3.4. load. Since series APF supplies active

3.4. Methodology
of UPQC Operating Conditions 24

Base on the above analysis the different modes of operation are
discussed conditions.

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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