Q. Tian∗ and X. Lin∗∗
[1] P. Kundur, Power system stabilitv and control (New York:McGraw-Hill, Inc., 1994). [2] C. Taylor, Power system voltage stability (New York: McGraw-Hill, Inc., 1994). [3] H. Kitamura, M. Shimomura, & J. Paserba, Improvement ofvoltage stability by the advanced high voltage control regulator,Proc. 2000 IEEE Power Engineering Society Summer Meeting,2000, 278–284. [4] T. Michigami, N. Onizuka, & S. Kitamura, Development ofadvanced generator excitation control regulator (PSVR) forimproving voltage stability of abulk power transmission system,The Institute of Electrical Engineers of Japan, 11 (110-B),1990, 887–894. [6]. We limited effective LDC to 25–30% of thestep-up transformer impedance because voltage stabilitystudies did not show benefit of higher compensation levels.For mitigating system voltage depressions, Xc should becalculated by some method for which RA has proved itsmerit. The performances of RA, which chose Xc = 4%, aresatisfactory while Xc may not always be the best in anykind of disturbance.4. Impact of Powerformer on Time to CollapseRecently, a completely new type of generator, Powerformer,presented by ABB, offers a direct connection to the powernetwork without the need for a step-up transformer [17].The voltage control regulator method of the Powerformeris HSVC inherent. The Powerformer is able to maintainan overload in its stator windings for a longer period thana conventional generator. The relationship between theoverload time and the Powerformer’s Armature current isgiven in (4), and the comparison between the conventionalcurve and the Powerformer curve is shown in Fig. 5.IaIan=754T+ 1 (4)Figure 5. Comparisons between conventional and Power-former armature overload capability curves.The overexcitation limiter used in this simulation is theinverse time curve model shown in Fig. 4. To replace an ex-isting generator with the Powerformer for comparison pur-poses, the step-up transformer impedance has been madesmall enough to be insignificant (less than 1/1,000 p.u.)and the inverse time curve for the stator is reconfiguredsuch that it follows the Powerformer curve, shown in Fig. 5,rather than the ANSI C50.13 curve [18]. The advantage ofthis method is that because the Powerformer is identical inall respects to the conventional generator it was replacing,except for the improved stator current overload capability,meaningful comparisons can be drawn. All 22 generatorsin the Nordic system were replaced with the Powerformerto see if there was any improvement on the RA. The resultsare illustrated in Fig. 6.From Fig. 6 it is clear to see that if the Powerformersreplace the conventional generators then the RA improves.It was also found that it was preferable to let the control77Figure 6. The value of RA for different Xc.of the system voltage to be less stringent via the use ofnegative compensation. This is improved by more than875 Mvar by making Xc = −20% compared with Xc = 10%,indicating that the control point of the Powerformer mustbe pushed back “into” the generators. The Xc value of−21% to −19% is sufficient for voltage stability, and theincremental benefit of lower LDC is small. Xc lower than−25% can cause the value of RA to decrease from theclimax.In Fig. 7, the variation in voltage at bus 4043 for thecase 1 contingency has been observed with the Xc at selectvalues in the Nordic test system when the Powerformersreplace generators.Figure 7. Case 1: bus 4043 voltage versus time for differentXc.Fig. 7 highlights that if the Powerformers replace theconventional generators, the improvement in the time tocollapse is optimum.The variation in voltage at bus 4043 for the case 4contingency has been observed with the Xc being the sameas in the case 1 contingency in the Nordic test system andthe result is shown in Fig. 8. If the control points arelocated in terminal of the Powerformer, the time to collapseis actually reduced. Xc must be negative or the