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Simulation of Dynamic Voltage Restorer Using
Embedded Z source inverter1.S.DEEPA 2.Dr.S.RAJAPANDIAN
1.Research Scholar, Sathyabama University, Chennai, India.
2.
Professor,
Panimalar Engineering College, Chennai, India.
Abstract:Dynamic Voltage Restorer (DVR) is one of the custom power devices that are used as an effective solution for the protection of sensitive
loads against voltage disturbances in power distribution system. The efficiency of the DVR depends on the performance of the efficiency
control technique involved in switching the inverters. Z-source inverters are recent topological options proposed for buck–boost energy
conversion with a number of possible voltage- and current-type circuitries. Common feature noted is their inclusion of an LC impedance
network, placed between the dc input source and inverter bridge. This impedance network allows the output end of a voltage-type Z-source
inverter to be shorted for voltage boosting without causing a large current flow and the terminal current of a current-type inverter to be
interrupted for current boosting without introducing over voltage oscillations to the system. Therefore, Z-source inverters are, in effect,
safer and less complex and can be implemented using only passive elements with no additional active semiconductor needed. Believing in
the prospects of Z-source inverters, this paper contributes by introducing a new family of embedded Z-source inverters that can produce the
same gain as the Z-source inverters but with smoother and smaller current/voltage maintained across the dc input source and within the
impedance network. . Simulation results are presented to illustrate and understand the performances of DVR with IEEE 30 -bus system in
supporting load voltages under voltage sags conditions.
Keywords: DVR, Z-source inverter, power quality
1 INTRODUCTION
Power quality problems like voltage sag, voltage swell
and harmonic are major concern of the industrial and
commercial electrical consumers due to enormous loss in
terms of time and money. This is due to the advent of a largenumbers of sophisticated electrical and electronic equipment,
such as computers, programmable logic controllers, variablespeed drives, and so forth. The use of this equipment often
requires very high quality power supplies. Some special
equipment are sensitive to voltage disturbances, especially if these take up to several periods, the circuit does not work.
Therefore, these adverse effects of voltage changes necessitate
the existence of effective mitigating devices. There are various
solutions to these problems. One of the most effective
solutions is the installation of a Dynamic Voltage Restorer (DVR). DVR is a series custom power device, which has
excellent dynamic capabilities. It is well suited to protect
sensitive loads from duration voltage sag or swell. A DVR is
basically a controlled voltage source installed between the
supply and a sensitive load. It injects a voltage on the system
in order to compensate any disturbance affecting the load
voltage. Voltage sag is defined as a sudden reduction of
supply voltage down from 90% to 10% of nominal. Accordingto the standard, a typical duration of sag is l0 ms to 1 minute.
On the other hand voltage swell, is defined as a sudden
increasing of supply voltage up1l0% to 180% in rms voltage
at the network fundamental frequency with duration from 10
ms to 1 minute. Voltage sag/swell often caused by faults such
as single line-to-ground fault, double line-to-ground fault on
the power distribution system or due to starting of large
induction motors or energizing a large capacitor bank. Voltagesag/swell can interrupt or lead to malfunction of any electric
equipment that is sensitive to voltage variation. Z-sourcetopological options have since been developed with either
voltage- or current-type conversion ability [1], [2].Among
them, the voltage-type inverters are more popular which are
tested for applications in motor drives [3]–[6] and fuel cell-
[6]–[9] and photovoltaic (PV)- [9]–[11] powered systems,
where the dc voltages generated by the sources are constantly
varying, determined solely by the prevailing atmospheric
conditions (e.g., intensity of solar irradiation). Although
traditional voltage-source inverters (VSI) can also be used for
such applications, their sole voltage step-down operationforces them to operate at a relatively low modulation depth
and, hence, poor harmonic performance in most cases. The
reason for using a low nominal operating ratio is because their
upper modulation range must be reserved for riding through
any surge in energy demand. On the other hand, Z-source
inverters can be designed with their maximum modulation
ratio set to the prevailing nominal case. Any surge in energydemand is then managed by varying the inverter shoot-through
time duration, which in effect is a third state introduced for
gaining voltage boosting in Z-source inverters, in addition to
their voltage-buck operation inherited from traditional VSI.
