A new technique for a static var compensator usingthe EMTP modeling environment

Nikolaos Athanasiadis

Abstract In this paper, a technique based on EMTP(electromagnetic transients program) is used to simulate astatic var compensator (SVC) model. The validation of thetechnique used in the paper demonstrates the effectivenessof the SVC controller. The firing method is capable of es-tablishing accurate firing, similar to that achieved using thephase locked loop method; consequently it can be used asan alternative to this method for static var compensators.

Keywords Static var compensator, Flexible AC transmis-sion systems, Phase locked loop method, Control system,Validation

List of symbolsU r.m.s. voltage across the TCR (thyristor controlled

reactor)XL fundamental frequency reactance on the TCR branchesr conduction angle of the thyristorsa firing angle of thyristorsIr fundamental reactive current on the TCR branches

1IntroductionThe static var compensator (SVC) is a major FACTScomponent. This device has been used in transmissionsystems in order to reduce temporary overvoltages andcontrol the reactive power flow [1–6]. SVCs are charac-terised by fast responses, high reliability and flexibility.The short response time of such systems make them ide-ally suited for applications in which fast control of tran-sient stability is required. The most prevalent type of SVCis the thyristor control reactor in parallel with a fixedcapacitor (TCR-FC).

This paper deals with the details of a method for con-trolling the SVC in a power network and the presentation

of results under transient system conditions. For the SVCmodel, a control strategy using voltage regulation isadopted.

For the SVC model presented in this paper, useful in-formation is taken from previously described publications[8, 10]. The modeling technique for the control system hasmany similarities with that in [8]. However, despite thecommon objective of both techniques, which is to derivethe required susceptance through the regulation of thevoltage, in the technique described in this paper thefunction of the voltage controller is to derive the appro-priate conduction angle by using a non-linear relationship[9], whereas in [8], the aim was to derive the firing angleand to switch on/off capacitor banks according to thevalue of the firing angle. These different approaches arebasically due to the different firing systems that are usedto generate the pulses for the thyristors. In the workreported here, AC filters are installed to absorb harmon-ics. These filters are generally shunt-connected branchesthat present a low impedance path to ground forharmonics. They also appear as large capacitors at thefundamental frequency, providing the reactive powercompensation needed.

From the research work that will be analysed in thispaper, comparison of the control and firing methods pro-posed with other control methods and firing systems, suchas the phase locked loop (PLL) method, will be made.Similar results to those of the PLL and the control methodsproposed in [8] and [10] are obtained in this paper usingthe EMTDC modeling package, and case studies are ana-lytically investigated and compared with those using thecontrol and firing methods proposed in this paper. Theresults presented using the proposed firing method dem-onstrate that it is capable of establishing accurate firingtiming similar to that obtained using the PLL method. Thesignificance of the paper is that using the proposed firingtechnique the problem that is caused by using the PLLmethod, as reported in [11], which is that the dynamics ofthe PLL require a very small time step which demands ahigh CPU time, can be avoided.

2Static var compensator (SVC) modelFor a SVC model, it is conventional for the controlledvariable to be the voltage at the point of connection of theSVC to the rest of the power system. During faults, thevoltage tends to decrease rapidly and the control systemtries to decrease the conduction angle, consequently de-creasing the admittance of the TCR.

Electrical Engineering 84 (2002) 85–89 � Springer-Verlag 2002

DOI 10.1007/s002020100105

85

Received: 27 June 2001/Accepted: 16 August 2001

N. AthanasiadisEnergy Consultant,Atlantis Research Organization,Leonidou 23 Xarilaou,54250, Thessaloniki, Greecee-mail: nathanasiadis@yahoo.comTel: +30-944156623

The author gratefully acknowledges Prof. J. McDonald, who hasbeen involved with the development of the modeling techniqueat Strathclyde University, Glasgow UK. The financial supportof the Rolls Royce Company is also appreciated.

Most control systems will generate a required reactivecurrent value which is converted to the conduction angleusing a non-linear interface.

Figure 1 shows a simplified diagram of a SVC voltageregulator. The main objective of this regulator is tomaintain the voltage controlled by a PI controller, whichsends an admittance order to a pulse generating unit. Thepulse generating unit (also called the firing system), issuesfiring pulses that permit a thyristor-controlled reactor tooperate in a thyristor phase-controlled mode, i.e. eachthyristor is fired once per cycle to control the effectivereactance.

Also in a SVC model, series tuned filters are used for thelow-order harmonics. A high-pass filter is used to elimi-nate the higher order harmonics. The filters appear as largecapacitors at the fundamental frequency, thus providingthe needed reactive compensation. The filters were repre-sented in the EMTP using simplified R–L–C models [3].

With respect to thyristor valves, such devices are usu-ally represented in EMTP as ideal AC switches in serieswith small resistors that account for part of the conductionlosses. In the actual thyristor valves, an RC snubber circuitmust be added to limit the recovery voltage transient peakto acceptable levels. In terms of modeling, the snubber isrepresented by an equivalent RC circuit across theswitches. The representation of the snubber circuit im-proves the thyristor valve modeling and avoids numericaloscillations associated with the switching of the inductivecurrents.

