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Paper accepted for presentation at PPT 20012001 IEEE Porto Power Tech ConferenceIO t h 131h September, Porto, Portugal

COMPENSATION OF DISTRIBUTION SYSTEMVOLTAGE SAG BY DVR AND D-STATCOMM.H. HaqueSchool of Electrical and Electronic EngineeringNanyang Technological UniversitySingapore

ABSTRACTThis paper describes the techniques of correcting the supplyvoltage sag in a distribution system by two powerelectronics based devices called DVR and D-STATCOM. ADVR injects a voltage in series with the system voltage anda D-STATCOM injects a current into the system to correctthe voltage sag. The steady state performance of both DVRand D-STATCOM is determined and compared for variousvalues of voltage sag, system fault level and load level. Theminimum apparent power injection required to correct agiven voltage sag by these devices is also determined andcompared. The maximum voltage sag that can be correctedwithout injecting any active power into the system is alsodetermined. Simulation results indicated that a DVR cancorrect a voltage sag with much less injected apparent powercompared to that of a D-STATCOM.

Keywords: D-STATCOM, DVR, power quality, voltage sag.

I INTRODUCTIONVoltage magnitude is one of the major factors thatdetermines the quality of power supply. Loads atdistribution level are usually subject to frequent voltage sagsdue to various reasons. Voltage sags are highly undesirablefor some sensitive loads, especially in high-tech industries.It is a challenging task to correct the voltage sag so that thedesired load voltage magnitude can be maintained duringthe voltage disturbances.Dynamic voltage restorer (DVR) or distribution STATCOM(D-STATCOM) can be used to correct the voltage sag atdistribution level [ I , 2, 3, 41. A DVR is a series device thatgenerates an ac voltage and injects it in series with thesupply voltage through an injection transformer tocompensate the voltage sag. The injected voltage and load

current determine the power injection of the DVR. On theother hand, a D-STATCOM is a shunt device that generatesan ac voltage, which in turn causes a current injection intothe system through a shunt transformer. The load voltagean d injected current determine the power injection of the D-STATCOM. For lower voltage sags, the load voltagemagnitude can be corrected by injecting only reactive powerinto the system [ 5 ] . However, for higher voltage sags,injection of active power, in addition to reactive power, isessential to correct the voltage magnitude. Note that bothDVR and D-STATCOM are capable of generating orabsorbing reactive power but the active power injection ofthe device must be provided by an external energy source orenergy storage system.The response time of both DVD and D-STATCOM is veryshort and is limited by the power electronics devices and thevoltage sag detection time. The expected response time isabout 25 ms [6], and which is much less than some of thetraditional methods of voltage correction such as tap-changing transformers. For simplicity, only the steady stateperformance of DVR and D-STATCOM is determined andcompared in this study.This paper investigates the steady state performance of aDVR and D-STATCOM when applied to correct the supplyvoltage sag in a distribution system. The maximum voltagesag that can be corrected without injecting any active powerinto the system is also determined. The minimum apparentpower injection required to correct a given voltage sag isalso calculated. The performance of both DVR and D-STATCOM for various values of voltage sag, system faultlevel and load level is also determined and compared.

I1 VOLTAGE SAG CORRECTION BY A DVRThe schematic diagram of a typical DVR is shown in Fig. 1.The circuit on left hand side of the DVR represents theThevenin equivalent circuit of the system. The system

0-7803-7139-9/01/$10.0002001 IEEE

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impedance (Zth= Rlh+j&) depends on the fault level of theload bus. When the system voltage (v,,) drops, the DVRinjects a series voltage VDVR through the injectiontransformer so that the desired load voltage magnitude VLcan be maintained. The series injected voltage of the DVRcan be written asvDVR = v L + ' r h l L -vrh (1)Here IL s the load current and is given by

PL+JQLI , = ( v, JWhen V L is considered as a reference, eqn. (1) can berewritten asV,,LCZ = V,LO +Z,,I,L(P -e) - h ~ 6 (3 )Here a, p and 6 are the angle of V D V R , h and Vth,respectively, and 8 is the load power factor angle( 0 = tan-'(Q, / P L ) ) .The complex power injection of theDVR can be written as' D V R = vDVR'*, (4)

I

Fig. 1 Schematic diagram of a DVRIt may be mentioned here that when the injected voltageVoVR s kept in quadrature with IL, no active power injectionby the DVR is required to correct the voltage. It requires theinjection of only reactive power and the DVR itself iscapable of generating the reactive power. Note that VDVRcan be kept in quadrature with IL only up to a certain valueof voltage sag and beyond which the qradrature relationshipcannot be maintained to correct the voltage sag. For such acase, injection of active power into the system is essential.The injected active power must be provided by the energystorage system of the DVR. On the other hand, when themagnitude of the DVR injected voltage is minimized, thedesired voltage correction can be achieved with minimumapparent power injection into the system. This aspect ofvoltage correction is also very important because it

minimizes the size of the injection transformer. The voltagecorrection by a DVR for the zero active power injection(ZAPI) and minimum apparent power injection (MAPI)cases is discussed in the following.

