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Application ofFACTS Devices for the hterconn ected LineBetween Fujian Network and Hutidong Network

Jinfu Chen XnghuaWang XianzhongDiuan(M) DaguangWang Ronglin B a n gDeparbnent of Electrical Engineering

Huazhong U niversityof Science and TechnologyWuhan, Hubei l?R.ChinaAbstract: In order to impmve the stability of Fujian power system andprovide power energy to Huadong power system, a 5OOkV interwnnecte.dline which is more than 5 O O h long will be laid to connect Fujian networkand Huadong network. For the interconnected line i s long and poor damping,power flow oscillations will arise on the interconnected line when there QZsome faults in Fujian or in Hudong. The oscillations will cause generatoisentering into asynchronous operations between Fujian and Huadong. In thispaper, two FACTS(F1eXibk AC Transmission Systems) devices areintroduced into the system to improve the stabilityof the interconnected line,one is Static Var cOmpensator(SVC) and the othn is Thyristor ControlledSeries Capacitor(TCSC). The abilities of the two devices improving thestabilityof the interconnected line are described and compared in the paper.Several related objects are presented to discussion at the end of this paper.Keywords: FACTS Power flow Oscillation Power Flow Control SVCTCSC Transient Stability

I. INTRODUCTIONChinese power industry has developed to a new stage, thecapacity of each power system is enlarged and the nationwideinterconnection of power systems is putting forward todiscussion. But the nationwide interconnection of powersystems also will bring in a series of problems. First of all,stability is one of the major issues affecting the secureoperation of a large power system, including voltage stab ilityand power angle stability. From the operating experiences ofpower systems in other countries, it is an important problemwhich can not be neglected. Secondly, it's difficult to controlthe power flow among the interconnected power systems. Atthe early stage of the nationwide interconnection of powersystems, the stability of the interconnected systems is themain concern. Power flow oscillation in the interconnectedline is one of the stability problems and it will affect thesecure operation of power systems. The comparatively wealcinterconnections between large power systems, will causepower flow oscillations in the interconnected linesunavoidably. The benefits of nationwide interconnection inpower systems will not be achieved unless these p roblems aresolved very well.Since 1986 Hingorani put forward FACTS concept andaimed to transport the thyristor-based control technology in tothe power system, many research works have been carriedout and lots of advanced FACTS devices emerged[ 11. Studieson FACTS aims at increasing transmission capabilities,damping the oscillations, providing the voltage support andexploring the potential of the power system stability[2-5]. At

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Fujian ElectricPowerTest and Research InstituteFuzhou,Fujian, P.R.China

the present tune, FACTS technology has already beenconsidered to 'bring a bright prospect for the modem powersystem.In this paper, two efficient and economical FACTSdevices, TCSC and SVC are employed to improve thestability of the interconnected line between Fujian network(in the following of this paper, it is called Fujian) andHuadong netw1Di-k (in the following of this paper, it is calledHuadong). In the second section, the str ucture s of Fujian andHuadong are dlescribed. The benefits and problems after thetwo systems interconnected are shown. The models ofFACTS devices are introduced in the follow ing section . Thenthe effects of TCSC and SVC o improve the stability of theinterconnected line are discussed in the fourth sectionthrough vary calculations, and the difference of the ab ility todamp the power flow oscillation between these two devices isshown. n the :Following section, several related objects havebeen put forward to discussion. Conclusions are presented atthe end of this paper.

11.DESCRIPTION OF SYSTEMSFujian and :Huadongare located in the Southeast of China,a one-line diagram of the 5OOkV transmission system inFujian and the interconnected line between them are shown

Figure 1 diagram of Fujian and Huadongin figure 1. As; a local network, Fujian has only 5000MWload. The main transmission system is the 220kV networkand they composed of a large loop with double lines (only

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few area is single line), so the structure of the 220kV systemis strong. The 500kV network shows as 1 liom south tonorth and its stability is lower than the 220kV network.Huadong is a regional system which consisted of zhejiangnetwork, Jiangshu network, Anhui network and Shanghainetwork. The load in Huadong is up to 20000MW. n thesystem, the 500kV network takes the main position, so thenetwork structure is stron g and its stability is much higherthan Fujian.With the development of power system, some 600MWgenerators will be introduced to Fujian. It will form anoperating condition which is called as big-generator vssmall-network in Fujian and the fkequency in Fujian willdecrease very quickly when faults have been occurred on the600MW generators. In order to improve the stability ofFujian, an interconnected line will be laid to connect Fujianand Huadong. The following benefits will be achieved afterthe two ystems are connected.a. Benefit of controlling peak-load: the proportion ofhydro-power is high in Fujian and there are remain capacityof controlling peak-load which can be sent to Huadong toloose the difficulty of c ontrolling peak-load in Huadong.b. Improving the stability of Fujian: connected toHuadong can enhance the ability in Fujian to antidisturbances, especially the problem that the frequencydecreasing caused by the failures n the 600MW generators.c. Decreasing the standby capacity of generators in thesystem: the whole systems standby capacity of generatorscan be decreased because the two networks can help eachother When disturbance arose in the system afterinterconnected.

