idvr

Simulation Modelling Practice and Theory 14 (2006) 989–999

www.elsevier.com/locate/simpat

Inter-line dynamic voltage restorer controlusing a novel optimum energy consumption strategy

M.R. Banaei a,*, S.H. Hosseini b, G.B. Gharehpetian c

a Electrical Engineering Department, Faculty of Engineering, Azarbaijan University of Tarbiat Moallem, Tabriz, Iranb Center of Excellence for Mechatronics, University of Tabriz, Tabriz, Iran

c Electrical Engineering Department, Amirkabir University of Technology, Tehran, Iran

Received 18 August 2004; received in revised form 30 January 2006; accepted 14 February 2006Available online 24 July 2006

Abstract

To restore the load voltage, Dynamic Voltage Restorer (DVR), which is installed between the supply and a sensitiveload, should inject voltage and active power from DVR to the distribution system during voltage sag. Due to the energystorage capacity limitation of DC link, it is necessary to minimize energy injection from DVR.

The techniques of the supply voltage sag compensation in a distribution feeder are reviewed in this paper. Then a newdevice which is named Inter-line Dynamic Voltage Restorer (IDVR) is discussed. This device consists of two conventionalDVRs which are installed in two different distribution feeders and the DC link capacitor. DVR which is installed in LVfeeder operates in voltage sag compensation mode. A novel control technique minimizes the energy flow from DC linkcapacitor to this feeder. The DVR, which is installed in MV feeder, controls the voltage of DC link capacitor. The pro-posed device and its new control strategy have been modeled and simulated by PSCAD/EMTDC. The results verifythe effectiveness of IDVR and the suggested control method.� 2006 Elsevier B.V. All rights reserved.

Keywords: Power quality; Dynamic voltage restorer; Control strategies; Active power minimization

1. Introduction

One of the most important power quality issues is voltage sag because the increasing usage of voltage sen-sitivity devices has made industrials processes more susceptible to supply voltage sags [1]. Voltage sag maycause tripping and shutdown of sensitive and industrial equipments and miss-operation of ASD (AdjustableSpeed Drive) systems. Voltage sags of down to 70% are much more common than complete outages [2].Dynamic voltage restorer (DVR) with energy storage can be used to correct the voltage sags [3–6]. A DVRis basically a controlled voltage source installed between the supply and a critical and sensitive load. It injectsa voltage to the distribution system in order to compensate any disturbance affecting the load voltage. Thecompensation capacity of a particular DVR depends on the maximum voltage injection ability and the real

1569-190X/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.simpat.2006.06.001

* Corresponding author. Tel.: +98 914 3134864; fax: +98 411 3300819.E-mail addresses: [email protected], [email protected] (M.R. Banaei).

Fig. 1. (a) Power circuit of DVR in distribution system and (b) three phase four-wire DVR.

990 M.R. Banaei et al. / Simulation Modelling Practice and Theory 14 (2006) 989–999

power, which can be supplied by the DVR because when DVR restorers voltage disturbances, active powershould be injected from DVR to the distribution system. DVR could maintain load voltage unchanged duringany kind of faults, if the capability of energy storage of DVR were infinite [3]. Energy storage devices, such asbatteries or Super-conducting Magnetic Energy Storage Systems (SMES) are required to provide this activepower. Considering the energy limitations of these devices, it is necessary to minimize energy injection.

Power circuit of a DVR in a distribution system is shown in Fig. 1(a). The main function of a DVR is theprotection of sensitive loads from voltage sags coming from network. Therefore, the DVR is located onapproach of sensitive loads. If a fault occurs on other feeders, DVR inserts series voltage, Vdvr and compen-sates load voltage to pre fault value. Distribution systems commonly use a delta–star or a star–star trans-former. If delta–star transformer is used in distribution system, zero-sequence voltages will not propagatethrough the transformer when earth faults occur on the higher voltage level. Therefore, restoration of positivesequence and compensation of negative sequence voltage are necessary. As shown in Fig. 1(b) the main ele-ments of the three-phase four-wire DVR are the energy storage system, the voltage source converter, theLC filter and the coupling transformer. The AC side of converter is connected to the line through a LC filter.Due to switching, the high order harmonics of converter must be removed by small high pass filters (repre-sented by Ls, Cs in Fig. 1(b)). The Sinusoidal Pulse Width Modulation (SPWM) technique is commonly usedto control forced-commutated converters. This method has been used in this paper too.