The input signals to the measurement systems are thethree line-to-neutral bus voltages to be regulated. Ther.m.s. voltages are summed and averaged to provide thep.u. voltage output.

The signal from the measurement system is comparedwith a reference to produce a voltage error signal for inputto the regulator. The regulator comprises a PI (propor-tional integral) regulator that operates continuously toreduce its input voltage error to zero.

The outputs from the proportional and the feedforwardpath are combined and inserted into an integrator, whichcalculates the corresponding reactive current.

This current is the input for the non-linear interface,which calculates the correct conduction angle though acurrent/conduction angle curve that will send a firingangle to the pulse generating unit. The relationship

between the fundamental current and the conduction an-gle is given by:

Ir ¼r � sin r

pXLU

where U is the r.m.s. voltage across the TCR, XL is thefundamental frequency reactance and r is the conductionangle.

The firing system consists of the gate pulse generator,the purpose of which is to provide firing pulses to thethyristors. The regulator calculates the conduction angle,which is passed to the gate pulse generator as a controlsignal. It is the function of the gate pulse generator togenerate the correct firing pulses to achieve the requestedconduction angle, r. It should be noted here that r is thetime the thyristor conducts, while the firing point relativeto the voltage across the thyristor is normally indicated bya. These two variables are related by r þ 2a ¼ 2p.

A typical gate pulse generator would calculate the firingangle a from the point of zero voltage crossings. Toachieve this firing at a requested a, the controller couldconsist of an integrator that starts integrating at zerovoltage and resets when it reaches a controlled threshold.At this point, a firing command would be generated for theappropriate thyristor. Control would be achieved bychanging the value of the threshold. This type of systemacts correctly if the points of zero voltage crossing areaccurately known. This is not the case since both har-monics and transients are expected.

In the case of a TCR, this problem can be solved bytaking advantage of the 90� phase shift between the currentand the voltage. This ensures that the conduction angle ris centered about the zero crossing of the voltage. In thatcase, the conduction current can be used to locate the zerocrossings of the voltage without the problems associatedwith harmonics. The modeling of the firing circuit followsthe principle described in [3] and [5], by taking advantageof the 90� phase shift between the current and the voltage.

3Description of the SVC power systemVarious case studies using the SVC controller presented inthis paper will now be examined. In this case, an ACsystem represented by a three-phase voltage source BUS1(rated close to 120 kV L–L r.m.s. voltage, depending on thestudy that is being undertaken) and a R–L equivalentsystem impedance (3.58 W for the series resistance,0.0358 H for the inductance and 1000 W for the parallel

Fig. 1. Control system based on PI regulator

Fig. 2. SVC system under investigation86

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resistance) is feeding a resistive load of 144 W (1.0 p.u.load). The circuit diagram is shown in Fig. 2.

For this system it is desired to control the voltage at bus2 by the addition of the SVC under various system dis-turbances. Results using the models presented in thispaper will be compared with results obtained using theEMTDC simulation package using the control method andthe firing technique (PLL) presented in [8] and [10] usingthe same rating for the SVC model. The parameters for thisSVC model using the PLL firing technique and the controlsystems are similar to those used in [8] and [10]. The SVCconsists of a 120/12.65 kV Y/D–Y transformer feeding a12-pulse TCR/TSC (thyristor-controlled reactor/thyristor-switched capacitor) with the thyristor-controlled reactorelements connected in delta and the thyristor switchesmodeled as changing resistances. The thyristor-controlledcapacitors are switched on/off according to a controlcommand.

The SVC controller for this example can be based on thefollowing principles [8]:

– to compare the measured voltage where the SVC isconnected to the reference voltage (both in p.u.)

– pass the error signal through a PI controller to pro-duce the alpha (firing angle) order

– use the alpha order signal to trigger control signals toswitch capacitors:when a > 3.1 rad, switch on one capacitor stagewhen a < 1.6 rad, switch off one capacitor stage

The instant of valve firing for the TCR is determinedwhen a reference angle derived from a PLL equals theorder angle. A built-in dq0- transformation-based PLL isused using the principles described in [8] and [10].

4Case studies and simulation results

Disconnected loadFor this case, the three-phase source (rated at 123 kV) inFig. 2 was used and a load rejection took place at 0.5 s forthe three phases at bus 2. The rating of the SVC was110 MVA inductive and 73 MVA capacitive. Figures 3–5

show the responses of the two SVC models. The total loadrejection causes AC bus overvoltages at bus 2, resulting ina reduction in the a (firing angle) order in Fig. 4. The newadjustment of a brings the voltage at bus 2 to 1.0 p.u. in avery short time. The results in the previous analysis showvery good similarity and prove the effectiveness of themodeling method used in this paper. These results are alsosimilar to those given in [10] for the same system condi-tions. Also, the time step of the simulation program usingthe method described in this paper was 10)5 where asmaller time step (10)6) was needed using the PLL methodin order to achieve satisfactory operation, somethingwhich demands a high CPU time. It took 30 s (real time)for the program to run using the proposed method andalmost 5 min using the PLL method.