Though here are som e benefits, a series of problems willarise for the interconnected line is weak compared to the twonetworks. The main problems are:a. For the interconnected line is long (about 300km, nd ifconsidering the lines fiom Fuzhou o Houshi, it is more than500km) nd the two networks are comparatively large, powerflow oscillations will be caused even by a little disturbancearose in the system. The oscillations will last for a longperiod because of the poor damping and the stabilityoperation o f the system will be broken.b. If there is power flow oscillation occurred in theinterconnected line, the voltage of 500kV bus in Fuzhoustation will sway very tempestuously and its minimumvoltage will lower than 1OOkV. The 500kV bus voltage inother stations will also change very large, especially in theShuikou station, the oscillation of the bus voltage will affectthe security operation of hydro generators connected to the

bus.111.MODEL DESCRIPTION

A. Model of generatorThe most of generators in Fujian and Huadong use the

model including the excitation control system and thegovemor system. Few generators that its rating is less thanlOOMvar and phase shiftenuse the m odel without excitationcontrol system and the gov emor system.B.Modeling of TCSC

Figure 2 is the diagram of TCSC model including itscontrol system. The T CSC is set to control active power flowof line i-j.

Figure 2 diagram of TCSC modelC. Modeling of SVC

Figure 3 diagram of SVC modelThe model of SVC is shown in figure 3 including its

control system. In this paper, the active power flow exchangebetween SVC and system is assumed to be zero and the DCvoltage in the capacitor is constant.IV. CASES STUDY

Calculations to the system have been carried out usingthose models described above. The stability of Fujian andHuadong is analyzed and according to the power flowoscillation in the interconnected line, TCSC and SV C will beintroduced to the system to show their abilities of improvingthe stability of the interconnected line.A. Without FACTS devices

Calculation resultsshow hat the generators in F ujian willnot enter into asynchronous operation between each otherunder many operation conditions, and the same in Huadong.So, if the interconnected line is cut and the whole system isdivided into two networks when there are some faults in thesystem, the fiequency will increase or decrease in Fujian but

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little change in Huado ng because of its strong structure. If theinterconnected line is exist, th e generato rs in F ujian will be inasynchronous to the generators in Huadong and power flowoscillation will be caused in the in terconnected line.When the power flow in the interconnected line is fkomlFujian to Huado ng and if the interconnected line is cut whenthere are some failures n the system, the fiequency in Fujian,will increased. The number of the increased fiequency is,related to the power transferring to Huadong. If the powertransferring to Huadong is IOOOMW, and the load in Fujian is5000MW, he fiequency will increase up to 51.3%. If thepower is 400MW, the frequency will increase up to 50.4Hz.Power flow oscillation will be found in the interconnected1line if the interconnected line still exists when system istfailure. The magnituae of power flow oscillation in the linewill increase up to 2500MW. The bus voltage in 500kVnetwork in Fujian will sway too, and the two networks willenter into asynchronous very quickly after sy stem failure.If the power flow in the interconnected line is fromHuadong to Fujian and the interconnected line is cut, thefiequency in Fujian will decrease because of power absence.The minimum frequency will lower than 43Hz and thenetwork will not be in operation unless some loads are shetedoff in time. If the interconnected line is not cut when systemis failure, power flow oscillation can also be found in it.Through he above calculations, the main problems afterFujian connected to Hua dong are:

a. If the interconnected line is cut (either fault or not),high frequency or low frequency will be found in Fujian.Some measures should be taken to confirm the stability of thenetwork, such as generators shedding or loads shedding. Forthere is not involved in improving the stability of theinterconnected l i e in this condition, the following analysiswill not consider about it.b. If the interconnected line is exist, when there are somefaults happen to it in the interconnected system, power flowoscillations will arise in it. Measures are also needed toimprove the stability of the interconnected line, such asFACTS devices.In order to analyze the abilities of TCSC and SVC toimprove the stability of the interconnected line, the secondcondition (the interconnected line is exist) and Fujian areconcerned in the following.