A Zero Active Power Injection (ZAPI)As mentioned earlier that, when the phase angle differencebetween VDVR and IL is kept at nI2, no active powerinjection into the system is required to correct the voltagesag. In this case, the angle a of the injected voltage V D V Rcan be written as

From eqn. (3), the angle a can also be expressed as

a = tan-' Z,I, sin(p -8)- V,, sin6[v,+z, , os(p - e ) -v,, os6By equating eqns. (5) and (6), the phase angle 6 of Vth canbe expressed as

where cl = V, + Z,I, C O S @ -8)c2= Z,I, tan8 sin@ - 8)c, =qhan8

and y = tan-'(c, / c 3 )It may be mentioned here that, for a feasible value of 6, thecondition

must be satisfied. After some mathematical manipulations,eqn. (8) can be expressed asv, 2 (v, ase + ZJ, cosp ) (9 )The right hand side of eqn. (9) depends on the load current,load voltage and system impedance. When the magnitude ofthe system or Thevenin equivalent voltage (Vth) satisfieseqn. (9), the desired voltage correction can be achievedwithout injecting any active power into the system. In thiscase, the complex voltage injection of the DVR can beobtained from eqn. (3 ) with a value of 6 found from eqn. (7).

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Once the value of VDVRs known, the complex powerinjection of the DVR can be found from eqns. (4) and it willhave only the reactive component.

B Minimum Apparent Power Injection (MAPI)For a given load current, the magnitude of the injectedapparent power of the DVR depends on the magnitude ofthe injected voltage. From eqn. ( 3 ) , the magnitude of theinjected voltage can be expressed as

Thus for the minimum voltage magnitude (or minimumapparent power) injection, the condition

must be satisfied. Eqn. (1 1) can now be solved for the valueof 6 and is given by

1, ,I , sin@ -6)v, + z,, , cos(p -e)= tan-'Once the value of 6 is known, the injected complex voltageand apparent power of the DVR can again be obtained fromeqns. ( 3 ) and (4), respectively.

I11 VOL TAGE SAG COR RECT IONBY A D-STATCOMThe schematic diagram of a D-STATCOM is shown in Fig.2. In this diagram, the shunt injected current Ish corrects thevoltage sag by adjusting the voltage drop across the systemimpedance &h. The value of Ish can be controlled byadjusting the output voltage of the converter. The shuntinjected current Ish can be written as

or, IShLq I LL -8 - - -L (6 -p )+ -L -pfh VL (13)' f h ' f h

The complex power injection of the D-STATCOM can beexpressed as' s h = L 1 ; h (14)It may be mentioned here that the effectiveness of the D-STATCOM in correcting voltage sag depends on the valueof Zh r fault level of the load bus. When the shunt injected

current Ish is kept in quadrature with VL, he desired voltagecorrection can again' be achieved without injecting anyactive power into the system. On the other hand, when thevalue of Ish is minimized, the same voltage correction can beachieved with minimum apparent power injection into thesystem. The voltage sag correction by a D-STATCOM usingthe above two techniques is discussed in the following.

D-STATCOM

Fb,[onverter-. .-

Fig. 2 Schematic diagram of a D-STATCOM

A Zero Active Power Injection (ZAPI)In this case, the D-STATCOM is not injecting any activepower into the system. Thus the entire load active power(P,)must be provided by the Thevenin equivalent of thesystem. The active power flow through the Theveninimpedance ofFig. 2(at load side) can be written as [7]

?h ' LPL=-' t h

From eqn. (1 5), the angle 6 can be expressed as

For a feasible value of 6 , the conditionIIcosp+-L ' L

K h F h Lmust be satisfied. The above constraint can be rewritten as

Thus, when the system voltage magnitude satisfies eqn.(18) , the D-STATCOM can correct the voltage sag withoutinjecting any active power into the system. For such a case,the injected complex current and apparent power of the D-STATCOM can easily be found from eqns. (13) and (14),respectively. Note that the injected apparent power will haveonly the reactive component.

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B Minimum Apparent Power Injection (MAPI)As mentioned earlier that, when the magnitude of theinjected current is minimized, the D-STATCOM can correctthe voltage sag with minimum apparent power injection intothe system. Thus the condition of minimum apparent powerinjection is

d bAn analytical expression of Is h can readily be obtained fromeqn. (13), and the solution of eqn. (19) provides thefollowing

Thus for a given load, the value of 6 can easily be foundfrom eqn. (20). Once the value of 6 is known, the complexcurrent and apparent power injection of the D-STATCOMcan again be obtained from eqns. (13) and (14), respectively.

IV SIMULATION RESULTSThe simple system of Fig. 3 is used to demonstrate thesteady state performance of a DVR and D-STATCOM whenapplied to correct voltage sags. It is considered that the loadof the system is 1.0 pu at 0.8 lagging power factor and thefault level of the load bus is 10 pu with a X/R ratio of 2. It isassumed that the voltage magnitude of the load bus is to bemaintained at 1 O pu during the voltage sag conditions. Thevarious results found for a DVR and D-STATCOM tosatisfy the above criterion are briefly summarized in thefollowing.