The inter-line dynamic voltage restorer (IDVR) is similar to the inter-line power flow controller (IPFC)which should be used in transmission system. In this device several DVRs protecting sensitive loads in differentdistribution systems share a common DC link energy storage [7]. In the simplest case, we consider two differentvoltage distribution system protected by two DVR. Low voltage (6.6 kV) DVR operates in voltage sag mitiga-tion mode and uses the suggested minimal energy control method of the paper to inject minimum active powerfrom DC link capacitor. In the same time medium voltage (20 kV) DVR keeps the voltage of DC link capacitorconstant in order to control the active power flow from distribution system to DC link capacitor.

2. Conventional control strategies for DVR

Several control techniques have been proposed for voltage sag compensation such as pre-sag method [5], in-phase method [5] and minimal energy control [3].

M.R. Banaei et al. / Simulation Modelling Practice and Theory 14 (2006) 989–999 991

It must be said that the characteristics of the sensitive load determine the control method and the compen-sation strategy for the DVR. For example, the linear loads are not sensitive to phase angle jump and onlymagnitude of voltage is dominant. The control techniques should consider the limitations of the DVR suchas the voltage injection capability (converter and transformer rating) and energy storage system limitation.The second limitation means that the minimization of exchanged active power from common DC link tothe distribution system must be considered.

2.1. Pre-sag compensation strategy

The most of nonlinear loads such as thyristor-controlled loads, which use the supply voltage angle as a setpoint, are sensitive to phase jumps. To overcome this problem, this technique compensates the differencebetween the sagged and the pre-sag voltages by restoring the instantaneous voltages to the same phase andmagnitude as the nominal pre-sag voltages. The disadvantage is the capacity limitation of energy storagedevice for the injection of active power. Fig. 2 shows the phasor diagram of the pre-sag compensation strategy.

In this diagram the subscript i has been used for the ith phase. Vs1i, VL1i, Vdvr1i, IL1i, U1 and Bi represent theleft hand side voltage of DVR, the load voltage, the DVR injected voltage, the load current , load power angleand the phase angle of ith phase, respectively.

When a fault occurs in other lines of Fig. 1(a), the left hand side voltage of DVR, Vs1i drops and the DVRinjects a series voltage, Vdvr1i through the transformer as

V dvr1i ¼ V L1i � V s1i; i ¼ 1; 2; 3 ð1Þ

2.2. In-phase compensation

In this control strategy the restored voltage, Vdvr1i, is in-phase with the left hand side voltage of DVR, Vs1i,regardless of the load current and the pre-fault voltage. The phasor diagram of this case is shown in Fig. 3.Assuming VL1i,pu = 1, Vdvr1i,pu can be obtained from

V dvr1;pu ¼ 1� V s1;pu ð2ÞThe advantage of this method is injection of voltage with minimum magnitude. Therefore, for a given loadcurrent and voltage sag the apparent power of DVR is minimized. The injected active power from energy stor-age to load for balanced sag is determined by the following equation:

P dvr1 ¼ 3ðV L1 � V s1Þ � IL1 � cosðU1Þ ð3Þ

Fig. 2. Phasor diagram of pre-sag control.

Fig. 3. Phasor diagram of in-phase control strategy.

Fig. 4. Phasor diagram of minimal energy control.