controlpoint of the generator could be pulled into the generator.By setting Xc to a value which can be found by RA, thepoint of control of the Powerformer will be moved intothe generator windings and the terminal voltage will beless stringently controlled. Fig. 8 illustrates the noticeableimprovement in time to collapse possible if Xc = −20%compared to Xc = 0%.Based on the studies for the Powerformer, LDC settinghigher than −16% can cause over-voltages at the generatorFigure 8. Case 4: bus 4043 voltage versus time for differ-ent Xc.terminals, and settings higher than −13% can cause insta-bility in reactive power sharing on parallel operation. Formitigating system voltage depressions, Xc should be calcu-lated by some method for which RA has proved its merit.The performances of RA for the Powerformer, which choseXc = −20%, are satisfactory while Xc may not always bethe best in any kind of disturbance.5. ConclusionIn this paper, the scheme of voltage control regulatorwhich utilizes both RCC and LDC has been discussed.The control point that the voltage regulator controls canbe acquired by RA method. The dynamic analyses werecarried out on the CIGRE Nordic test system, and itwas proved that the new scheme can improve the voltagestability in all kinds of disturbances. The additionaloverload capability of the Powerformer and its impact ofthe compensation scheme chosen for the Powerformer havealso been discussed. It has been shown that in the case ofcertain system contingencies in the Nordic test system, itwas preferable to let the control of the system voltage tobe less stringent via the use of compensation.References[1] P. Kundur, Power system stabilitv and control (New York:McGraw-Hill, Inc., 1994).[2] C. Taylor, Power system voltage stability (New York: McGraw-Hill, Inc., 1994).[3] H. Kitamura, M. Shimomura, & J. Paserba, Improvement ofvoltage stability by the advanced high voltage control regulator,Proc. 2000 IEEE Power Engineering Society Summer Meeting,2000, 278–284.[4] T. Michigami, N. Onizuka, & S. Kitamura, Development ofadvanced generator excitation control regulator (PSVR) forimproving voltage stability of abulk power transmission system,The Institute of Electrical Engineers of Japan, 11 (110-B),1990, 887–894.[5] J.B. Davies & L.E. Midford, High side voltage control atmanitoba hydro, Proc. 2000 IEEE Power Engineering SocietySummer Meeting, 2000, 271–277.[6] D. Brandt, R. Wachal, R. Valiquette, & R. Wierckx, Closed looptesting of a joint VAR controller using a real time simulator,IEEE Transactions on Power Systems, 6 (3), 1991, 1140–1146. [7] A. Murdoch, J.J Sanchez-Gasca, & M.J. D’Antonio, Excitationcontrol for high side voltage regulation, Proc. 2000 IEEEPower Engineering Society Summer Meeting, 2000, 285–289. [8] D. Kosterev, Design, installation and initial operating experi-ence with line drop compensation at John Day Powerhouse,IEEE Transactions on Power Systems, 16 (2), 2001, 261–265.78 [9] P. Kundur, J. Paserba, V. Ajjarapu, G. Andersson, A. Bose, C.Caizares, N. Hatziargyriou, D. Hill, A. Stankovic, C. Taylor,T.V. Cutsem, & V. Vittal, Definition and classification ofpower system stability, IEEE Transactions on Power Systems,19 (2), 2004, 1387–1401. [10] J.P. Paul, C. Corroyer, P. Jeannel, J.M. Tesseron, F. Maury,A. Torra, Improvements in the organisation of secondaryvoltage control in France, CIGRE Session Paris 1990, Report38/39-03. [11] V. Arcidiacono, S. Corsi, A. Natale, C. Raffaelli, & V. Menditto,New developments in the application of ENEL transmissionsystem voltage and reactive power automatic control, CIGRESession 1990, Report 38/39-06. [12] J.P. Piret, J.P. Antoine, M. Stubbe, N. Janssens, & J.M.Delince, The Study of a centralized voltage control methodapplicable to the belgian system [Online]. Available athttp://www.eurostag.be/download/PUB_PSO_4N_52052_001_00.pdf [13] A.S. Rubenstein & W.W. Walkley, Control of reactive KVAwith modern amplidyne voltage regulators, AIEE