DVR orD-STATCOMRest of thesystem

Fig. 3 A simple system with a DVR or D-STATCOM

A DVRFirst a DVR is used to correct the system voltage sags. Thevariation of apparent power injection (Sjnj )against thevoltage sag to maintain the load voltage magnitude of 1O pufor both ZAPI and MAPI cases is shown in Fig. 4.It can beobserved in Fig. 4 that, for the MAPI case, Sinj varieslinearly with the voltage sag. However, for the ZAPI case,

Sinj increases rapidly with voltage sag. For this system it isfound that the maximum voltage sag that can be correctedby the DVR without injecting any active power into thesystem is 0.236 pu. It can also be observed in Fig. 4that, forlower voltage sags (< 0.236 pu), the voltage correctionwithout injecting any active power is achieved at an expenseof higher apparent power injection into the system(compared to the MAPI case). The injected apparent powerof the DVR for various values of fault level, load level, loadpower factor and X/R ratio of system impedance is alsostudied in detail. It is found that, for a given load level, theminimum apparent power injection of the DVR isindependent of system fault level, X/R ratio of systemimpedance and load power factor. However, Sinj for theMAPI case depends on the load level as can be seen in Fig.5. Fig. 5 indicates that, for a given voltage sag, the injectedapparent power of the DVR increases as the load level isincreased.

0.6I

0 0 1 0 2 0 3 0 4 0 5Voltage sag, pu

Fig. 4 Variation of injected apparent power of the DVRagainst voltage sag.a: ZAPI case; b: MAPI case.

0 5

5Voltage sag , P"

Fig. 5 Minimum apparent power injection of the DVR forvarious load levels (S).

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B D-STATCOMFig. 6 shows the variation of injected apparent power of theD-STATCOM against the voltage sag. Results for bothZAPI and MAPI cases are shown in the Figure. It can beobserved in Fig. 6 that, S,nj is again varies linearly withvoltage sag for the MAPI case. However, for the ZAPI case,Sinj s slightly higher than that of MAPI case. For this systemthe D-STATCOM can correct the voltage sags of up to0.563 pu without injecting any active power into the systemand which is much higher than that found for the DVR (only0.236 pu). It is also found that Sinj, for the MAPI case, isindependent of load level, load power factor andX/R ratio ofsystem impedance. However, Sinj is very sensitive to thesystem fault level as can be seen in Fig. 7. It can beobserved in Fig. 7 that Sinj increases with the increase ofsystem fault level (FL).

Voltage sag. pu

Fig.6 Variation of injected apparent power of the D-STATCOM against voltage sag.a: ZAPI case; b: MAPI case.

Voltage sag. P

Fig. 7 Minimum apparent power injection of the D-STATCOM for various fault levels (FL).

V CONCLUSIONSThe steady state performance of a DVR and D-STATCOMto correct the supply voltage sags is determined andcompared in this paper. Techniques of correcting the voltagesags with zero active power injection as well as minimumapparent power injection are also discussed. Simulationresults on a simple system indicated that the amount ofapparent power injection required by a D-STATCOM tocorrect a given voltage sag is much higher than that of aDVR. The main reason of that is a DVR corrects the voltagesag only on the downstream side, whereas a D-STATCOMcorrects voltage on both sides. It is also found that a D-STATCOM can correct much higher voltage sags withoutinjecting any active power into the system compared to thatof a DVR. The minimum apparent power injection of the D-STATCOM is found to be very insensitive to load level butsensitive to the system fault level. On the other hand, theminimum apparent power injection of the DVR is observedto be very insensitive to the fault level but sensitive to thesystem load level. In terms of minimum apparent powerinjection or size of the coupling transformer, theperformance of a DVR is found to be superior to a D-STATCOM.

VI REFERENCESWoodley, N.H., Morgan, L, and Sindaram, A.,Experience with an inverter-based dynamic voltagerestorer, IEEE Trans. on Power Delivery, Vol. 14,NO.3, 1999, pp. 1181-1186.Chen, S . and Joos, G., Series and shunt active powerconditioners for compensating distribution systemfaults, Proc. of the Canadian Conf. on ECE, Vol. 2,2000, pp. 1182-1 186.Jenkins, N. , Power electronics applied to thedistribution systems, IEE Colloquium, Flexible ACTransmission Systems, Ref. No. 19981500, 1998, pp.311 -317.Song, Y.H. and Johns, A.T., Flexible ac transmissionsystems (FACTS), IEE, 1999.Choi, S.S. , Li, B.H. and Vilathgamuwa, Dynamicvoltage restorer with minimum energy injection,IEEE Trans. on Power Systems, Vol. 15, No. 1, 2000,Weissbach, R.S., Karady, G.G. and Farmer, R.G.,Dynamic voltage compensation on distributionfeeders using flywheel energy storage, IEEE Trans.on PD, Vol. 14, No. 2, 1999, pp. 465-471.Saadat, H., Power system analysis, McGraw Hill,1999.

pp. 51-57.