2.3. Minimal energy strategy

For a given load and a balanced sag if voltage phasor, Vdvr1, is perpendicular to the load current, IL1, thenthe active power injection is not required to restore the voltage by the DVR [3]. Fig. 4 shows the phasor dia-gram for this control strategy. In this diagram, d,a are the angle of VL1 and Vdvr1, respectively. In this case, acan be obtained from

a ¼ p2� U1 þ d ð4Þ

and the d is calculated as follows:

d ¼ U1 � cos�1 V L1: cosðU1ÞV s1

ð5Þ

If the supply voltage parameters satisfy the following condition then the d can be determined.

V L1 � cosðU1Þ 6 V s1 ð6Þ

Inequality (6) means that the level of voltage sag is shallow sag. Therefore, injected active power of DVR isequal to zero and the optimum a is obtained from (4). If inequality (6) is not satisfied then level of voltage sagwill be deep sag and active injected power is not equal to zero.

2.4. Suggested control strategy

As mentioned earlier, for shallow sag injected power, Pdvr is zero. However, for deep sag Pdvr1 is not zero.Considering Fig. 4, for a given Vs1 and load current (e.g., cosU1 = 0.8 and IL1,pu = 1) increase of d (forVL1,pu = 1) results increase of Vdvr1 (i.e., both magnitude and phase angle). As a result, the injected activepower, Pdvr1, can be easily determined for each Vs1 and known load. Considering these points the relationshipbetween, Pdvr1, and the injected voltage, Vdvr1 is determined and illustrated in Fig. 5(a). The parameter of thiscurve is a balanced three-phase voltage sag, Vsag,pu, for a known load (e.g. here cosU1 = 0.8 and IL1,pu = 1). Itis obvious that for a 0.2 pu voltage sag the minimum value of Pdvr1 can be equal to zero. While, for the shallowsag (less than 0.2 pu) minimum value of Pdvr1 is negative and for deep sags (more than 0.2 pu) minimum valueof Pdvr1 is positive.

In the in-phase compensation strategy the apparent power of DVR is small and injected active power ofDVR is considerable. But in the minimal energy strategy injected active power is minimum and the apparentpower of DVR is considerable.

In this paper, the proposed strategy is the minimization of the injected active power for different sags. Forexample, for a DVR with a apparent power of 0.6 pu and constant load of IL1,pu = 1 and cosU1 = 0.8, theinjected voltage must be equal to 0.6 pu. Now using the Fig. 5(a), it is possible to determine the minimumof Pdvr1 if Vdvr1=0.6 pu and Vsag,pu is between 0.05 pu and 0.3 pu. The result of calculations are given inFig. 5(b). In this figure, it is obvious that the negative value of minimum injected active power is assumed

Fig. 5. (a) Injected active OWC1 vs. injected voltage and (b) minimum injected active power vs. voltage sag (apparent power ofDVRO.6 pu).


to be zero and the positive value of minimum injected active power can be approximated using a curve fittingas follows:

P dvr1;pu ¼ ðV sag;pu � 0:2Þ ð7ÞIt must be said that the negative Pdvr1 gives no economical advantage because DVR requires additional stor-age facility for power absorption [3]. As a result, this control strategy, as shown in Fig. 6, makes Pdvr1 zeroduring shallow sag. But for deep sag and using Fig. 5(b), this method can control and minimize the Pdvr1

of DVR for a given apparent power. The voltage sags considered earlier were balance three-phase. For unbal-ance voltage sages, the three-phase instantaneous values of Vs1, i.e., (Vs1a,Vs1b and Vs1c) generate negative andpositive sequence voltages.

The synchronous d–q reference frame algorithm is applied to extract the reference voltage for the DVR i.e.,Vdvr1a,ref, Vdvr1b,ref and Vdvr1c,ref from the measured Vs1 (or Vs1a, Vs1b and Vs1c) as shown in Fig. 6.

Fig. 6. The suggested control strategy for DVR.


