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Power Flow Control with Static Synchronous Series Compensator (SSSC)

Abdul Haleem, Project Manager,VISION KREST Embedded

Technologies, Hyderabad [email protected]

Chandra babu NayuduDept. of Electrical EngineerngCollege of engineering, Pune

[email protected]

Dr. N. Gopala KrishnanDept. of Electrical EngineerngCollege of Engineering Pune

[email protected]

Abstract The series compensation technique of long andmedium transmission lines is extensively employed in manycountries including India as it offers considerable advantagesand better use of transmission lines. It can also be a techniquein improving power system stability and power flow throughthe intended transmission network. However, technicalproblems such as reliability of capacitors and their protectiveequipments do exist; and more recently the problem of subsynchronous resonance (SSR) has surfaced. To remove thesedrawbacks, recently a series compensation technique fortransmission line which uses a synchronous voltage source(SVS). The static synchronous voltage source utilizes a powerelectronic voltage source (VSC) converter employing GTO orIGBT depending upon power requirements. The VSC mayemploy a two level or multilevel converter. In this paper astatic synchronous series compensator (SSSC) using a 6-pulseVSC employing sinusoidal pulse width modulation is examined.The steady state performance and P- characteristics areobtained for a given transmission network embedded withSSSC. A control circuit for the operation of SSSC is developedand the performance of the control circuit is investigated in

MATLAB-SIMLINK Platform.

Keywords-6-pulse VSC, SSSC, FACTS, Power Flow Control, Series compensation

I. I NTRODUCTION Series capacitive compensation is widely used in long

transmission lines to maintain the overall impedance of thetransmission line. The capacitive series compensationincreases the power transfer capacity as well as the transientstability. The series dielectric capacitors have been installedall over the world as efficient economical way of providingcapacitive series compensation [1]. With the new advancesin the generation of the power electronics devices based on

voltage source converter (VSC) known as flexible actransmission system (FACTS), more flexible operation andcontrol of the transmission networks are possible. FACTScontrollers can be classified as shunt, series, or phase anglecompensating devices or devices which are a combination of the above three types such as unified power flow controller (UPFC) [1]. These FACTS devices enable fast responseusing the phase locked loop (PLL) with minimum inherenttime delay during severe disturbances, transient power swings, thus allowing the transmission system operatingsafely and close to the theoretical stability limit. TwoFACTS devices can provide capacitive series compensation,they are :(1) thyristor controlled series capacitor (TCSC) and(2) static synchronous series compensator [2,3] .

There are several TCSCs are widely installed. The TCSCis used in practice to significantly improve the smalldisturbance and transient stability of the power system.Although the TCSC can provide the capacitive seriescompensation, it has several disadvantages. It injects loworder harmonic components (typically third, fifth, seventh

and ninth) into the power system because of phase control of the thyristors. Transient response of the circuit is rather slow, because of controlling thyristor firing pulse is available onlyonce in each half cycle. Deriving a closed-loop model of TCSC is complicated. Furthermore, it is susceptible to

parallel resonance due to the presence of inductors andcapacitors in parallel paths.

The SSSC is one of the most important FACTS devicesfor power transmission line series compensation. It is a

power electronic-based VSC that generates a nearlysinusoidal three phase voltage which is in quadrature withthe line current [2,3] .The SSSC converter block is connectedin series with the transmission line by a series coupling

transformer. The SSSC can provide either capacitive or inductive series compensation independent of the line current.Unlike other series compensators, an ideal SSSC isessentially a pure sinusoidal ac voltage source at the systemfundamental frequency. Its output impedance at other frequencies is ideally zero. Thus, SSSC does not resonatewith the inductive line impedance to initiate sub synchronousresonance oscillations. This paper deals with a 6 pulse (twolevel) VSC.

The objective of this paper is to analyze and investigatethe steady state performance of the SSSC for providingdynamic series compensation, voltage regulation. A controlcircuit is proposed for the operation of the SSSC. The

proposed control scheme for the SSSC is fully validated in both capacitive and inductive modes of operation bysimulation.

II. PRICIPLE OF OPERATION OF SSSCThe SSSC is generally connected in series with the

transmission line with the arrangement as shown in Fig.1.The SSSC comprises a coupling transformer, a magneticinterface, voltage source converters (VSC) and a DCcapacitor. The coupling transformer is connected in serieswith the transmission line and it injects the quadraturevoltage into the transmission line. The magnetic interface isused to provide multi-pulse voltage configuration toeliminate low order harmonics.

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Figure 1. static synchronous series compensator

The VSCs are either two-level converter or three levelconverters. One side of the VSC is connected to the magneticinterface while the other side is connected to the DC bus.The VSC generates six-pulse voltage waveform and it iscombined into multi-pulse (12 pulses) voltage waveform byWye-Delta connection of the magnetic interface. More

pulses (24 or 36 pulses) can be achieved if zigzagtransformers are used as the magnetic interface. The DCcapacitor is used to maintain DC voltage level on the DC bus.This DC capacitor is selected to meet harmonic andeconomic criteria of the SSSC and the power system.