For controlling the Z-source inverters, many pulse width
modulation schemes [12], [13] have also been reported withsome achieving a lower switching loss and others realizing an
optimized harmonic performance. This paper illustrates the
analysis of the embedded impedance source inverter for DVR.
2. Dynamic Voltage Restorers
A DVR is a device that injects a dynamically controlledvoltage Vinj(t) in series to the bus voltage by means of a
booster transformer as depicted in Figure1. The amplitudes of
the injected phase voltages are controlled such as to eliminate
any detrimental effects of a bus fault to the load voltage VL(t).
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An equivalent voltage generated by the converter and injected
on the medium voltage level through the booster transformer
will compensate this means that any differential voltage
caused by transient disturbances in the AC feeder. The DVR
works independent of the type of fault or any event that
happens in the system, provided that the whole system
remains connected to the supply grid, i.e. the line breaker doesnot trip. For most practical cases, a more economical design
can be achieved by only compensating the positive andnegative sequence components of the voltage disturbance seen
at the input of the DVR. This option is reasonable because for
a typical distribution bus configuration, the zero sequence part
of a disturbance will not pass through the step down
transformers because of infinite impedance for this
component. For most of the time the DVR has, virtually,"nothing to do," except monitoring the bus voltage. This
means it does not inject any voltage (V inj(t)= 0) independent of
the load current. Therefore, it is suggested to particularly focus
on the losses of a DVR during normal operation. Two specific
features addressing this loss issue have been implemented in
its design, which are a transformer design with low
impedance, and the semiconductor devices used for switching
Fig. [2] Equivalent circuit of DVR
Mathematically expressed, the injection satisfies
VL(t)=Vs(t)+Vinj(t)
Where VL(t) is the load voltage, Vs(t) is the sagged supply
voltage and Vinj(t) is the voltage injected by the mitigation
device as shown in Fig. 2. Under nominal voltage conditions,
the load power on each phase is given by
SL = ILVL* = PL - jQL
Where I is the load current, and, PL and QL are the active and
reactive power taken by the load respectively, during a sag.
When the mitigation device is active and restores the voltage
back to normal, the following applies to each phase:
SL =PL-j QL=(PS-j Qs) +(Pinj-jQinj)
Where the sag subscript refers to the sagged supplyquantities. The inject subscript refers to quantities injected by
the mitigation device. The real and reactive power is given by
Pp=|Vp|∑=
n
q 1
|Vq|(Gpq Cosδ pq + Bpq Sinδ pq)
Qp=| Vp|∑=
n
q 1
|Vq|(Gpq Sinδ pq - Bpq Cosδ pq)
3. Embedded Z source inverter
Z-source inverters are recent topological options proposed for
buck–boost energy conversion with a number of possible
voltage- and current-type circuitries. common feature noted is
their inclusion of an LC impedance network, placed between
the dc input source and inverter bridge. This impedance
network allows the output end of a voltage-type Z-sourceinverter to be shorted for voltage boosting without causing a
large current flow and the terminal current of a current-type
inverter to be interrupted for current boosting without
introducing over voltage oscillations to the system. Therefore,Z-source inverters are, in effect, safer and less complex and
can be implemented using only passive elements with no
additional active semiconductor needed. Believing in the
prospects of Z-source inverters, this paper contributes by
introducing a new family of embedded EZ-source inverters
that can produce the same gain as the Z-source inverters but
with smoother and smaller current/voltage maintained across
the dc input source and within the impedance network.
4. Modeling of DVR in MATLAB
. This section will briefly highlight one way of modeling
a DVR in MATLAB against balanced voltage sags based on
published literature and show the result of mitigation obtained.
There are typically four main components to model a DVR:
• Coupling transformer
• DC voltage source
• Multi-pulse bridge inverter
• Control system
Fig.[1] Schematic diagram of DVR System
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A typical DVR built in MATLAB and installed into a
simple power system to protect a sensitive load in a large
distribution system is presented. The coupling transformer
with either a delta or wye connection on the DVR side is
Fig(3) IEEE 30 BUS SYSTEMS WITH DVR
Fig(4) EMBEDDED Z-SOURCE BASED DVR
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installed on the line in front of the protected load. Filters
can be installed at the coupling transformer to block high
frequency harmonics caused by DC to-AC conversion to
reduce distortion in the output [5]. The DC voltage source is
an external source supplying DC voltage to the inverter to
convert to AC voltage. The optimization of the DC source
can be determined during simulation with various scenariosof control schemes, DVR configurations, performance
requirements, and voltage sags experienced at the pointDVR is installed.
