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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).



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



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



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



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



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:

[1] F. Z. Peng, “Z-source inverter,” IEEE Trans. Ind. Appl.,

vol. 39, no. 2,pp. 504–510, Mar./Apr. 2003.

[2] P. C. Loh, D. M. Vilathgamuwa, C. J. Gajanayake, L. T.

Wong, and C. P. Ang, “Z-source current-type inverters:

Digital modulation and logic implementation,” IEEE Trans.

Power Electron., vol. 22, no. 1,

pp. 169–177, Jan. 2007.[3] R. Antal, N. Muntean, and I. Boldea, “Modified Z-

source single-phase inverter for single-phase PMsynchronous motor drives,” in Proc. OPTIM , 2008, pp.

245–250.

[4] L. Sack, B. Piepenbreier, and M. von Zimmermann,

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[6] F. Z. Peng, M. Shen, and K. Holland, “Application of Z-

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ultracapacitors,” in Proc. IPEMC , 2004, pp. 1587–1591

[8] N.H.Woodley, L.Morgan and A.Sundaram, “Experience

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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.

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