Figure.2 shows a single line diagram of a simpleTransmission line with an inductive transmission reactance,XL, connecting a sending-end voltage source, and a receivingend voltage source, respectively [3].

Figure 2. an Elementary Power Transmission System

The real and reactive power (P and Q) flow at thereceiving-end voltage source are given by eq (1) and (2)

(1)

(2)

Where Vs and V r voltage magnitudes and arethe phase angles of the voltage sources. The voltagemagnitudes are chosen such that Vs = Vr =V and thedifference between the phase angles is

An SSSC, limited by its voltage and current ratings, iscapable of emulating a compensating reactance, Xq, (bothinductive and capacitive) the expression of power flow givenin equation (1) and equation (2) becomes

2 2

sin sin(1 )

qq

L L

V V P

X Xeff X

X

G G

2 2

(1 cos ) (1 cos )(1 )

qqeff

L L

V V Q

X X X

X

G G

Where X eff is the effective total transmission linereactance between its sending and receiving power systemends, including the equivalent variable reactance inserted

by the equivalent injected voltage (Vq) (Buck or Boost) bythe SSSC. The compensating reactance is defined to benegative when the SSSC is operated in inductive mode and

positive when SSSC operated in capacitive mode. Fig.3shows an example of a simple power transmission systemwith an SSSC and the related phasor diagrams.

Figure 3. Two machine system with SSSC

Figure 4. Phasor diagram

The SSSC injects the compensating voltage in series withthe line irrespective of the line current. The transmitted

power P q therefore becomes a parametric function of theinjected voltage and it can be expressed as follows:

The normalized power P q versus angle plots are shownin Fig.4.6 as a function of Vq These values are calculated for the system whose specifications are given earlier in AProgram in MATLAB has been developed to obtain thesecharacteristics for V q= 0, 0.353, 0.707 and these are shown inFig.5

0 20 40 60 80 100 120 140 160 180-1

-0.5

0

0.5

1

1.5

2

TRANSMISSION ANGLE (DEGREES)

T R A N

S M I T T E D P

O W E R

( p . u

)

Vq=0.707

Vq=0.353

Vq=0Vq=-0.353

Vq=-0.707

Figure 5. Transmitted power versus transmission angle as a function of

the degree of series compensating voltage V q by the SSSC.

2

sin( ) sin s r s r L L

V V V P

X X G G G

2

(1 cos( )) (1 cos ) s r s r L L

V V V Q

X X G G G

S r G G G

S r V V V

(1 cos ) s r qeff

Q V V X

G

eff L q X X X

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From the plots given Fig.5 we can say that the SSSCincreases the transmitted power by a fixed fraction of themaximum power transmittable by the uncompensatedline,independently of transmission angle and SSSC not onlyincrease the transmittable power but also decreases it.

The transmittable active power, P, and the reactive power,Q, supplied by the receiving end bus can be expressed for tTwo-machine system as functions of the (actual or effective)reactive line impedance, X L the line resistance, R, andtransmission angle, as follows:

The normalized active power P and reactive power Q versusangle transmission characteristics described by equations

and are plotted as a parametric function of the X L/R ratiofor 7.4, 3.7, and 1.85 in Fig.6 . These values are calculatedfor the system whose specifications are given earlier. AProgram in MATLAB has been developed to obtain thesecharacteristics for X L/R = , 3.7 , 7.4, 1.85.

Figure 6. Transmitted real and reactive power versus transmission angleas a function of ratio of X L/R.

These plots clearly show that the maximum transmittableactive power decreases, and the ratio of active to reactive

power increases, rapidly with decreasing X L/R ratio.

III. CONTROL CIRCUITIntroduction

An advanced control scheme is introduced by Akagi [4]used for SSSC. The development of this control scheme isdiscussed in this chapter.

Development of Control circuit for SSSC

Figure 7. System Configuration of SSSC

The following assumptions are made in the analysis1) The sending-end voltage is equal to the receiving endvoltage2) The SSSC device is assumed to be an ideal controllable

The injected voltage is independent of the line currentand controlled by using the pulse width modulationswitching techniques. The voltage source converter usesPWM switching techniques to ensure fast response and togenerate a sinusoidal wave form. The output of The PLL isangle, , which is used to transform the direct axis andquadrature axis components of the ac three phase voltagesand current. The measured quadrature voltage is comparedwith the desired reference constant quadrature voltage to theinput of the AC voltage regulator which is a PI controller.Thus the voltage regulator provides the quadraturecomponent of the converter voltage. Also the Measureddirect axis component voltage is compared with thereference voltage; this driven error is an input to the voltageregulator which is a PI controller to compute the directcomponent of the converter voltage. The injection voltage isgenerated by transforming these direct axis and quadratureaxis components into three phase voltage and is applied tothe VSC to produce the preferred voltage, with the help of

pulse width modulation (PWM).

