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Comparison of STATCOM, SVC, TCSC, and SSSC

Performance in Steady State Voltage StabilityImprovement

Shravana Musunuri

Student Member, IEEEMississii State University

USAsm354@msstate.edu

Abstract- The paper presents a comparison of four FlexibleAC Transmission Systems (FACTS) controllers, the Static VarCompensator (SVC), the STATic synchronous COMpensator(STATCOM), the Thyristor Controlled Series Compensator(TCSC) and the Static Synchronous Series Compensator (SSSC)

on power system steady state voltage stability. The choice of thelocation and sizing of these devices is also presented and ananalysis is made on the 14 bus system using Power SystemAnalysis Toolkit (PSAT) software. A cost comparison of theseFACTS devices with traditional reactive power/voltage stability

devices is also presented.

I. NTRODUCTON

The resent day ower system is a lrge comlexinterconected network that consists of thousads of buses and

hunreds of generators. The network is increasing everydaywith the increase n demnd nd to meet this, eier newinstallation of ower generating stations and transmissionlnes is requred or he existng inastrcture oeration has to

be extended to limits. The laying of new lines or nstallation of new generatng stations imoses many envronental ndeconomical constraints. As a result, the existng trnsmissionlnes e more heavily loaded thn ever before nd oneconsequence of is is e theat of losing stability following adisturbnce. It was found at e voltage instability was oneof e main reasons for the recent Nor American blackout inAugust 2003 [1]. Voltage nstability causes system voltagecollase, which mes the system voltage to decrease to alevel that is unacceptable and is unable to recover leading tointeuption of the ower suly in e system. The only way to counteract this problem is by reducing the reactive powerload in the system or by addg new reactive generationsystems in the weakest oints in e system, ereby,

increasing e voltage at tose onts. The stability could getmuch worsened as e percentage of power generated om te renewable energy systems lke wind power ncrease [2]. The recent otential grid instability caused by too much wd ower generated om the wnd systems n Oregon area is one

Gholamreza Dehnavi

Student Member, IEEEMississii State University

USAgd24@msstate.edu

of e mny problems that could be caused due to the random natre of the wind [2]. Mny otential roblems like lowvoltage ride though, unbalanced faults impact etc [3] need tobe analyzed.

The recent develoment and use of Flexible AlteatingCuent Trnsmission Systems (FACTS) in the bulk ower trsmission system has led to mny alications where thesedevices are not only able to imrove the voltage nd nglestability but are also able to rovide exible oerationcaabilities. Several distinct models have been roosed to reresent FACTS static and dynamic alysis of the system[4]. This paer mentions e alication of four such FACTSdevices that are more used for voltage stability roblem. StaticSynchronous Comensator (STATCOM), Static VARComensator (SVC), Thyristor Controlled SeriesComensator (TCSC), Static Synchronous SeriesComensator (SSSC) are e FACTS devices at are used forhis uose. The oerational characteristics and caabilitiesof each of ese devices imrovg the steady state voltagestability of a selected test system are dealt in this aer alongwith e simulation results.

The paer is organized as follows: Section II troducessteady state voltage stability in general. A brief ntroduction of e stability of FACTS devices is resented in Section III.The selected test case system nd soware tool is en brieystated n Section IV. In Section V, the location and ratngs ofFACTS device d e results obtaned are resented alongwi a discussion.

II. STEADY STATE VOLTAGE STABLTY

Steady state voltge stability nd ynamic voltagestability re two types of voltage stability deed based on e time ame of simulation [5]. Since e steady state nalysisonly nvolves the solution of algebraic equations it iscomutationally less extensive than dynmic analysis. Also, this nalysis is ideal for the bulk of stdies in which voltagestability limit for many re-contgency and ost-contngency

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cases must be deteined [5]. Slow variations n the powersystem that eventually lead to voltage collapse is analyzed in the steady state voltage study. This can be seen om the plotof the power with the voltage at the bus also known as the "PV curve or "nose curve. Figure 12 is a typical P-V curve plot. It can be seen om the gure that as the power transferred increases, the voltage at the receiving enddecreases, eventually reaching a nose point where any furher

increase in the power transfer leads to a rapid declne involtage magnitude. Before reachng the critical point, the largevoltage op due to heavy reactive power losses can beobserved [5]. The region above the nose point is referred to as the stable regon and regon below s the unstable region.Analysis of the power ow equations reveal that the nose point occurs at the value at which the corresponding Jacobianis singular and is mathematically associated to saddle-node bircation pont. This nose point is also known as the maximum loadability point and hence the voltage collapse problem could also be dened as an optization problem,where the objective is to maximize ceran system parameters typically associated to load levels [5]. Hence, voltage collapseanalysis can also be used to compute the maxum power thatcan be transmitted through the transmission system [4].