B. ncluding TCSCTCSC can rapidly adjust its reactance to control flexiblythe power flow in transmission systems, improve the

dynamic and transient performance of the power system.Under the operating condition of l O O OMW power flowfrom Fujian and there is one fault happened in Fu jian, powerflow in the interconnected line is shown as solid line in fig4aIn fig4b, the solid line shows a generator in Fujian comparedto generators in Huadong. It is obviously that two networksare in asynchronous. The solid line in fig4c show s the 500kV

bus voltage in Ih h o u station, and sometime it is lower than1OOkV. The low and swing bus voltage causes other buses'voltage in Fujia n oscillation too, and the stabil ity operation inFujian is damaged.20151050-5-10-15 0 1 2 3 4 5 6 7 8 9

Time (second)(a)

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-160

h

1 -2400 1 2 3 4 5 6 7 8

Time (second)@)

0 1 2 3 4 5 6 7 8 9Time (second)

(clFiigun4 Calculationresults with TCSC

If there is iB TCSC in the interconnected line, such asinstalling the 'KSC in the interconnected line close toFuzhou station ;and the maximum co mpensation to the line is75%. We can use the TCSC to damp the power flowoscillation in the interconnected line. Under the samecondition as solid line in Fig 4, power flow in theinterconnec ted line is shown as dashed line in fig4a. It shows

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that the power flow oscillation in the interconnected line issmaller with TCSC than without TCSC. n fig4b, the dashedline shows the same generator in Fujian compared toHuadong. From it we can see that Fujian and Huadong arenot in asynchronous again, the all generators keep insynchronous. The bus voltage of Fuzhou station is shown infig4c as the dashed line. It shows better than without TCSC

can pull the two networks which may be operated intoasynchronous to synchronous. Other calculation results alsoshow that inother operating conditions, TCSC has the similareffects. It can improve the stability of the interconnected lineand prevent Fujian &om operating into asynchronous toHuadong.

(solid line). c. ncluding svc

0 1 2 3 4 5 6 7 8 9Time (second)(4

80-40-160-280-400

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(b)I 21.110.83 0.73 0.6

* 0.5 0.40.3

s 0.9

0 1 2 3 4 5 6 7 0 9Time (second)

(c)Figure 5 Calculation results with SVC

From the comparisons above we can find that TCSC hasgood effect to improve the stability of interconnected line. It

From the calculations above we can see that Fuzhoustation is about in the m id-point of two groups of generatorswhich maybe operate into asynchronous. If installing a SVCon he F ujian station, the stability of interconnected line willbe improved by SVC. he rating of the SVC s BOOMVar.

4

1312111098765430 1 2 3 4 5 6 7 8 9

Time (second)(a)140

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1.11

0.90.80.70.60.50.4

0 1 2 3 4 5 6 7 8Time (second))

(b)

0 1 2 3 4 5 6 7 8 9Time (second)

(c)Figure 6 Calculationresultswith SVC

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Under the same condition as figure 4, calculation resultswith the SVC are shown in figure 5. In figure 5, the solidlines are the power flow oscilla tion in the interconnected lint:,the generator angle in Fujian compared to Huadong and the500kV bus voltage of Fuzhou station without SVCrespectively, and the dash ed lin es are the same variables withSVC. From figure 5, it is concluded that under a bigdisturbance, the SVC has not th e efficient ability to damp thepower flow osciilation except that it has some effects oncontrolling the bus voltage.If the power flow in the interconnected line is small, suchas 400MW, n the same system failure, what about the SVC'seffects? Calculation resu lts are shown in figure 6. The powerflow in the interco nnec ted line is shown in figure 6% thedashed lines are with SVC and solid lines are without SVC.The magnitude of power flow oscill ation with SVC is smallerthan that of without SVC. It can be seen that there still havesome effects on damping the power flow oscillation withSVC. In figure 6b, it is the generator angle in Fujian. Fromfigure 6b, the oscilla tion of gen erator with SVC (dashed line)is smaller than hat without SVC (solid line). With SVC, the500kV bus voltage has been improve well, as dashed lineshown in figure 6c and the so lid line is without SVC.Compared with figure 5and figure 6, we can find that theability of SVC has some l imited . For it is a shunt type device,its effects on controlling the system are through regulatingthe bus voltage. Under a big disturbance in the system, theSVC has great difficulty to sho w its function.

V. D ISCUSSIONA. Influence of the different position of TCSC

From the results of calculation, there is little effect to thestability of the interconn ected line when TCSC is installed indifferent position of the interco nnec ted line. But the ability todamp the power flow oscillation will be decrease if thedevice is not installed in the interconnec ted ine. In thispaper,TCSC is suggested to install in Fuzh ou station.B. nfluence of the differen t posi tion of SVC

There is an obviously influence to realize he function s ofSVC when it is installed in different position, because theSVC can improve the stability of network through regulatingthe bus voltage and the influence on the stability of networkis different among the different buses in the system. K eFuzhou station is about a mid-point in the interconnected lineof Fujian and Huadong. So Fuzhou station is the mostsuitable position for SV C to improve the stability of theinterconnected line than other stations.