For the unbalance Vs1, which contain negative sequence components, the transformation to the d–q-axesresults in [8]

V s1d

V s1q

264

375 ¼

ffiffiffi2

3

rT

V s1a

V s1b

V s1c

264

375;

V s1d

V s1q

264

375 ¼

V s1d;dc

V s1q;dc

264

375þ

V s1d;ac

V s1q;ac

264

375 ð8Þ

where T is

T ¼cosðxtÞ cosðxt � 2p=3Þ cosðxt þ 2p=3ÞsinðxtÞ sinðxt � 2p=3Þ sinðxt þ 2p=3Þ

� �ð9Þ

The fundamental frequency components of Vs1 are transformed into Vs1d,dc,Vs1q,dc (DC quantities) and neg-ative sequence components are converted to AC components, Vs1d,ac and Vs1q,ac.

As shown in Fig. 6 the high pass filter (HPF) has been used to block DC quantities and the differencebetween the per unit magnitude of load voltage, VL1,pu has been calculated by (10) and the output of PI con-troller is added to the d-axis AC component of voltage.

V L1;pu ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiV 2

L1d þ V 2L1q

q=V bas1 ð10Þ

In this equationVL1d and VL1q are d–q components of load voltage and Vbas1 is base voltage.In the case of full compensation of load voltage for a unbalance voltage sag, DVR injects DC component of

active power Pdvr1,dc which restores fundamental components of voltage sag and in the same time injects ACcomponent of active power, Pdvr1,ac which restores negative sequence components of voltage sag.

The DC component of active power, Pdvr1,dc can be calculated as follows:

P dvr1;dc ¼ V dvr1d;dci1d þ V dvr1q;dci1q ð11Þwhere Vdvr1d,dc and Vdvr1q,dc are

V dvr1d;dc ¼ V L1d � V s1d;dc

V dvr1q;dc ¼ V L1q � V s1q;dc

ð12Þ

The per unit magnitude of fundamental components of voltage sags, Vs1,pu can be obtained by the follow-ing equation:

V s1;pu ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiV 2

s1d;dc þ V 2s1q;dc

q=V bas1 ð13Þ

As shown in Fig. 6, the per unit magnitude of fundamental components of voltage sag, Vsag,pu, has beenapplied to nonlinear function, shown in Fig. 5(b) and the output of nonlinear function is the reference DCcomponent of injected active power, Pdvr1,dc,ref.

The difference between reference values, Pdvr1,dc,ref and the feedback value, Pdvr1,dc passes through the PIcontroller and the output of the PI controller is added to the q axis AC component of Vs1.

Fig. 7 shows the inner loop controller of the gate pulse generator for this DVR.

Fig. 7. The gate pulses generator for DVR.


3. Inter-line dynamic voltage restorer (IDVR)

The equivalent circuit on the left hand side of DVR shown in Fig. 1(a) can be presented by thevenin voltagesource, Vth1and thevenin impedance, Zth1, as shown in Fig. 8.

When a fault occurs in other lines, the left hand side voltage of DVR, i.e., Vs1 drops and the DVR injects aseries voltage, Vdvr1 through the transformer. The active power flow is from energy storage to the distributionsystem.

In the general form inter-line dynamic voltage restorer uses several DVRs in different distribution feedersand all DVRs have a common DC voltage link. An IDVR with two DVRs is shown in Fig. 9.

The voltages of feeder 1 and 2 are 6.6 kV and 20 kV, respectively. A voltage sag in feeder 1 may not besensed in feeder 2 and therefore they can be supposed as two independent distribution feeders [7]. In this casethe active power flow is from common DC link to one feeder and the other DVR which is in normal conditioninjects active power from the other feeder to the common DC link.

Assume that the voltage sag is in the feeder 1 and to restore the voltage, the DVR1 must inject the activepower, Pdvr1. The other DVR, i.e. DVR2 must inject the active power, Pdvr2 from feeder 2 to the system. Inthis case we have

P dvr2 ¼ P dvr1 þ P loss ð14Þwhere Ploss is the power losses of two converters. The load 2 (in feeder 2) is not sensitive to the voltage phaseangle. The control strategy of DVR2 is active power injection to the common DC link and in the same time,load voltage magnitude compensation at VL2,pu = 1, as shown in Fig. 10.