As explained n the ntroduction section of this paper, thevoltage reduction can be improved by either decreasing the reactive load or by increasing the reactive power supply before voltage collapse point. Flexible control and operationof various FACTS devices can be effectively used for this puose. Of various existing FACTS devices, this paperadresses the improvement by installng STATCOM, SVC,TCSC and SSSC at the weakest bus.

III. FACTS DEVCES

Each of the above mentioned FACTS devices have therown characteristic and limitations. They are represented by

different models and mathematical equations depending on theissue under consideration and the time ame nvolved. Thissection gives a brief introduction to each of these devices.

A. Static V AR Compensator

Static VAR Compensator (SVC) is a shunt conectedstatic Var generator/load, whose output is adjusted according the requed capacitive or inductive current. The basicstructure of SVC is shown in Fig. It can be seen that the

model of a SVC is represented by a controllable reactor andxed capacitors. Though a suitable coordination of thecapacitors and the controlled reactor, the bus reactive powerinjected (or absorbed) by the SVC can be contnually varied inorder to control the voltage or to maintain the desirable powerow in the transmission network either over normal operatingor under disturbances conditions [1,5]. For steady stateanalysis, SVC is represented as a conollable susceptance. Itcontans the equivalent of automatic voltage regulator system to set and maintain a target voltage level. Steady statecharacteristic of SVC in Fig. 2 shows that there are upper andlower limits for SVC susceptance [1,4,6].

2

-L ilte -'

- "Figure I. Basic structure of SVC

Figure 2. Steady state V- characteristics of SVC

B. Static Synchronous Compensator

STATCOM is a Voltage-Source Inverer (VSI), whichconvers a C input voltage nto AC output voltage in order tocompensate the active and reactive power needed by thesystem [6]. Figs. 3 and 4 show the basic structre and termnalcharacteristic of STA TCOM, respectively. It can be seen omFig. 3 that STA TCOM is a shunt-conected device, whichcontrols the voltage at the conected bus to the reference value by adjustng voltage and angle of inteal voltage source.From Fig. 4, it is clear that STA TCOM exhibits constantcurrent characteristics when the voltage is lowhighunder/over the limit. This allows STATCOM to deliverconstant reactive power at the limits compared to SVC [1,5].

+ c-

Figure 3. Basic structure of STATCOM

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v

\-Figure 4. Steady state V characteristics of ST ATCOM

C. Thyristor Controed Series Capacitor

TCSC device uses Thyristor-Controlled Reactor (TCR) n parallel with capacitor segments of series capacitor bank. The basic structure of this device is shown n Fig. 5 and it can beseen that the combination of TCR and capacitor allows thecapacitive reactance to be smoothly controlled over a wide range. The value of susceptance (Be) of the line can becontrolled according to a specic controlled variable [5] hence

controlling the voltage. Fig. 6 gives the impedancecharacteristics of TCSC with the rg angle.

-IFigure 5. Basic structure of TCSC

(rd)'=x

Figure 6. TCSC impedance characteristics [7]

D. Static Synchronous Series Compensator

SSSC is based on a solid-state synchronous voltage sourceemployig an appropriate DC to AC verer, which can be

used for series compensation of transmission les. The SSSCis based on a DC capacitor fed Voltage Source Inverter (VSI)[4] that generates a three-phase voltage at ndamentalequency, which is then injected in a transmission line

though a transformer connected in series with line. The maincontrol objective of the SSSC is to directly control the current,and indirectly the power, owig though the le bycontrollg the reactive power exchange between the SSSCand the AC system. Fig. 7 shows the representing model ofSSSC and state vriables and Fig. 8 shows its operationalchracteristics [1,5].

3

r 1

Figure 7. Stability model of SSSC

,ja

Figure 8 Power-angle characteristics in constant reactance mode [8]

IV. TET YTE AND OFTWAR

A standard I 14-bus test system as shown in Fig. 9 is

used for this stdy. The test system consists of vesynchonous machines, icludg thee synchonouscompensators used only for reactive power support and twogenerators located at buses 1 and 2. In the system, there are

twenty branches and foureen buses with eleven loads totaling259 MW and 81.4 Mvar. The simulation is based on PSATsimulation soware [9]. PSAT is power system analysissoware, which has many features including power ow andcontuation power ow.