In order to test the effectiveness of the firing methodproposed in this paper, simulation results are presented inFig. 5 and compared with results found in [10].

For the results in Fig. 5 it is clear that from the graphsof the thyristor currents and the generated sawtoothwaveform (used for the firing of thyristors), the responseof the system to a load rejection (at 0.5 s) on the TCRbranches is practically stabilised in one cycle. A similarresponse is illustrated for the same conditions using thePLL method by examining the thyristor currents in [10].This is due to the immediate response of the firing system

Fig. 3. Voltage at bus 2 in p.u. using the SVC model describedin the paper for the disconnected load case. Voltage at bus 2 inp.u. using the SVC modeling technique described in [8, 10] forthe disconnected load case

Fig. 4. Firing angle in rads using the SVC model described inthe paper for the disconnected load. Firing angle in rads using theSVC modeling technique described in [8, 10] for the disconnectedload case

Fig. 5. Current and sawtooth waveform for the branch BA ofthe TCR

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N. Athanasiadis: A new technique for a static var compensator using the EMTP modeling environment

to the change in system conditions in a way similar to thePLL method. Moreover, by using the firing techniquepresented in this paper we can avoid a problem that iscaused by using the PLL method s reported in [11], whichis that the dynamics of the PLL require a very small timestep, which demands a high CPU time.

Voltage dropFor this case study the three-phase voltage source in Fig. 2was rated at 120 kV with a voltage reduction of 8% (from120 kV down to 110 kV) at 0.5 s for the three phases. Therating of the SVC was 100 MVA inductive and 167 MVAcapacitive. For this case two capacitor stages were used(with a rating of 83 MVA each). A control signal was re-sponsible for the switching on/off of the capacitor stagesdepending on the value of the firing angle a, as was ad-dressed before. Figures 6 and 7 show the responses of thetwo SVC modeling techniques. By examining the graphs itis clear that the source voltage reduction at 0.5 s causes thevoltage at bus 2 to drop below 1.0 p.u. and the firing anglea to increase up to the level for switching in two capacitor

stages (Fig. 7). After this, only a small adjustment of abrings the voltage back to 1.0 p.u. The switching on/off ofthe capacitor stages due to the value of the firing angle a isalso noted during the start up in the figures above.

The comparison of the results plotted in Figs. 6 and 7shows that the modeling techniques presented in this paperare capable and accurate enough to control SVC models.

Three-phase faultIn this case there is a complete loss of the synchronizingvoltages where the SVC is connected at bus 2 at 0.38 s witha duration of 30 ms. The three-phase voltage source inFig. 2 was rated at 120 kV and the rating of the SVC was100 MVA inductive and 83.0 MVA capacitive (only onecapacitor stage was used, with a rating of 83.0 MVA).Figs. 8 and 9 show the responses of the SVC models. Bylooking at these graphs we notice that the very severe dropat voltage bus 2 (Fig. 8) causes the firing angle to increaseup to a maximum value and then gradually to drop back toits pre-fault condition, bringing in this way the voltageback to 1.0 p.u. in a short timescale. Earlier results showvery good similarity and prove the effectiveness of the

Fig. 6. Voltage at bus 2 in p.u. using the SVC model describedin the paper for the voltage drop case. Voltage at bus 2 in p.u.using the SVC modeling technique described in [8, 10] forthe voltage drop case

Fig. 7. Firing angle in rads using the SVC model described in thepaper for the voltage drop case. Firing angle in rads using theSVC modeling technique described in [8, 10] for the voltagedrop case

Fig. 8. Voltage at bus 2 in p.u. using the SVC model describedin the paper for the three-phase fault case. Voltage at bus 2 inp.u. using the SVC modeling technique described in [8, 10] forthe three-phase fault case

Fig. 9. Firing angle in rads using the SVC model described inthe paper for the three-phase fault case. Firing angle in rads usingthe SVC modeling technique described in [8, 10] for the three-phase fault case

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control and firing methods used in this paper in com-parison with those used in [10].

5ConclusionsThe work presented in this paper has described a static varcompensator model for use in power system transmissionnetworks. The SVC response under various power systemdisturbances in power system networks has been analysed.

Comparison of the control and firing methods proposedin this paper with other control methods and firing sys-tems (such as PLL) was also made. The simulations showthat the SVC control system is rapid enough and the firingtechnique is capable of establishing accurate firing timing.Moreover, by using the firing technique presented in thispaper, the problem caused by using the PLL method,which is that the dynamics of the PLL require a very smalltime step, which demands a high CPU time, can beavoided. All simulation results are in accordance with thetheory and were validated using similar systems presentedin the reference literature.

Future work may include the application of the pro-posed SVC model to a transmission network with otherFACTS controllers, in order to test the response of thecontrol system in an integrated power network.

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