Transactions,1957, 961–970. [14] R.A. Schlueter, A voltage stability security assessment method,IEEE Transactions on Power Systems, 13 (3), 1998, 1423–1438. [15] C.A. Aumuller & T.K. Saha, Determination of power systemcoherent bus groups by novel sensitivity-based method forvoltage stability assessment, IEEE Transactions on PowerSystems, 18 (3), 2003, 1157–1161. [17].The voltage control regulator method of the Powerformeris HSVC inherent. The Powerformer is able to maintainan overload in its stator windings for a longer period thana conventional generator. The relationship between theoverload time and the Powerformer’s Armature current isgiven in (4), and the comparison between the conventionalcurve and the Powerformer curve is shown in Fig. 5.IaIan=754T+ 1 (4)Figure 5. Comparisons between conventional and Power-former armature overload capability curves.The overexcitation limiter used in this simulation is theinverse time curve model shown in Fig. 4. To replace an ex-isting generator with the Powerformer for comparison pur-poses, the step-up transformer impedance has been madesmall enough to be insignificant (less than 1/1,000 p.u.)and the inverse time curve for the stator is reconfiguredsuch that it follows the Powerformer curve, shown in Fig. 5,rather than the ANSI C50.13 curve [18]. The advantage ofthis method is that because the Powerformer is identical inall respects to the conventional generator it was replacing,except for the improved stator current overload capability,meaningful comparisons can be drawn. All 22 generatorsin the Nordic system were replaced with the Powerformerto see if there was any improvement on the RA. The resultsare illustrated in Fig. 6.From Fig. 6 it is clear to see that if the Powerformersreplace the conventional generators then the RA improves.It was also found that it was preferable to let the control77Figure 6. The value of RA for different Xc.of the system voltage to be less stringent via the use ofnegative compensation. This is improved by more than875 Mvar by making Xc = −20% compared with Xc = 10%,indicating that the control point of the Powerformer mustbe pushed back “into” the generators. The Xc value of−21% to −19% is sufficient for voltage stability, and theincremental benefit of lower LDC is small. Xc lower than−25% can cause the value of RA to decrease from theclimax.In Fig. 7, the variation in voltage at bus 4043 for thecase 1 contingency has been observed with the Xc at selectvalues in the Nordic test system when the Powerformersreplace generators.Figure 7. Case 1: bus 4043 voltage versus time for differentXc.Fig. 7 highlights that if the Powerformers replace theconventional generators, the improvement in the time tocollapse is optimum.The variation in voltage at bus 4043 for the case 4contingency has been observed with the Xc being the sameas in the case 1 contingency in the Nordic test system andthe result is shown in Fig. 8. If the control points arelocated in terminal of the Powerformer, the time to collapseis actually reduced. Xc must be negative or the controlpoint of the generator could be pulled into the generator.By setting Xc to a value which can be found by RA, thepoint of control of the Powerformer will be moved intothe generator windings and the terminal voltage will beless stringently controlled. Fig. 8 illustrates the noticeableimprovement in time to collapse possible if Xc = −20%compared to Xc = 0%.Based on the studies for the Powerformer, LDC settinghigher than −16% can cause over-voltages at the generatorFigure 8. Case 4: bus 4043 voltage versus time for differ-ent Xc.terminals, and settings higher than −13% can cause insta-bility in reactive power sharing on parallel operation. Formitigating system voltage depressions, Xc should be calcu-lated by some method for which RA has proved its merit.The performances of RA for the Powerformer, which choseXc = −20%, are satisfactory while Xc may not always bethe best