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CHAPTER 6
MITIGATION OF VOLTAGE SAG, SWELL AND SINGLE
PHASE OUTAGE USING MULTI WINDING
TRANSFORMER
6.1
INTRODUCTION
From the literature survey it is observed that the DVRs based on
direct converters are not able to mitigate single phase outage. In this chapter a
DVR topology based on direct converter is proposed which is able to
compensate single phase outage. The DVR consists of a direct converter, a
multi winding transformer at the input side and a series transformer at the
output side of the converter. The direct converter is fabricated using three bidirectional controlled switches. The DVR can properly compensate long
duration, balanced and unbalanced voltage sag and swell by taking power
from the grid. To compensate the voltage sag in any one phase, the other two
phase voltages are added using a multi winding transformer. The added
voltage is modulated using controlled switches to compensate the sag. To
compensate the swell, power from the same phase is taken and the
corresponding phase voltage is attenuated. The switches are controlled by
ordinary pulse width modulation (PWM) technique. The simulation and the
hardware results validate the idea that the proposed topology can mitigate
balanced sag, balanced swell, unbalanced swell and single phase outage
effectively.
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6.2
PROPOSED
TOPOLOGY
USING
MULTI
WINDING
TRANSFORMER
Each DVR consists of a direct converter with LC filter, a multi
winding transformer with a turns ratio of 1:1 and a series transformer as
shown in the Figure 6.1. Each three-phase-to-single-phase converter is
structured by three bidirectional power switches. The topology of the
bidirectional switch used is shown in the Figure 6.2.
Figure 6.1 Proposed topology using multi winding transformer
Figure 6.2 Bidirectional switch topology
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The switches are controlled by ordinary pulse width modulation
(PWM) technique. The bypass switches are connected across the series
transformers which remain closed when the DVR is not activated. However
when compensation takes place, the bypass switches are modulated by the
PWM signals.
6.3
CONTROL PROCEDURE
As mentioned earlier, the voltage sag and swell is identified using
single phase d-q theory. In this section switching pulse generation for the
mitigation of sag and swell is explained in detail.
6.3.1
Voltage Sag Mitigating Scheme
In order to compensate the voltage sag, power from the alternate
phases is utilized by the converter. So if there is voltage sag in a-phase then
are added using a multi winding
transformer such that resultant voltage will be out of phase with a-phase
voltage. We know that under balanced condition
(6.1)
So
(
(6.2)
Where va, vb and vc are three phase voltages. This added voltage is modulated
using bi-directional switches S , Sga and added to the a-phase voltage using
the series transformer which is connected in appropriate polarity.
The block diagram of switching pulse generation for the mitigation
of voltage sag is shown in the Figure 6.3. Ugmax has been obtained from single
phase d-q transform as explained.
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Figure 6.3 Block diagram of switching pulse generation
Uref is the desired terminal voltage, which is a user specified
constant value set in the micro controller program. The difference between the
reference voltage value Uref and the peak value of the grid voltage Ugmax gives
the amount of voltage sag or swell in the grid. The error signal is used for
generating the PWM signals as shown in the Figure 6.3.
6.3.2
Voltage Swell Mitigating Scheme
In order to compensate the voltage swell, power from the same
phase is utilized by the converter due to the availability of a large voltage in
the phase itself where the swell occurs. Whenever there is voltage swell in
aa
and Sga will be alternatively
modulated to mitigate the voltage swell. Switching pulse generation is similar
to that of sag.
6.4
SIMULATION RESULTS
The MATLAB/SIMULINK software has been used for simulation.
RL loads of 0.8 power factor lag, 240VA per phase were connected to the
lines. The desired terminal voltage has been set at 60 V rms (1p.u), 50 Hz.
The switching frequency of the converters is 8 kHz. Passive LC filters were
used on the output of the converters (1.732mH and 15µF) to filter the
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harmonics in the compensating voltage. The turns ratio of the injection
transformers is 1:1.
The ability of the DVR to mitigate balanced voltage sag of 50% in
all the phases is shown in Figure 6.4 and the single phase outage
compensation is shown in the Figure 6.5.
The compensation of balanced swell of 100% is illustrated in
Figure 6.6.
The ability of the DVR to mitigate unbalanced voltage swell of
100% in a-phase, 50% in b-phase, and 25% in c-phase can be observed in
Figure 6.7.
Figure 6.4
Mitigation of balanced (50%) sag (a) Grid voltage (b) Load
voltage (c) DVR compensation voltage
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Figure 6.5
Mitigation of single phase outage (a) Grid voltage (b) Load
voltage (c) Compensation voltage produced by the DVR
Figure 6.6
Mitigation of balanced voltage swell (a) Grid voltage
(b) Load voltage (c) Compensation voltage produced by the
DVR
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Figure 6.7
6.5
Mitigation of unbalanced voltage swell (a) Grid voltage
(b) Load voltage (c) Compensation voltage produced by the
DVR
SUMMARY
The three-phase DVR presented is based on direct converters which
do not require the dc link as in the conventional DVRs. The absence of the dc
link reduces the cost, weight, and volume of the DVRs and also avoids the
maintenance of energy storage devices. The DVR in each of the three phase
lines is constructed using only three bidirectional switches. During the
occurrence of voltage swell, as more voltage is available in the line itself, for
compensating the voltage swell power is fed from the same line. So it enables
unlimited swell compensation. As this topology aims at mitigating single
phase outage, for the voltage sag compensation in any one phase, power
derived from the other two phases. The other two phase voltages are added
with the help of a multi-winding transformer and then the compensating
voltage is synthesised using the converter. During the unbalanced sag, if other
two phase voltages are balanced then the sag compensation range could be
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more than 50%. The control of DVR is done by using a very simple PWM
procedure. The DVR is able to mitigate 50% of balanced voltage sag,
unlimited voltage swell and single phase outage effectively. The topology
presented uses an additional multi-winding transformer for each phase with
only three switches, thereby effectively compensating single phase outages
with simple control logic.