International Journal of Computer Science and Software Technology, Vol. 4, No. 1, January-June 2011, pp. 5-13© International Science Press, ISSN: 0974-3898

MITIGATION OF FLICKER BY SYNCHRONOUS MACHINEEXCITATION CONTROL

Abdul-Latif G. Raji and John VanCollerUniversity of the Witwatersrand, School of Electrical and Information Engineering, Post Bag 3, Johannesburg, South Africa

E-mails: raji@dept.ee.wits.ac.za and John.VanColler@wits.ac.za

ABSTRACT: The operation of electric arc furnaces in steel mills results in the generation of flicker which causes deteriorationin the quality of supply to other customers served from the point of common coupling. The objective of this paper was toinvestigate into mitigation of flicker produced by an UHP electric arc furnace by means of synchronous machine excitationcontrol. By applying a linear control law to the error signal obtained from the feedback of terminal voltage of a synchronouscondenser connected to the point of common coupling, appropriate thyristor firing angles were generated at every instantfor the necessary modulation of its field voltage. EMTP simulation of the network controlled in this way produced thedesired mitigation of flicker. In addition, limits were imposed on the firing angle to safeguard against total loss of excitation.Key Words. Flicker; mitigation,; excitation

1. INTRODUCTION

The operation of electric arc furnaces in steel millsresults in, among other things, the generation of voltageflicker. Flicker causes deterioration in quality of supplyfrom the utility especially to other customers servedfrom the point of common coupling and ways must befound to mitigate its undesirable effects. A number ofmethods have been utilized by various utilities andresearchers in the past to mitigate flicker. These includethe use of a series reactor, the use of Static VarCompensators (SVC) – in particular, the thyristor-controlled reactor (TCR), and very recently theinstallation of a STATCOM [1-4 ]. Synchronouscondensers with rotating machine exciters were widelyused also by some investigators and utilities. However,the use of synchronous condensers experienced adecline due to better flicker reduction achieved fromthe other afore-mentioned methods. Very recently,there has been a renewed interest in synchronouscondensers following the advent of synchronouscondensers with static excitation systems and thesuccessful construction and testing of superconductingsynchronous condensers [5]. This provided themotivation for this paper the objective of which is toinvestigate into the mitigation of flicker by controllingthe excitation of a modern synchronous condenserfitted with a high-speed static exciter and connected tothe point of common coupling of an ultra high power(UHP ) electric arc furnace.

The schematic diagram of the power system usedfor the study is shown in section 2, the excitationrequirements for synchronous condenser operation isderived in section 3. A linear control law governingthe thyristor firing angles is developed in section 4. Insection 5, the simulation results are presented togetherwith the calculation of the improved Pst, 99% . Finally,section 6 contains the conclusion.

2. THE POWER SYSTEM

Fig. 1 shows the power system used for the study. Agas turbine generator rated 110 MVA, 10.5 kV,supplies power to a 60 MVA UHP electric arc furnaceand other customers, via a 165 MVA, 10.5 / 330 kVstep-up transformer, a 63 kilometre 330 kVtransmission line, and a 140 MVA, 330 / 33 kVstep-down transformer. Connected to the pointof common coupling via a 45 MVA, 33 / 13.8 kVstep-down transformer, is a 25 MVA, 13.8 kVsynchronous condenser the excitation of which iscontrolled in order to mitigate the flicker produced bythe arc furnace. Specifications of all the componentsof the power system, except the synchronouscondenser, are contained in reference [6]. In addition,reference [6] contains the derivation of a mathematicalmodel for flicker which is used for theEMTP simulation below. Specifications of thesynchronous condenser are taken largely fromAppendix D of [7].

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3. DETERMINATION OF EXCITATIONREQUIREMENT

The excitation required by the synchronous condensercan be determined from the circle diagram as shownin Fig. 2. For synchronous condenser operation, theload angle, δ, tends to zero, and the power-factor angle,ϕ, tends to 90° % .Thus:

d d

VE VkVAR VI

X X

2

= − =

where: E= internal machine voltageV = terminal voltageXd = synchronous impedanceI = stator current

Simplifying the above equation:

E = V + IXd

= V + I(Xl + Xad)

In the absence of load, there will be no armaturereaction, hence Xad = 0, therefore:

E = V + IXl

E is produced by the total field current of thesynchronous condenser.

4. THE CONTROL LAW

The linear control law is obtained from feedback ofthe a-phase of the terminal voltage of the synchronouscondenser by means of a 13.8/√3 / 0.11/ √3 kV voltagetransformer. As illustrated in Fig. 3, the per-unit RMSvalue of the VT secondary output voltage is compared

with a reference to produce the error signal. Theaddition of the error signal to a unity reference yieldsthe desired cosine value of the firing angle, subject toan under-excitation limit of 1.0 and an over-excitationlimit of zero. To guard against total loss of excitation,however, a small value close to zero is utilized for theover-excitation limit. Thus:

cos α = 1 + ERROR, 0 ≤ cos α ≤ 1Where:

ERROR = 1 – RMSpu (va)

5. EMTP SIMULATION [8, 9]

The results of the simulation are shown below. Fig. 4shows the improved waveform of the voltage at thepoint of common coupling and Fig. 5 shows thewaveform of the steady-state current supplied tocustomers connected to the PCC. Fig. 6 shows themodulation of synchronous condenser field voltage.