Figure 8. Control circuit of SSSC

Simulation resultsSimulation of the SSSC is performed in MATLAB

SIMULINK using the Akagis control technique.

Steady state characteristics of SSSC.

Figure 9. Simple system taken for simulation

P = [ sin -R (1-CosQ = [Rsin + (1-Cos

voltage source. Output voltage vector is equal to itsreference 3) The three phase voltages at sending end are

balanced Fig.7 shows a block diagram of the control circuit[4]. The three- to two-phase transformation obtains and from the three-phase currents and. The d-qtransformation yields and from and the phaseinformation is generated by a phase lock-loop (PLL).

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Fig.9 shows the simple system taken for simulation. Themain circuit of the SSSC device consists of three phasevoltage-fed pulse width modulation (PWM) inverters. APWM control circuit compares reference voltage with atriangle carrier signal in order to generate gate signals. Theac terminals of the PWM inverters are connected in seriesthrough step-up transformers because injecting voltage isvery small compare to transmission line voltage. A three-

phase diode rectier is employed and reactor L and resistor R representing the impedance of the transmission line areinserted between sending end and receiving end. DCcapacitor used for the charging and discharging purpose. Thefunction of the control system is to keep the injecting voltagein quadrature with the transmission line current and onlycontrol the magnitude of injected series reactance to meet thedesired reactance compensation level.

Figure 10. Static Synchronous Series Compensator Model in MATLAB

Figure 11. injecting voltage

The fig.11 shows that the injecting voltage of the SSSCand this injected voltage will be in quadrature with the linecurrent. The SSSC can provide either capacitive or inductiveseries compensation independent of the line current. Bycontrolling the magnitude of injected voltage the amount of series compensation can be adjusted.

When an SSSC injects an alternating voltage lagging theline current as shown in the Fig.12, it emulates a capacitivereactance in series with the transmission line causing the

power flow as well as the line current to increase as the levelof compensation increases and then SSSC is operating in acapacitive mode. The emulating capacitive reactance is 0.22ohms.

0.62 0.63 0.64 0.65 0.66 0.67 0.68 0.69 0.7-100

-80

-60

-40

-20

0

20

40

60

80

100

Time in seconds

C a p a c

i t i v e c o m p e n s a t

i o n

voltage (V)current (A)

Figure 12. SSSC Operating in Capacitive Mode (Capacitive Compensation)

The emulating reactance value calculated by usingfollowing relation is .where Vq is the rms value of the injecting voltage and I is the current flowing in the line(rms value).

When an SSSC injects an alternating voltage leadingthe line current as shown in the fig.13, it emulates aninductive reactance in series with the transmission linecausing the power flow as well as the line current todecrease as the level of compensation increases and theSSSC is operating in an inductive mode. The emulatinginductive reactance of 2 ohms

2.04 2.05 2.06 2.07 2.08 2.09 2.1-40

-30

-20

-10

0

10

20

30

40

Time in seconds

I n d u

c t i v

e C

o m p e n s a t

i o n

voltage (V)current (A)

Figure 13. SSSC Operating in Inductive Mode (Inductive Compensation)

0 0.5 1 1.5 2 2.5 3-1.5

-1

-0.5

0

0.5

1

1.5x 10

4

Time in Seconds

I n j e c t e d

A c t

i v e

P o w e r

( W a t

t )

Figure 14. Performance of a SSSC Operating in Capacitive Mode

(Capacitive Compensation) and Inductive Mode (Inductive Compensation)in the case of injected Active Power

0 0.5 1 1.5 2 2.5 3-1500

-1000

-500

0

500

1000

1500

Time in Seconds

I n j e c t e d

R e a c t

i v e

P o w e r

( V A R )


(Capacitive Compensation) and Inductive Mode (Inductive Compensation)in the case of Injected Reactive Power

Fig 14 and 15 shows the simulation results when anSSSC emulates a reactance in series with the transmissionline. At the time 0 seconds, the SSSC injects no voltage. At0.2 seconds, capacitive reactance compensation is requested.The injecting voltage lags the line current, by almost 90 0.Due to the capacitive reactance there is an increase in theline current and the power flow in the transmission lineincreases. At 0.8 seconds coming into the no injected state.The time interval between 0.8 to 1.6 seconds SSSC does notinject any voltage. At 1.6 seconds, the inductive reactance isrequested. The inverter voltage leads the line current, byalmost 90 0. Due to the inductive reactance there is a decreasein the line current and the power flow in the transmissionline. At 2.5 seconds its again coming into the no injectedstate so it does not emulates any reactance.