Figure 9. 14 bus system [6]

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V. RESUTS AND DSCUSSON

A. Location /Compensation Devices

The best location for shunt reactive power compensationfor steady state voltage stability margin is the weakest bus of the system [5]. Usng continuation power ow (CPF) featureof PSAT, the voltage prole of the test system is determinedand the weakest bus i.e., the bus where the voltage collapse

rst occurs is identied. Shunt compensation is provided n that bus to improve the system performance. The ideallocation for Series compensation is still under investigation[5]. However, a widely used and accepted method is by using

he continuaion power ow, the performance is stdied by placing the series compensator one at a time in the lines between weakest buses, and determining the best location.This method is followed in this paper.

B. Device Ratings

The required shunt compensation capacity is deteed by knowing the amount of reactive power suppor needed at the weakest bus. is could be determed by placg asyncronous condenser without any limit on the reactivepower at the weakest bus and observng the amount of reactive power generated at the maximum loadg pont. Another method is by ding the relationship between the maximumloading point and the corresponding capacities that the devicescan deliver without having the voltage collapse [5]. The seriescompensation sizing can be determined om the voltagestabili stdy. The active and reactive power requirement at the collapse point gives the rating of the required seriescompensation [1]. In this paper, rating of shunt and seriesdevices is deterined under intact system. Fig. 10 gives theow char of the procedure followed as was explained in the

previous paragraphs.

Select the Standard Nework

from V curves

Determi te FACTS ratigaccordg to reactve power

reqremnts obtad rom power

flow

Comare FACTS accord to Vcuves

Figure 10. Flowchar of procedure followed

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C. Simulation Results

/ Base caseAs described the earlier section, the continuation power

ow analysis using PSAT is rn for the test system and thevoltage prole without any FACTS devices is obtained. It wasfound om the results that bus 14 was the weakest bus and thefollowg Fig. 11 gives the voltage prole and Fig.12 gives

the PV curve for the weakest bus which is bus 14. It can beseen om the gure that the maximum loading parameter A =2.7699 pu for the base case.

l M l

#

Figure I I. Voltage prole for the base case without FACTS devices

0.9

08

07

06

051-

\.04!--'5,!-"'5-'5

f k b

Figure 12. "Nose curve for Bus 14 without FACTS

2) With STATCOMIt is seen om Fig. 12 that the voltage collapse occurs at

the maximum loadg of A = 2.7699 and bus 14 is the bestlocation for shunt compensation. Usng the proceduredescribed in the earlier section, a STATCOM of 0.7717 pu

was placed at bus 14 and CPF analysis was performed. Fig. 13gives the voltage prole at bus 14 with STATCOM. It can be

seen om the gure that the maxum loadg factor A =2.8507 pu and has increased when compared to the base case.

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105.0

1005

0995

099

0985

098

0975

0970.9 50!="0.5 1.5 m (pu)

Ss 14

Figure 13. "Nose curve with STA TOM

3) With SVC

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The STATCOM is now replaced with an SVC whose requed capacity was detenined in a similar way asSTATCOM. Fig. 14 gives the voltage prole at bus 14 with

S. It can be seen om the gure that the maximum loadngfactor A = 2.8203 pu has increased when compared to the base case but is less than STATCOM case. It can also be seen that the SVC has a relatively at prole until the voltagecollapse pont, and then decreases suddenly. This is nherentto the SVC approachng its operatng characteristics limits.

09 \08 /0

05 0- ,05---15-- - 5 m I (pu)

B 4

Figure 14. "Nose curve with

4 With SSSC

For the series compensation, as was described in the earliersection, the SSSC was placed in the weakest bus lines one ata time and it was found that for this system bus line 13-14series compensation has better results. Hence, a 0.2 pu SSSCwas placed in that bus line. Fig. 15 gives the voltage prolefor the system with SSSC. It is seen that the maximum

loading factor A = 2.8047 has increases when compared to the base case but is less than the shunt compensation devices.

5

1_09

08

0

7

06

05

0405 15

m/ (pu)us

25

Figure 15. "Nose curve for Bus 14 with at bus line 13-14

5) With TCSC

TCSC, as was described n the earlier section, the TCSCwas placed in the weakest bus lines one at a time and it wasfound that for this system bus lne 13-14 series compensation has better results. Hence, a 0.2 pu TCSC was placed in that bus line. Fig.16 gives the voltage prole for the system with

TCSC. It is seen that the maximum loading factor A = 2.8031 has increases when compared to the base case but is lesserthan the shunt compensation devices.

,-09

08

07

06

05

04

05 5 m I (u)

Bus 14

25

Figure 16 "Nose curve for Bus 14 with TS at bus line 13-14

D. Analysis of Results

Based on the results above, it can be seen that the maximum loading factor with STA TCOM is highest while that with SSSC is lowest when compared to the base case. Itcan also be seen that the voltage reduction is lowest in case ofST ATCOM. Shunt compensation device iects the reactive power at the connected bus but series compensation deviceinsers the reactive power at the connected lne. The testsystem needs reactive power at the load bus more than theline; hence shunt compensation gives beter results. It can also

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be seen that the SVC has a at voltage prole until thecollapse point and then rapidly decreases as it approaches theoperational lits. From g. 18 in appendix, it can also beseen that with the addition of FACTS devices with appropriateratngs the overall system performance can be improved.

