Design and Control of Single Phase DynamicVoltage Restorer

Amit Meena, Shirazul Islam, Sandeep AnandDepartment of Electrical Engineering,

Indian Institute of Technology Kanpur, Kanpur - 208016, India.amitkmn@iitk.ac.in, shirazul@iitk.ac.in, me.sandeepanand@gmail.com

Yogesh Sonawane and Sanjay TungareEmerson Network Power India Pvt. Ltd.

yogesh.sonawane@emerson.comsanjay.tungare@emerson.com

Abstract—Dynamic Voltage Restorer (DVR) is a custom powerdevice used in electrical distribution system for power qualityimprovement. The main application of DVR is for voltagecompensation of sensitive loads against voltage disturbances suchas voltage sag and voltage swell in distribution lines. It is aseries connected device and is able to compensate voltage sagand voltage swell by injecting a voltage with the help of seriestransformer. The injection of an appropriate voltage componentin the event of a voltage disturbance requires a certain amountof real and reactive power. Conventionally, DVR consists of anenergy storage device which supplies the required power overthe limited duration of the sags. Large magnitude and longduration of sags, lead to heavy financial investment in energystorage unit. To overcome this limitation, a single phase back-to-back converter based DVR is implemented in this paper, whicheliminates energy storage requirement. In the event of voltagesag, front-end converter acts as active rectifer and other converteracts as inverter, injecting the voltage in series with grid voltage.Real power flows from rectifer to inverter. In case of voltageswell condition, the operation of two converters interchanged andenergy is fed back to the shunt converter from the series converter.The effectiveness of control schemes, protection schemes andplug and play operation of DVR is verified through detailedsimulation studies. The viability of above schemes is confirmedby the experimental results generated from laboratory prototypedeveloped.

Keywords—Dynamic Voltage Restorer, In-phase compensation,Voltage Sag, Voltage Swell.

I. INTRODUCTION

Voltage, sag and swells are considered to be one of themost severe disturbances to the sensitive loads [1]. DynamicVoltage Restorer (DVR) is a power electronics device whichprotects sensitive loads from disturbances in the power supply.It ensures high power quality for sensitive loads. The dynamicvoltage restorer has become popular as a cost effective solutionfor the protection of sensitive loads from voltage sag andvoltage swell. It injects voltage in series and synchronism withthe grid supply voltage in order to compensate for voltage sagand voltage swell [2]. DVR is connected in series with linethrough injection transformer. Fig.1 shows the single phaseDVR, which is connected in series with feeder. When shortcircuit fault occurs in load 1, voltage at the distribution busdecreased and voltage sag occurs across the sensitive load 2.To restore the voltage across this load, DVR is used [3].

During the period of voltage sag or swell, DVR injectsthe voltage so as to restore the load voltage to its normalvalue. During this operation, the DVR exchanges the activeand reactive power with the load. In case of voltage sag, active

power has to be supplied by DVR. This motivates the use ofenergy storage element in the DVR. Various energy storagedevices like batteries, capacitors, flywheels etc are used inDVR [4]. However, this increases the cost of DVR. Further,due to presence of battery, regular maintenance is required.

Three voltage injection techniques are popularly used inDVR, to restore the phase and magnitude of voltage across thesensitive load. These are pre-sag voltage compensation, energyoptimization technique and in-phase voltage injection tech-nique. In presag compensation both real and reactive power arerequired. This method provides the compensation for voltagemagnitude and its phase. The difference between sag and pre-sag voltage is detected by DVR and injected voltage, providescompensation for voltage amplitude without any phase shift[5], [6]. This technique mainly used for loads equally sensitivefor voltage magnitude as well as phase shift. This techniquelead to increment in capacity of energy storage device as themagnitude of voltage to be injected is more. Other technique iszero active power injection technique [7]. This method reducesthe energy storage size. Active power PDVR supplied by theDVR depends on the angle between the load current IL andthe injected voltage Vinj . In this technique, these two vectorsare maintained in quadrature, thereby, ensures PDVR remainsalmost zero. Only reactive power has to be supplied. This leadsto reduction of the size of battery. However, the magnitude ofinjected voltage is governed by following relation,

∆Vsag ≤ VL(1 − cosφ) (1)

From the above relation, it is clear that for the loads withpoor pf, it is easier to compensate voltage sag without realpower injection. Therefore, proposed technique has limitationto compensate the voltage sag with loads having leadingpower factor [5], [8], [7]. In-phase voltage injection techniquerequires, minimum voltage injection to compensates either sagor swell [3]. However, the compensated voltage suffers a phaseshift with respect to presag voltage. This technique is notsuitable for loads sensitive to voltage phase shift.