Figure 1: The Power System

Figure 2

Mitigation of Flicker by Synchronous Machine Excitation Control 7

Figure 3: Synchronous Condenser Excitation Control

Figure 4: Voltage at the Point of Common Coupling

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The RMS voltage of the VT secondary, representativeof the synchronous condenser terminal voltage is shownin Fig. 7 while the per unit error signal is shown inFig. 8. The cosine of the thyristor firing angle, obtained

from the linear control law described above, is shownin Fig. 9. Finally, the instantaneous flicker signaland its integral are shown in Figs, 10 and 11respectively.

Figure 5: Current Supplied to Customers Connected to Point of Common Coupling

Figure 6: Modulation of Field Voltage

Mitigation of Flicker by Synchronous Machine Excitation Control 9

Figure 7: RMS Voltage of VT Secondary

Figure 8: The Per Unit Error Signal

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Figure 9: Cosine of the Thyristof Firing Angle

Figure 10: The Instantaneous Flicker Signal

Mitigation of Flicker by Synchronous Machine Excitation Control 11

Figure 11: The Integral of the Instantaneous Flicker Signal

Figure 12: Power System with Synchronous Condenser Compensation with Series Reactance

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Figure 13: Voltage at Point of Common Coupling after Insertion of Series Reactance

Applying the methods spelt out earlier in [6], thenew Pst, 99% was evaluated. The result obtained wasPst, 99% = 0.88. Recalling that Pst, 99% = 1.19 wasobtained for the same network without compensation,then the Flicker Improvement Factor (FI) may becalculated as:

,99%

,99%

st without

st without

PFlicker Improvement Factor

P=

6. EFFECT OF LONG FIELD TIMECONSTANT

One drawback of the synchronous condenser is thelong field transient time constant. It causes slowresponse to flicker. This can be remedied by theinsertion of a series reactance between thesynchronous condenser and arc furnace on one sideand the remainder of the system on the other as shownin Fig. 12. The value of the series reactance is usuallyof the order of the condenser’s sub-transientimpedance [10]. For this project, the value was 0.304pu or 2.316 Ω, and an improved waveform is obtainedat the point of common coupling as shown in Fig. 13(compare Fig. 4).

7. CONCLUSION

The results of the foregoing EMTP simulations havedemonstrated that via synchronous condenserexcitation control, irritating flicker can be reducedsubstantially; a Flicker Improvement Factor of 1.35 wasobtained. The tangible effect, as can be seen from Figs4 and 5, is a better quality of power supply for othercustomers connected to the point of common couplingtogether with enhanced electric arc furnace operations.

References[1] Robert, A. and Couvreur, M., “Recent Experiences of

Connection of Big Arc Furnaces with Reference to FlickerLevel”; International Council on Large Electric Systems(CIGRE ) Proceedings 36-305, pp.1-8, 1994 Session, Aug.28-Sept. 3, 1994.

[2] Bisiach, L., Campestrini, L., and Malaguti, C., “Technicaland Operational Experiences for Mitgating Interferencesfrom High-Capacity Arc Furnaces”,International Councilon Large Electric Systems (CIGRE) 36-204, pp. 1-6, Aug.30–Sept. 5, 1992 Session.

[3] Astorg, M., Bergeal, J., Moller, M., and Dubreuil, M., “AStatic Var Compensator on the French Public Network:Reduction of Flicker or Voltage Control at Les Ancizes”;International Council on Large Electric Systems (CIGRE)36-05 pp. 1-8.

Mitigation of Flicker by Synchronous Machine Excitation Control 13

[4] “Static Synchronous Compensator (STATCOM) for ArcFurnace and Flicker Compensation”, International Councilon Large Electric Systems (CIGRE ) Working Group B4.19,Publication No. 237, December 2003.

[5] Sweet, William, “Winner Adrenalin for the Grid: NovelSuperconducting Device Provides Essential VoltageSupport”; IEEE Spectrum, pp. 42-45, January 2006.

[6] Raji, A. G. , and VanColler, John M., “A Model of FlickerProduced by an UHP Arc Furnace for Mitigation Studies”,School of Electrical and Information Engineering,University of the Witwatersrand, Johannesburg, SouthAfrica. (To be published).

[7] Anderson, Paul M. and Fouad, A. A., “Power SystemControl and Stability”, Revised Printing, IEEE Press, NewYork, 1994.

[8] K. U. Leuven EMTP Center, Belgium, “ATP Rule Book” ,June 1992.

[9] Hermann W. Dommel, “EMTP Theory Book”, SecondEdition, Microtran Power System Analysis Corporation,Vancouver, BC, May 1992.

[10] D. D. Stephen, “Synchronous Motors and Condensers”,Chapman and Hall, London, 1958.