Table I gives the average costs of various FACTS devices[10, 11] used for steady state voltage stability. The cost of

capacitor-based compensation is also provided for reference.

TABE . COT COAON O ACT DECE

Compensation Device Cost (US $)

SYC 40-60/ kYar

TCSC 25-40/ kYar

STATCOM 55 - 70/ kYar

Shunt Capacitor 8-12/kYar

Series Capacitor 12-15/kYar

It can be seen om the table that even though the cost for

FACTS is more than the capacitor-based compensation, the benets of exible operation, and safety extend theadvantages. It is nteresting to note that the average cost forsynconous condensers used for dynamic reactive powercompensation varies $10-40 per kVAR and mantenance costis about $-0.8/kVAR per year [10].

VI. CONCUSON

The paper gives a comparison of STATCOM, SVC, SSSC,and TCSC devices for steady state voltage stabilityimprovement. A technique to identi the location and sizes of the devices based on the continuation power ow is given d the results obtaned have clearly shown the effectiveness of these FACTS devices in mproving the transmission

capabilities of the system. It is clear om the analysis of the results that the shunt devices lke STATCOM and SVC gives the best performance for reducing the voltage collapse whencompared to the series compensation. It can also be seen that the benets or savings obtained by nstalling these FACTSdevices compensates for the additional cost a reasonabletime.

ACKNOWEDGMENTS

The authors would le to thank Dr. Suresh Srivastava,visitg research professor, Mississippi State University for hisguidance and encouragement for working on this topic.

REFERENCES

[I] C A. Ca\izares, "Power ow and transient stability models of FACTScontrollers for voltage and angle stability studies, Proc 2000IEEEPower Eng Soc Winter Meeting, pp 8, Jan. 2000.

[2] Shravana Musunuri, Herb Ginn, "Comprehensive Review of WindMaximum Power Extraction Algorithms, to be submitted at PESGeneral Meeting, July 2011

[3] Baggu, MM.; Watson, .D.; Kimball, J.W.; Chowdhury, B.H, "DirectPower Control of double fed generator based wind turbine converters toimprove ow voltage ride through during system imbalance,AppliedPower Electronics Conference, March 2010.

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[4] C. A Canlzares, Z. T. Faur, Analysis SVC and TCSC Controllers inVoltage Collapse, EEE Trans. Power Systems, Vol. 14, No. I,February 1999, pp. 158-165.

[5] Arhit Sode-Yome, Nadarajah Mithulananthan and Kwang Y. ee, "AComprehensive comparison of FACTS Devices for Enhancing StaticVoltage Stability, IEEE Power Engineering Socie General Meeting,June 2007.

[6] Mehrdad Ahmad, Mostafa Alinezhad, "Comparison of SVC andSTATCOM in Static Voltage Stability Margin Enhancement,

Proceedings of World Academy of Science, Engineering andTechnology, Vol. 38, Feb. 2009

[7] ennar ngquist*, Gunnar ngestrm, Hans-ke Jnsson, "Dynamicperformance of TCSC schemes, ABB Power Systems, 1996

8] Understanding FACTS by N.Hinorani .Gyugyi: EEE Press

[9] F. Milano, "Power System Analysis Toolbox, Version 1.3.4, Sowareand Documentation, July 14, 2005.

[10] John Kueck, Brendan Kirby, Tom Rizy, Fangxing i and Ndeye Fall,"Reactive power from Distributed energy, Electricity Journal, Dec.2006

[II] N.Acharya, "Facts and Figures abour FACTS, Training workshop ofFACTS application, EPSM, Energy, Dec. 2004

APPENDIX

For comparison of results with and without FACTSdevices, a plot of the four weakest bus voltages without anyFACTS and the voltage prole of the same buses aer placga ST ATCOM in bus 14 is shown the following gures.

1.1,-09

08

0

06

0.40;-5-1'5=-:-:5- m, (pu)Figure 17. Bus voltages result for NO FACTS caseIn the above gure the light blue color curve coesponds

to the bus 14, which is the weakest bus. It is clear om g. 17and g. 18 that by placing STATCOM at bus 14, not only thevoltage prole at that bus has improved signicantly, but alsothe voltage prole of other buses has also improved.

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-0.9

OB '07

06

0.5r. __04

0.5 1.5 2.5Loading Pater (pu

Figure 18. Result for STA TOM case with same buses

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