Various circuit configurations and topologies are suggestedfor DVRs in literature. A detailed comparison of various DVRtopologies with their control strategies is discussed in [4].Enhanced voltage sag compenstor scheme used for DVR isproposed in [9], which uses dc link capacitor at the input side.This scheme enables the DVR to increase sag support timecompared with existing phase jump compensation techniques.However, the suggested method does not provide support fordeep and longer duration sags. DVR based on multilevelinverter with adjustable dc link voltage is proposed in [10]. A

Fig. 1. Schematic of Basic DVR circuit

battery is connected at the input of DVR to provide active andreactive power support. The output of DVR has various outputvoltage levels which improves the quality of output voltageduring deep voltage sags. However deeper and longer durationsags, lead to increased size of battery. A DVR topologywith no energy storage is proposed in [11]. The suggestedscheme uses a matrix converter to compensate voltage sags oflonger duration. The energy required by the DVR during thevoltage sag duration is derived from residual supply. However,this technique is usable only for three phase system due tothe use of matrix converter. Two types of basic topologiescategorized as Supply-Side-Connected Shunt Converter andLoad-Side-Connected Shunt Converter for voltage sag andswell compensation are suggested. These two topologies areimplemented with help of two back to back connected H-bridge converters connected through common dc link. Theback to back connected converters can also be implementedwith the help of half bridge or full bridge topology or combi-nation of the two.

This paper deals with the design of single phase DVRbased on source-side-connected shunt converter topology. Thisconverter topology consists of two back to back connectedH-bridge converters to eliminate the energy storage require-ment. The load side converter, called series converter, is asingle phase PWM inverter, injecting the voltage in serieswith grid voltage. The in-phase voltage injection technique isused to provide compensation against voltage sag or voltageswell across the sensitive load. Detailed design of laboratoryprototype of DVR and its performance on linear and non-linear loads is discussed in this paper. This paper is organizedas follows. Section II describes the circuit topology, controltechnique, and protection schemes used for DVR. Section IIIincludes the simulation results of DVR for linear and non-linear loads. Section IV includes discussion of experimentalresults for voltage sag and voltage swell compensation. SectionV includes the conclusion of this paper.

II. DESIGN OF DVR

This section discusses the operation, design and protectionscheme of DVR.

A. Operation of Single Phase DVR with back to back con-nected converter

The DVR consists of shunt and series converter connectedvia a common dc link capacitor as shown in Fig 2. The seriesconverter is used to inject the voltage in series with grid