5. SIMULATION RESULTS:
Digital simulation is done using the blocks of
Matlab simulink and the results are presented here Fig-3
shows the IEEE 30 bus system with DVR devices. At bus
no.13 and 27 load demand will occur. This additional load
is added to the circuit at T=0.2sec. Thus the load changeoccurs at the system. As a result voltage sag occurs at bus
no.12, 13, 22, 27. Power requirements also increased. After
0.25 sec DVR circuit are added to system two DVR are
connected in two different places. DVR1 is connected
across bus no.9 and 13. DVR2 is connected across bus 18and bus 27. It provides sufficient voltage & power
compensation at bus 21, 22, 26.
Fig 4 shows the DVR model. Fig 5(a),5(b)&5(c)
shows the voltage, real and reactive power with load
disturbance and compensation at bus 13.Fig 6(a),6(b)&6(c)
shows the voltage, real and reactive power with load
disturbance and compensation at bus 22.
Tabulation shows the relationship between voltage, real and
reactive power without and with DVR compensation. Fromthis the voltage, real and reactive power are improved after
connecting the DVR to the system. Fig7(a),7(b)&7(c)shows the voltage, real and reactive power
without and with DVR system. Figure 8(a) &8(b) shows the
total harmonic distortion at number 13 and 22 is 0.68% and
0.10% respectively.
Fig.5(a)Voltage across bus-13
Fig 5(b)Real Power In Bus 13
Fig.(5c) Reactive Power In Bus 13
Fig6(a)Voltage across bus-22
Fig6(b)Real Power In Bus 22
Fig6(c) Reactive Power In Bus 22
Fig7(a) Bus voltage vs bus number
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Fig7(b)Bus voltage vs real power
Fig7(b)Bus voltage vs reactive power
Fig 8(a) FFT Analysis for voltage of bus 13
Fig8(b) FFT Analysis for voltage of bus 22
6. EXPERIMENTAL WAVEFORMS
Fig 9 shows the prototype has been built to further verify the
operation, the critical relationships of voltage boost, andsimulation results of the presented Z-source DVR system. TheCapacitor inductor used in the Z-source has the similar effect onthe harmonic reduction, which was confirmed in the above
simulation results. For a DVR system, the required dc capacitanceis relatively small for a tolerable voltage ripple mainly resultedfrom rectification. Figure shows experimental waveforms under the nominal voltage of 30-V rms. The voltage across the inverter
bridge was boosted to 38V.Also, it can be seen that the outputvoltage contains much less harmonics
Fig 9(a )Prototype of the EZ-source inverter
Fig 9(b )Input voltage
Fig 9(c )Rectifier output
Fig 9(d )Driving pulse
Fig 9(e )Inverter output
Fig 9(f )After LC filter output
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7. CONCLUSION
This paper has presented a new DVR system based on the
Embedded Z-source inverter. The operating principle,
analysis and the harmonic contents are presented.
Simulation results verified the operational and promising
features. In summary, the Embedded Z-source inverter
DVR system has several unique advantages that are very
desirable for many DVR applications,
* it can produce any desired output ac voltage, even greater
than the line voltage
* Provides ride –through during voltage sags without any
additional circuits and energy storage;
* reduces in-rush and harmonic current.
*unique features include buck-boost inversion by single
power-conversion stage, improved reliability, strong EMI
immunity, and low EMI
* the Impedance source technology can be applied to theentire spectrum of power conversion.
REFERENCES:
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About Authors
S.DEEPA has obtained her B.E degree from PeriyarUniversity in 2003. She has obtained her PG degree from
Annamalai University in 2005. Presently she is doing her
research at Sathyabama University. Her research interest is in
the area of Power quality.
Dr.S.RAJAPANDIAN has obtained his B.E degree fromMadras University in 1966. He obtained his M.E degree and
PhD from IISc, Bangalore in 1969 and 1974. Presently he is a
Professor at Panimalar Engineering college, Chennai, India.
His research interest is in areas of High voltage Engineering
and power quality improvement.