In this figure Vs2, VL2, Vdvr2, IL2, U2 and c represent the left hand side voltage of DVR2, the load 2 voltage,the DVR2 injected voltage, the load 2 current, load 2 power angle and power injected phase angle respectively.The harmonics of common DC link voltage are neglected. As a result the harmonics of feeder 2 can beneglected too.

Fig. 8. Equivalent circuit of power system.

Fig. 9. The simplest IDVR with two converter.

Fig. 10. Phasor diagram of DVR2.


The DVR2 injected active power can be controlled by DC link voltage control at a constant value, i.e.Vc = cte. As shown in Fig. 11, Pdvr2 can be controlled by comparing Vc and the reference DC link voltage Vcref

and applying voltage error to the PI controller.The synchronous d–q reference frame algorithm is applied to extract the reference voltage for the DVR2

i.e., (Vdvr2a,ref,Vdvr2b,ref and Vdvr2c,ref) from the measured Vl2 (or Vl2a, Vl2b and Vl2c) as shown in Fig. 11.The difference between the load 2 voltage magnitude, VL2,pu which can be calculated by (15) and the ref-

erence load voltage, VL2,pu,ref, which is normally 1pu, passes through the PI controller and the output ofthe PI controller is added to the d-axis AC component of voltage.

V L2;pu ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiV 2

L2d þ V 2L2q

q=V bas2 ð15Þ

Fig. 11. The IDVR with two converter.

Table 1System parameters

Parameter Value

Load 1 voltage 6.6 kVLoad 1, 2 apparent power 250 kV ALoad 1, 2 power factor 0.8Load 2 voltage 20 kVBase power 250 kV ACommon DC link voltage 20 kVCommon DC link capacitor 14,000 lFLs1, Ls2 5 mHRs1, Rs2 0.07 XCs1, Cs2 50 lF

Fig. 12. (a) Left hand side voltage of DVR1, Vs1, (b) DVR1 injected voltage, Vdvrl, (c) Load 1 voltage, VL1, (d) Load 2 voltage, VL2, (e)DC component of DVR1 injected active power Pdvrl, (f) common DC link voltage for balance deep sag (t = 0.1 till t = 0.2) and shallow sag(t = 0.25 till t = 3.5).



DVR2 injected active power is obtained from

P dvr2 ¼ 3V s2IL2 cosðU2 � cÞ � 3V L2IL2 cosðU2Þ ð16Þ

The per unit value of DVR2 injected active power can be rewritten in the following form:

P dvr2;pu ¼ cosðU2 � cÞ � cosðU2Þ ð17ÞIf the power factor and apparent power of both feeders were the same then from (14) and (17) c can be

obtained as follows:

c ¼ U2 � cos�1½ðP dvr1;pu þ P loss;puÞ þ cosðU2Þ� ð18ÞUsing (14) and (18) in case of cmax = U2 the maximum of DVR2 injected active power can be obtained from

P dvr2;pu;max ¼ 1� cosðU2Þ ð19Þ

4. Simulation results

To verify the efficiency of suggested control strategy for the IDVR system shown in Fig. 9 simulations car-ried out by the PSCAD/EMTDC [9].

The system parameters are listed in Table 1.It is assumed that the voltage magnitude of the load bus in feeder 1, 2 must be set at 1 pu during the voltage

sag conditions in feeder 1. The results of the most important simulations are presented in Fig. 12.Left hand side voltage of DVR1, injected voltage of DVR1, load 1 voltage, load 2 voltage, injected active

power of DVR2 and common DC-link voltage have been shown in Fig. 12(a)–(f), respectively. Considering(7), (14) and (19), the maximum balanced voltage sag which can be mitigated by DVR1 is 0.36 pu in minimumactive power control mode.