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0 0.5 1 1.5 2 2.5 30

0.5

1

1.5

2

2.5x 10

4

Time in Seconds

L i n e

A c t

i v e

P o w e r

( W a t

t )

]


(Capacitive Compensation) and Inductive Mode (Inductive Compensation)in the case of Line Active Power

0 0.05 0.1 0.15 0.2 0.25-100

-80

-60

-40

-20

0

20

40

60

80

100

Time in seconds

V o

l t a g e

( V )

C u r r e n

t ( A )

voltagecurrent

Figure 17. injected voltage and line current

0 0.5 1 1.5 2 2.5 30

500

1000

1500

2000

Time in Seconds

L i n e

R e a c

t i v e

P o w e r

( V A R )


(Capacitive Compensation) and Inductive Mode (Inductive Compensation)in the case of Line Reactive Power

In the fig.16 from the time 0 seconds, the SSSC did notemulate any reactance compensation. At 0.2 seconds,capacitive reactance compensation is requested. Due to the

capacitive reactance there is an increase in the line currentand the power flow in the transmission line increases from12 kW to 22 kW. At 0.8 seconds coming into the no injectedstate. The time interval between 0.8 to 1.6 seconds SSSCdoes not injecting any voltage. At 1.6 seconds, the inductivereactance is requested. Due to the inductive reactance thereis a decrease in t the power flow in the transmission linefrom 12 kW to 2 kW. At 2.5 seconds its again cominginto the no injected state so it does not emulates anyreactance.

Therefore, from the figures 16 and 18 when an SSSCemulates a reactance in series with the transmission line, the

power flow in the transmission line always decreases if theemulated reactance is inductive. Also, the power flow

always increases if the emulated reactance is capacitive.The parameters of the test systemControllable Power rating (P) =10 kWUtility line to line Voltage=200VLine inductance (L) = 1.0 mHLine resistance (R) = 0.04 ohmFrequency = 60 HzPhase difference=100Rms voltage of Vc =12VPI controller gains areKp =0.5Ki = 100Capacitor = 200 F

IV. CONCLUSION The static synchronous series compensator offers an

alternative to conventional series capacitive line

compensation. Whereas the series capacitor is impedancethat produces the required compensating voltage as the linecurrent flows through it, the SSSC is a solid-state voltagesource that internally generates the desired compensatingvoltage. However the voltage is in quadrature to line current(Leading or lagging as per requirement) independent of theline current. The voltage source nature of the SSSC

provides the basis for its superior operating and performance characteristics not achievable by seriescapacitor type compensators.

R EFERENCES [1] N.G Hingroni and L Gyugyi. Understanding FACTS: Concepts and

Technology of flexible AC Transmission System , IEEE Press, NewYork, 2000.[2] L.Gyugyi, C. D. Schauder, K. K. Sen. Static synchronous series

compensator: a solid-state approach to the series compensation of transmission lines, IEEE Trans. on Power Delivery , vol. 12 ,no. 1 ,1997 , pp. 406-417.

[3] K .K.Sen, SSSC -static synchronous series compensator: theory,modeling and applications, IEEE Trans. On Power Delivery, v.13,no.1, 1998, pp.241-246.

[4] Hideaki Fujita, Yasuhiro Watanabe Hirofumi Akagi Control andAnalysis of a Unified Power Flow Controller IEEE Trans.On Power Electronics vol.14, no.6, November 1999, pp. 1021-1027.

[5] Hideaki Fujita, Yasuhiro Watanabe Hirofumi Akagi DynamicPerformance of a Unified Power Flow Controller for Stabilizing ACTransmission SystemsIEEE Trans.On Power ElectronicsVol.14,No.6,November 1999,pp.81-87.

[6] B.Geetalakshmi, A.Saraswathi, P.Dananjayan Comparing andevaluating the performance of SSSC with Fuzzy Logic controller andPI controller for Transient Stability Enhancement Proceeding of IndiaInternational Conference on Power Electronics 2006.

[7] M.S. El- Moursi, A.M. Sharaf, Novel reactive power controllers for the STATCOM and SSSC, Electric Power Systems Research 76(2006) 228-241.

[8] Mohammed El Mours A.M.Sharaf KhalilEl- Arroud Optimal controlschemes for SSSC for dynamic series compensation Electric Power Systems Research 78 (2008) 646 656.

[9] Bruce S. Rigby and Ronald G. Harley An Improved Control Schemefor a Series-Capacitive Reactance Compensator Based on a Voltage-Source Inverter IEEE Transactions on industryapplications,vol.34,no.2, march/april1998.

[10] C.J. Hatziadoniu, Member, A.T. Funk, Student Member,Development of a control scheme for a Series -Connected Solid-StateSynchronous Voltage Source IEEE Transactions on Power Delivery.Vol. 11, No. 2, April 1996.

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