If we installed two SVCs the same rating in total todifferen t stations, for example, in Fuzhou station and Xiamenstation, the two SVCs with +lOOMVar are installed dividually.In this condition vary calculation results show that the bus

voltage of network canbe reg ulated flexibly, but the effect toimprove the stability of the interconnected line is not betterthan hat all ralhg are located in Fuzhou station.C. Influence oft he different control of SV C

In general, the SV C can be used to control the bus voltagewhen system i:r in its normal operation. If so, when the powerflow oscillation arose in the interconnected liie after systemfaults, the SV C only can damp th e oscillation by regulatingthe bus voltage which it connected . If we change the controlof the SVC to damp the oscillation when system was fault,could the ability of SV C be improved? Through a lot ofcalculation results we found th at the effect of SVC to dampthe oscillation is the same. If we analyze further we couldfind that the bus voltage swing of Fuzhou station is similar topower flow oscillation in the interconnected liie, so theircontrolling effects are the same.D . Influence of different system condition

The system condition mainly includes the number of loadin Fujian and the power flow in the interconnected line. Thesmaller load in Fujian and more power flow in theinterconnected line, the greater effect the power flowoscilla tion to :Fujian.Thus Fujian network can not be stabledvery quickly and the power flow oscillation will last a littlelong time in the interconnected line. The simulation resultscan verify that.Ther e also' is great influ ence by the different directions ofpower flow in the interconnected line. If the same power toFujian when i t absence and w hen it overdose, the power flowoscillation will last different time because of he differentnetwork operating.E. Other func1:ions of TCSC

In the steady state, the TCSC could be used to regulatethe bus voltage of Fuzhou station and damp the lowfrequency oscillation occurred in the interconnected linebesides damping the power flow oscillation. And when theinterconnectica is strengthen (such as laying new lines) andthe power f l ow oscillation could not be a problem any more,the TCSC could be used to improve the capability of powerflow transferring of this line. In the open power market in thefuture, it can also be used to control the exchange of powerflow between the two netw orks.VI. CONCLUSION

From the calculation results and discussions above, thefollowing conclusions will be achieve dThe stability of Fujian and the interconnected line isanalyzed. The stability of the interconnected line betweenFujian and Huadong can be improved either with TCSC or

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with SVC.The interconnected line is the weakest part when twosystems have weak interconnected l i e . The stability of thewhole system can be improved greatly if some measures aretaken to improve the stability of the interconnected line.TCSC has good effect to damp the power flow oscillationoccurred on the long interconnected line. But the good effectmust be based on a suitable position and control system, andthe variables of control system must be set properly.SVC has little effect to damp the big power flowoscillation on the interconnected line. But it can control thebus voltage in the network very well, and in this area it isbetter than TCSC.

Vm. EFERENCE[ I ] N.G Hinporani. High P o w Electronicsand Flexible AC Transmission

System,IEEEP o w Eng.REV. July 1988.[2] E.V.Lason, J.J. Sanchez-Gasca. J.H. Chow, Concepts for desigh of

FACTS controllers to damp power swings,DEEE Trans. PowerSystems,[3] A.E. Hammad, Analysis of powcr system stability enhancement by

static Var compensators, IEEE Trans. Power Systems, PWRS-I, 1986,[4] M.Norwzian, G Andersson. Damping of Power System Oscillations by

Vol. 10.N0.2,1995,pp.948-956.

pp. 28-35.

Use of Controllable Components, IEEE Trans. Powr Delivery, Vol. 9,No. 4,1994.

(5) F.J. Swift. H.F.Wag, Application of the Controllable SeriesCompensa tor in Damping PowerSystem Oscillations, E E Proc. 4ener.Tmsm. Distrib., Vol. 143,No. ,1996, pp. 359-364.

JX. IOGRAPHIESJinfu Chea was born in Fujian, China in 1972. He received his B.Engdegree from HUST n 1996.He currently is working his toward hi s Ph.D inHUST. is areas of intercst arc power flow control with FACTS devices anddynamic character analysis of FACTS devices.

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Xingbua Wang was bom in Hubei, China in 1972. He received his B.Engdegree from Tqinghua University in 1996 and M.S degree hom HUST n1999. He currently works BS a lecture at HUST and his research areas arevoltage stability and application of IT in the power system.Xianzhong Dum wasbom inHunan. China in 1966.He eceived his B.Engand Ph.D de- f b m Huazhong University of Science and Technology(HUST) n 1987 and 1992 respectively. H e currentlyworks as a professor inthe same university. Dr.Du m is currently with the D epartment of ElectricalEngineering at HUST. is fields of interest include voltage s tability, FACTSand application of IT in the power system.

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