Left hand side voltage of DVR1, injected voltage of DVR1, load 1 voltage, load 2 voltage, injected activepower of DVR1 and common DC-link voltage have been shown in Fig. 12(a)–(f), respectively for balancedthree phase fault.

Fig. 12(a) shows that 0.3 pu deep voltage sag has occurred from t = 0.1 till t = 0.2 and 0.2 pu shallow saghas occured from t = 0.25 till t = 0.35. In this case minimum component of DVR1 injected active power fordeep sag is 25 kW or 0.1 pu and for the shallow sag 0 kW, respectively.

These simulation results indicate that based on the suggested control strategy, DVR1 consumes zero activepower during shallow sag and minimizes active power injection during deep sag. DVR2 active power injectionis controlled by keeping constant the common DC link voltage with a good dynamic response and in the sametime the DVR1 injected voltage is equal or less than 0.6 pu (according to Fig. 5(a)). The magnitude of loadsvoltage in the both feeders is set to 1 pu.

5. Conclusion

In order to compensate voltage sag it is possible to use dynamic voltage restorer (DVR) in distribution sys-tem for a sensitive load. Due to the limit of energy storage capacity of DC link, it is necessary to minimizeenergy injection from DVR. In this paper the control strategies for the compensation of the supply voltagesag is reviewed presented. In addition, a new concept of restoration strategy is proposed to inject minimumenergy for unbalance sags. This control strategy has been applied to an inter-line dynamic voltage Restorer(IDVR) which consists of two DVRs with a common DC link capacitor. Simulation results indicate thatthe DVR1 consumes zero active power during shallow sag and minimizes the active power injection duringdeep sag. In the same time the other DVR injects energy to common DC-link by keeping the voltage of com-mon DC-link constant. The results verify the capability of proposed control strategy.

References

[1] S.S. Choi, B.H. Li, D.M. Vilathgamuwa, Design and analysis of the inverter-side filter used the dynamic voltage restorer, IEEETransactions on Power Delivery 17 (3) (2002).


[2] X. Lei, D. Retzmann, M. Weinhold, Improvement of power quality with advanced power electronic equipment, in: Electric UtilityDeregulation and Restructuring and Power Technologies, 4–7 April, IEEE (2000).

[3] Il-Yop Chung, Dong-Jun Won, Sang-Young Park, Seung-Il Moon, Jong-Keun Park, The DC link energy control method in dynamicvoltage restorer system, Electrical Power and Energy Systems 25 (2003) 525–531.

[4] Hongfa Ding, Shuangyan Shu, Xianzhong Duan, Gao Jun, A novel dynamic voltage restorer and its unbalanced control strategy basedon space vector PWM, Electrical Power and Energy Systems 24 (2002) 693–699.

[5] V.K. Ramachandaramurthy, C. Fitzer, A. Arulampalam, C. Zhan, M. Barnes, N. Jenkins, Control of a battery supported dynamicvoltage restorer, IEE Proc.-Gener. Transm. Distrib 149 (5) (2002).

[6] Changjiang Zhan, Atputharajah Arulampalam, Nicholas Jenkins, Four-wire dynamic voltage restorer based on a three-dimensionalvoltage space vector PWM algorithm, IEEE Transactions on Power Electronics 18 (4) (2003).

[7] H.M. Wijekoon, D.M. Vilathgamuwa, S.S. Choi, Interline dynamic voltage restorer: an economical way to improve interline powerquality, IEE Proceedings – C Generation Transmission and Distribution 150 (5) (2003).

[8] Jianjun Gu, Dianguo Xu, Hankui Liu, Maozhong Gong, Unified power quality conditioner (UPQC): the principle, control andapplication, in: Power Conversion Conference, PCC-Osaka, IEEE 1 (2002) 80–85.

[9] Manitoba HVDC Research Center, PSCAD/EMTDC: Electromagnetic transients program including dc systems, 1994.

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