Current Controller

Voltage Controller

AC

Source

L

O

A

D

Cdc

Rs Ls

Lf

S1

S4

S3

S2

S5 S7

S8S6

Injecting

Transformer

Shunt Converter Series Converter

KP+KI /sKP+KI /s + +- VLref

Iref

PWM

Pulses

+-

VOref

-

KP+KI /s+-

PWM

Pulses

Rf

Voltage Controller

KP+KI /sILref

IL

Vo

VL

Current Controller

Bypass

Switch

Cs

Cf

X

sine

PLL

V_grid

SCR1

SCR2

Fig. 2. Series and shunt converters with their control schemes

voltage. The voltage injection takes place with the help ofinjection transformer connected between the source and load.During voltage sag, shunt converter is used to control the dclink voltage and works as unity power factor rectifier. Theseries converter acts as inverter and generate constant ac outputvoltage. The injected voltage is in phase with sag voltage.During the sag duration, the active power required by DVRis harnessed from source and flow of energy takes place fromshunt converter to series converter. Similarly, during voltageswell, power flows from series converter to shunt converter.The injected voltage is out of phase with swell voltage. Duringvoltage sag and voltage swell, both converters generate switch-ing frequency voltages along with the fundamental component.These harmonics deteriorates the power quality of injectedvoltage. To filter out these switching harmonics, LC filtersare used at input and output of shunt and series converters,respectively. This DVR is able to compensate both voltagesag and swell in range of 185 V (rms) to 265 V (rms), andmaintains the load voltage to 230 V. The key advantage ofthis topology is that no energy storage is required. Figs. 3 and4 show the phasor of the injected voltage in case of sag andswell, respectively. Fig 3 shows that in case of voltage sag, theinjected voltage is in phase with the sag voltage. From Fig. 4,it is clear that when voltage swell occurs, injected voltage isout of phase with the swell voltage. The DVR is designed tocater to 5 kVA load. The rating of the injection transformeris decided by the voltage to be injected and the load currentflowing through the secondary of injection transformer. Forthe voltage compensation in the range of 185-265 V (rms),1.5 kVA rating transformer is used for voltage injection.

B. Control Scheme

Linear controllers are used for shunt and series converters.The commonly used controllers are outer voltage controlloop and inner current control loop. These controllers are

Vsag Vinj

Iload

ΦVload comp

Vload comp = Vsag + Vinj

Fig. 3. Voltage injected in series with grid voltage during voltage sagcondition

Vinj

Vswell

Vpresag

Vload comp

Iload

Vload comp= Vswell-Vinj

Fig. 4. Voltage injected out of phase with grid voltage during voltage swellcondition

implemented with the help of conventional PI controllers.The dynamics of current controller are faster than voltagecontroller, which can be realized by maintaining the band-width of current control loop to be more than voltage controlloop. The dc link voltage Vo is to be regulated to its referencevalue Vref by the voltage controller. The output of the voltagecontroller is multiplied with ac signal to generate the referencecurrent. This reference current is compared with inductorcurrent of shunt converter. Phase locked loop (PLL) is usedto generate the ac signal of same frequency and phase as thatof the grid.

Similarly, for series converter, outer voltage and innercurrent controllers are used and implemented with conven-tional PI controllers. These controllers are used to adjustthe voltage injected by series converter to maintain constantvoltage across the load. For sag and swell operation, the loadvoltage is regulated by controlling the phase and magnitude ofthe injected voltage. For making transient response of DVR tobe fast, optimal tuning of the PI controller is required. Bodeplots are used for tuning of PI controller.

C. Protection Schemes

Different protection schemes are incorporated in DVR forits safety and rendering reliable services to the operator in caseof fault.

Due to overload, the load current exceeds its rated value.A bypass scheme is incorporated in DVR. During bypassoperation, the secondary of the injection transformer is shortedthrough anti-parallel SCR pair. During this mode transformerworks in zero voltage injection mode. Control logic is imple-mented which turns on the SCRs in case load current exceedsits safe limit. limit. The upper limit of load current selected forbypass operation is 125% of its rated load current. Howeverfor this range of overload current, DVR bypass operation takesplace after a duration of 10 s. The time duration after whichbypass operation takes place, goes on decreasing with increasein value of load current above its rated value.

In case of short circuit fault on load side, a large currentflows through the secondary of injection transformer. The

corresponding current on the primary side may damage thepower electronic switches (IGBTs, MOSFETs etc.)of inverter.To avoid this, fuse is connected in series with output to stopthe flow of large current.

During starting of the DVR, dc-link capacitor is completelydischarged. Due to high value of capacitor, a large magnitudeof current can flow through the shunt converter at starting ofDVR. To limit this initial current, a resistor is connected inseries with shunt converter at starting of DVR called pre-charging resistor Rs. As dc-link voltage become equal tothe peak of ac voltage, Rs is bypassed with help of a relayconnected in parallel with Rs. The DVR compensates thevoltage in the range 185-265 V (rms). If the voltage to becompensated exceeds this limit, the DVR is turned off. To turnoff DVR, the pulses supplied by the DSP to the converters arestopped and load is supplied by the bypass SCRs.

D. Plug and Play Operation of DVR

As soon as DVR, is plugged in load circuit at starting,the following steps are adopted for its reliable and faultfree operation. At starting the IGBTs of both the convertersare kept off and bypass SCRs are kept on. Charging of dclink capacitor takes place through pre-charging resistance. Ascapacitor voltage reaches to peak value of ac voltage, pre-charging resistor is bypassed through the relay. At the sameinstant, soft start operation to build dc link voltage up to 400V is initiated. Voltage and current controller determine theswitching sequence of the shunt converter. As soon as dc linkcapacitor reaches at 400 V, the switching pulses to the loadside converter are released. The DVR is connected to the circuitand bypass SCRs are turned off.