in any kind of disturbance.5. ConclusionIn this paper, the scheme of voltage control regulatorwhich utilizes both RCC and LDC has been discussed.The control point that the voltage regulator controls canbe acquired by RA method. The dynamic analyses werecarried out on the CIGRE Nordic test system, and itwas proved that the new scheme can improve the voltagestability in all kinds of disturbances. The additionaloverload capability of the Powerformer and its impact ofthe compensation scheme chosen for the Powerformer havealso been discussed. It has been shown that in the case ofcertain system contingencies in the Nordic test system, itwas preferable to let the control of the system voltage tobe less stringent via the use of compensation.References[1] P. Kundur, Power system stabilitv and control (New York:McGraw-Hill, Inc., 1994).[2] C. Taylor, Power system voltage stability (New York: McGraw-Hill, Inc., 1994).[3] H. Kitamura, M. Shimomura, & J. Paserba, Improvement ofvoltage stability by the advanced high voltage control regulator,Proc. 2000 IEEE Power Engineering Society Summer Meeting,2000, 278–284.[4] T. Michigami, N. Onizuka, & S. Kitamura, Development ofadvanced generator excitation control regulator (PSVR) forimproving voltage stability of abulk power transmission system,The Institute of Electrical Engineers of Japan, 11 (110-B),1990, 887–894.[5] J.B. Davies & L.E. Midford, High side voltage control atmanitoba hydro, Proc. 2000 IEEE Power Engineering SocietySummer Meeting, 2000, 271–277.[6] D. Brandt, R. Wachal, R. Valiquette, & R. Wierckx, Closed looptesting of a joint VAR controller using a real time simulator,IEEE Transactions on Power Systems, 6 (3), 1991, 1140–1146.[7] A. Murdoch, J.J Sanchez-Gasca, & M.J. D’Antonio, Excitationcontrol for high side voltage regulation, Proc. 2000 IEEEPower Engineering Society Summer Meeting, 2000, 285–289.[8] D. Kosterev, Design, installation and initial operating experi-ence with line drop compensation at John Day Powerhouse,IEEE Transactions on Power Systems, 16 (2), 2001, 261–265.78[9] P. Kundur, J. Paserba, V. Ajjarapu, G. Andersson, A. Bose, C.Caizares, N. Hatziargyriou, D. Hill, A. Stankovic, C. Taylor,T.V. Cutsem, & V. Vittal, Definition and classification ofpower system stability, IEEE Transactions on Power Systems,19 (2), 2004, 1387–1401.[10] J.P. Paul, C. Corroyer, P. Jeannel, J.M. Tesseron, F. Maury,A. Torra, Improvements in the organisation of secondaryvoltage control in France, CIGRE Session Paris 1990, Report38/39-03.[11] V. Arcidiacono, S. Corsi, A. Natale, C. Raffaelli, & V. Menditto,New developments in the application of ENEL transmissionsystem voltage and reactive power automatic control, CIGRESession 1990, Report 38/39-06.[12] J.P. Piret, J.P. Antoine, M. Stubbe, N. Janssens, & J.M.Delince, The Study of a centralized voltage control methodapplicable to the belgian system [Online]. Available athttp://www.eurostag.be/download/PUB_PSO_4N_52052_001_00.pdf[13] A.S. Rubenstein & W.W. Walkley, Control of reactive KVAwith modern amplidyne voltage regulators, AIEE Transactions,1957, 961–970.[14] R.A. Schlueter, A voltage stability security assessment method,IEEE Transactions on Power Systems, 13 (3), 1998, 1423–1438.[15] C.A. Aumuller & T.K. Saha, Determination of power systemcoherent bus groups by novel sensitivity-based method forvoltage stability assessment, IEEE Transactions on PowerSystems, 18 (3), 2003, 1157–1161.[16] Long Term Dynamics Phase II, Cigr´e, Cigr´e TF 38-02-08, 1995.[17] C. Aumuller & T. Saha, Investigating the impact of pow-erformer on voltage stability by dynamic simulation, IEEETransactions on Power Systems, 18 (3), 2003, 1142–1148.[18] M. Leijon, K.N. Srivastava, B. Franken, & B. Berggren,Generators connected directly to high voltage network, Proc.Third International R&D Conf. of Central Board of Irrigationand Power, CBIP, Jabalpur, India, 2000, 1–8.
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