF

Figure3.3: Reactive

power flow of diagrams

CASE II: Active Power Flow during Voltage Sag

Condition

If k

0, i.e. vt > vL, then PSr will be negative,

this means series APF is absorbing the extra real power from the source. This

condition is possible during the voltage swell problem. Again is will

be less than the normal rated current. Since VS is increased, the dc link voltage can

increase. To maintain the dc link voltage at constant level the shunt APF

controller reduces the current drawn from the supply. In other words we can say

that the UPQC feeds back the extra power to the supply system. Since

series APF absorbs active power, termed as Active Power Absorption Mode.

The overall active power flow is shown in the Figure3.5.

Figure 3.5:

Active power flow during voltage swell conditions

Key:

Ps”-power

supplied by the source to the load during voltage swell condition

Psr”-power

injected by series APF in such a way that sum Ps”- Psr” will be the required load power during normal

working condition

Psh”=power

delivered by shunt APF during voltage sag condition Psr”= Psh

CASE

IV: Active Power Flow during Normal Working Condition

If k = 0, i.e. vt = vL, then there will

not be any real power exchange though UPQC. This is the normal operating

condition and active power flow is shown in the Figure 3.6.

Figure 3.6: Active Power Flow during Normal Working

Condition

Key:

Ps-power

supplied by the source to the load during voltage swell condition

Psr-power

injected by series APF in such a way that sum Ps”=Psr” will be the required load power during

normal working condition

Psh=power

delivered by shunt APF during voltage sag condition Psr= Psh

CASE V: Voltage Harmonic Compensation Mode

If the supply voltage is distorted which containing several

harmonics, in such cases the series APF injects harmonic voltages equal to the

sum of all harmonics voltage at PCC but in opposite direction. Thus the sum of

voltage injected by series APF and distorted voltage at PCC will get cancelled

out. During this voltage harmonic compensation mode of operation the series APF

does not consume any real power from sources since it injects only harmonics

voltage. Here UPQC works in zero active power consumption modes.

CASE VI: Current Harmonic Compensation Mode

If the load is a nonlinear one producing harmonics, in such cases

the shunt APF injects current equals to the sum of harmonics current but in

opposite direction, thus cancelling out any current harmonics generated by

nonlinear load. During this current harmonics compensation mode of operation

the shunt APF does not consume any real power from the source since it injected

only harmonics currents. Here UPQC works in zero active power

consumption modes.

3.5. Dynamic dqo transformation

The d-q-model convenience for control system design stationary

symmetrical AC variables to DC reference frame.

Figure

3.7……………………….

Park’s (abc_to_dq0)

transformation for the reference current source

The inverse dq0 transformation

The Park’s and its inverse is the same for the reference voltage

source.

3.6. CONTROL STRATEGY OF UPQC

The control strategy proposed here

aims to generate reference signals for both shunt and series APFs of the UPQC.

The proposed control technique is capable of

extracting most of the load current and source voltage distortions. The series

APF is controlled to eliminate

the supply voltage harmonics; whereas the shunt APF is controlled to the supply

current harmonics and negative sequence current. In

this paper, d-q frame theory is used to control both series and shunt

controller.

3.6.1. Description

of implementation of series controller

The control strategy of series

controller is shown in figure{?} in which voltage from the load and source is

converted to its equivalent dqo components, by using the angles from the

discrete three phase PLL. The angles for the calculation are generated by using

load voltage. The resultant voltage is then transferred back to the 3 phase

component using reverse transformation. The resultant abc component is the fed

to the discrite pulse width modulation generator (PWM) to produce gate pulses.

The dqo transformation is done by parks transformation. The same formula can be

used for current transformation. Inverse parks transformation for generation of

reference signal.

Figure 3.8: Control Strategy for Series Controller

3.6.2.

Description of implementation of shunt controller

The control strategy of shunt controller

is shown in Figure 3.8. it is same as that for series controller the difference

lies in the fact that input in place of control voltage wave having magnitude

of 1 p.u controlled by the angle drawn

from the pll(phase lock loop). Load control is given as input to pll. The angle

for the calculation is generated by using load current by parks transformation

equation. Resultant reference signal is fed to PWM generator which produce gate

signal.

Figure

3.9: Control Strategy for Shunt Controller

3.7.

General Simulink Representations

In this model, the load is supply

from the utility of 200V and 50 Hz as a source. A step up transformer is used

to step up the utility voltage of 200V to

440V. Non-linear loads are chosen for the purpose of investigation of both

single line to ground fault and double-line to ground fault.

The series and shunt compensator of

UPQC is connected through an inductor so that to remove the harmonics from the

injected voltage and to remove the

distortions in the injected current. Here, two compensators of UPQC works as

Dynamic Voltage Restorer (DVR) and

Distribution Static Compensator (DSTATCOM), the series compensator works as DVR

and the shunt compensator works as

DSTATCOM.

Figure

3.10: Matlab/Simulink model of UPQC 1