III. SYSTEM PARAMETERS

TABLE I. PARAMETERS FOR LC FILTER FOR BOTH SIDE CONVERTERS

Parameter Inductor Capacitor ResistanceSource side converter 1.693 mH 10 µF 0.1 mΩLoad side converter 0.9 mH 10 µF 0.2 mΩ

TABLE II. CONTROLLER PARAMETERS

Voltage loop Current loopParameter KP KI KP KI

Source side controller 1 100 0.5 0.1Load side controller 0.01 1000 0.1 100

IV. SIMULATION RESULTS

To implement the control strategies and protection schemesdiscussed in previous sections, a detailed simulation studyof DVR is accomplished in MATLAB/Simulink. Parametersof the system and controller are given in Tables I and II,respectively. The performance of the DVR during voltage sagand swell condition is elaborated both for linear and nonlinearloads. The linear load taken into consideration is a simpleresistive load. Fig. 5 shows the result with resistive load whenvoltage sag and swell occurs. Up to 0.4 s, value of grid voltageis 230 V (rms). A sag in source voltage occurs at the t = 0.4s. The value of sag voltage is 185 V (rms)and the durationof the voltage sag is from 0.4 s to 0.6 s (shown by bluetrace). However, the load voltage remains constant at 230 V(rms) (shown by red trace). During voltage sag, real power

flows from shunt converter to series converter. Similarly, Fig.6 shows the performance of DVR with swell in grid voltagefor resistive load. The duration of voltage swell is from 0.4 to0.6 s shown by blue trace. The load voltage remains constantequal to 230 V (rms) shown by red trace. During swell, thepower is transferred from series converter to shunt converter.

Fig. 7 shows the performance of the DVR with electronicload (non linear load) for voltage sag. The duration of voltagesag is same as that for linear load. The electronic load is asingle phase full wave diode bridge rectifier. The diode bridgerectifier is designed for load current having crest factor of 3.08.The load voltage is compensated and maintained at the rmsvalue of 230.5 V shown by red trace.

Fig. 8 shows the transient response of the DVR for suddenchange in load. To test the transient performance of DVR,sag in grid voltage of same magnitude and duration as forlinear load, is provided. Load is changed from no-load to full-load and vice versa at 0.45s and 0.55s, respectively. Duringtransients, load voltage remains constant equal to 230 V (rms).However, due to load shedding, the dc link voltage risesmomentarily. After a duration of 70 ms, dc link voltage isrestored to its normal value of 400 V. Therefore from abovewaveforms, it is clear that bidirectional AC/DC/AC converteris able to compensate both voltage sag and voltage swell inlinear and non linear loads. The simulation results reflect thesuccessful operation of the DVR implemented with back toback connected H-bridge converters.

V. EXPERIMENTAL RESULTS

To validate the simulation results, a 5 kVA prototype ofDVR is developed, and shown in Fig. 9. The system parametersare shown in Table III. This section discuss the hardwareresults and performance analysis of DVR. The experimentalresults are presented for shunt converter, series converter andwith back to back connected converter. The set up is also tested

Fig. 5. Voltage sag with resistive load: Trace (i) (Red) load voltage (200V/div), (Blue) grid voltage (200 V/div), Trace (ii) DC link voltage (10 V/div),Trace (iii) (Blue) grid voltage of source side converter (100 V/div), (Red)source current (10 A/div), X-axis 20 ms/div

Fig. 6. Voltage swell with resistive load: Trace (i) (Red) load voltage (200V/div), (Blue) grid voltage (200 V/div), Trace (ii) DC link voltage (2 V/div),Trace (iii) (Blue) grid voltage of source side converter (100 V/div), (Red)source current (10 A/div), X-axis 20 ms/div

Fig. 7. Voltage sag with electronic load: Trace (i) (Red) load voltage (200V/div), (Blue) grid voltage (200 V/div), Trace (ii) DC link voltage (10 V/div),Trace (iii) (Blue) grid voltage of source side converter (200 V/div), (Red)source current (10 A/div), Trace (iv) (Blue) load voltage (200 V/div), (Red)load current (10 A/div), X-axis 20 ms/div

for bypass operation implemented with anti-parallel SCRs.The 5 kVA DVR prototype is realized by using three voltagesensors, three current sensors, two IGBT modules, one anti-parallel SCR pair for bypass operation and relay with pre-charging resistor. The voltage sensors are used for sensinggrid voltage, voltage of dc link capacitor and output voltageof series converter. The current sensors are used for sensinginductor current of shunt converter, output inductor currentof series converter and load current. Digital signal processor

Fig. 8. Voltage sag with resistive load(transient): Trace (i) (Red) load voltage(200 V/div), (Blue) grid voltage (200 V/div), Trace (ii) DC link voltage (20V/div), Trace (iii) (Blue) grid voltage of source side converter (200 V/div),(Red) source current (10 A/div), Trace (iv) (blue) load current (20 A/div),X-axis 50 ms/div

(DSP) is used to implement all control algorithms and forgenerating the switching pulses for both side converters. DSPis used because of its advantages in cost, size, speed andreliability over analog controllers and other digital platforms.To control the converters, 32 bit TMS320F2808 Texas DSPis used. Switching frequency used for both converters is 7.5KHz.

TABLE III. PARAMETERS FOR LC FILTER FOR BOTH SIDECONVERTERS

Parameter Inductor Capacitor ResistanceSource side converter 2.5 mH 10 µF 0.19 mΩLoad side converter 2.3 mH 10 µF 0.112 mΩ

The shunt converter is separately tested for resistive loadof 1 kW. Fig. 10 shows the waveform for grid voltage, gridcurrent and voltage between IGBT legs of shunt converter.For a grid voltage of 215 V (rms), the output dc voltage ofactive rectifier is 400 V. The value of DC link capacitor is8 mF. The series converter is a single phase voltage source

Fig. 9. Laboratory prototype of experimental set up

Fig. 10. Waveforms of shunt converter with 1 kW dc load rectifier outputvoltage (250 V/div), Grid voltage (250 V/div), grid current (10 A/div), X-axis10 ms/div

Fig. 11. Waveforms of series converter on no load, output ac voltage (250V/div), inverter output voltage (250 V/div), inverter inductor current (2 A/div),X-axis 10 ms/div

inverter and also tested separately for no load. Fig. 11 showsthe waveforms for inverter output voltage, inductor current andinverter output available after LC filter. For the dc link voltageof 250 V, the fundamental component of output voltage ofinverter is 177 V(rms). After filtering, the output is almostsinusoidal in nature.

The DVR including both converters connected togetheris tested for a resistive load of 2.5 kW rating. The DVR istested for voltage sag of 187 V (rms) with same loading asin previous case. Fig. 12 shows the waveform for the gridvoltage , gird current, load voltage and inductor current ofshunt converter. For the grid voltage of 187 V(rms), the loadvoltage is regulated at 229 V (rms). The inductor current hasa rms value of 3 A.

Similarly, the DVR is tested for voltage swell of 267 V(rms) with same loading as with voltage sag. Fig. 13 showsthe waveform for the grid voltage , grid current, load voltageand inductor current of shunt converter. For the grid voltageof 267 V(rms), the load voltage is regulated at 232 V (rms).The inductor current has a rms value of 1.322 A.

Table IV shows THD of grid voltage, grid current, load

TABLE IV. THD VALUES OF THE DVR FOR INPUT CURRENT, INPUT VOLTAGE, OUTPUT CURRENT, OUTPUT VOLTAGE, POWER FACTOR, EFFICIENCYAND VOLTAGE REGULATION

Source Voltage(rms)THD(%)

(Input Current)

THD(%)

(Input Voltage)

THD(%)

(Output Current)

THD(%)

(Output Voltage)

Power factor(%)

(Input side)

Efficiency(%)

Voltage Regulation(%)

185 V 5.2 2.7 2.7 2.7 1 94.2 1230 V 5.4 3 3.1 3.2 1 98.52 0.8265 V 5.6 3 3.6 3.5 0.99 93.95 1.3

Fig. 12. Waveforms of shunt and series converter with sag in grid voltage,source voltage (250 V/div), load voltage (250 V/div), rectifier inductor current(5 A/div), grid current (5 A/div), X-axis 10 ms/div

Fig. 13. Waveforms of shunt and series converter with swell in grid voltage,source voltage (250 V/div), load voltage (250 V/div), rectifier inductor current(5 A/div), grid current (5 A/div), X-axis 10 ms/div

voltage and load current for normal value of grid voltage, sagand swell in grid voltage. Power factor of the grid supply, andefficiency of DVR are evaluated for normal, sag and swell ingrid voltage and shown in table IV. The power factor of gridsupply is almost unity in all the three cases.

VI. CONCLUSIONS

Power quality problems, such as sag and swell can haveadverse impact on the performance of critical loads. Thesepower quality problem can even cause the tripping of these

loads. To address this issues, a Dynamic Voltage Restorerwith no energy storage requirement to compensates the voltageacross load is designed. H-bridge converters connected in backto back configuration with a common dc link capacitor is usedto compensate the load voltage against sag and swell. The loadvoltage is compensated by injecting a voltage in series withsupply voltage with the help of injection transformer connectedbetween supply and load. Various protection schemes are alsoincorporated in DVR. The performance of DVR is tested bydetailed simulation results in MATLAB. Simulation results areshown for voltage sag and swell with linear and nonlinearloads. To validate the simulation results, a 5 kVA laboratoryprototype of DVR is developed. From the experimental results,it is concluded that DVR is successfully able to compensatefor voltage sag and swell. Voltage regulation across the loadis found to be within ±2%.

ACKNOWLEDGMENT

This work is supported by Emerson Network Power IndiaPvt. Ltd.

REFERENCES

[1] M. H. J. Bollen, “Understanding Power Quality ProblemsVoltage Sagsand Interruptions,” New York, NY, USA: IEEE Press, 2000.

[2] A. Ghosh and G. Ledwich,“ Power Quality Enhancement Using CustomPower Devices,” London, U.K.: Kluwer, 2002,.

[3] Kasuni Perera, , Daniel Salomonsson, Arulampalam Atputharajah, andSanath Alahakoon “ Automated Control Technique for a Single PhaseDynamic Voltage Restorer” in Proc. IEEE Int. Conf. Inform. Auto.(ICIA-2006 , no. 1, pp. 63-68, 2006.

[4] J. D. Nielsen and Frede Blaabjerg “A detailed comparison of systemtopologies for dynamic voltage restorers,” IEEE Trans. Ind. Appl., vol.41, no. 5, pp. 1272-1280, 2005.

[5] D. Mahinda Vilathgamuwa, A. A. D. Ranjith Perera, and S. S. Choi, “,Voltage Sag Compensation With Energy Optimized Dynamic VoltageRestorer,” IEEE Trans. Power Del., vol. 18, no.3, July. 2003

[6] S. S. Choi, J. D. Li, and D. M. Vilathgamuwa “A generalizedvoltage compensation strategy for mitigating the impacts of voltagesags/swells,” IEEE Trans. Power Del., vol. 20, no. 3, pp. 22892297,Jul. 2005.

[7] M. Rostami “Employing of Minimum Active Power Injection Strategyto Compensate Voltage Sag by DVR,” Majlesi Journal of ElectricalEngineering5.,no. 3, 2011.

[8] H.K. Al-Hadidi and A. M. Gole, “Minimum power operation of cascadeinverter based dynamic voltage restorer,” in Proc. 3rd Inst. Elect. Eng.Int. Conf. PEMD, pp. 301-306, Mar. 2006.

[9] Abdul Mannan Rauf and Vinod Khadkikar, “An Enhanced Voltage SagCompensation Scheme for Dynamic Voltage Restorer,” IEEE Trans. Ind.Eletrons., vol. 62, no.5, May. 2015

[10] Ebrahim Babaei, Mohammad Farhadi Kangarlu, Mehran Sabahi, “Dy-namic voltage restorer based on multilevel inverter with adjustable dc-link voltage,” IET Power Eletrons., July. 2013

[11] Jos M. Lozano, Juan M. Ramirez and Rosa Elvira Correa,“A noveldynamic voltage restorer based on matrix converters,” in Proc. ModernElect. Power Syst. (MEPS) Symp., pp. 1-7, Sept. 2010.