2006 International Conference on Power System Technology

Analysis and Implement of Thyristor-basedSTATCOM

Jianye Chen, ShanDepartment of Electrical Engineering,

Abstract- As an important member of FACTS family, STAT-COM has got more and more widely application. However,conventional STATCOM is based on self-commutated devices,the price and unavailable homemade devices limit its applicationin China. Based on detailed analysis of commuting process ofconventional STATCOM, the authors find that as STATCOMoperated in the state of absorb reactive power, only turn-onsignals is available, and turn-off signals have no effects. And whenSTATCOM generates reactive power, only turn-off signals areavailable. Hence, this paper proposed a new kind of STATCOM,where thyristors instead of self-commutated devices are usedas switching devices. Simulation and experimental results showthat such a thyristor-based VSC (Voltage source converter)STATCOM can absorb inductive reactive power by adjustingits fire angle and has the same characteristics as ordinarySTATCOM within its inductive operating range, so the thyristor-based STATCOM can be used for static var compensation inpower systems.

Index Terms- STATCOM, Static Var Compensation, FlexibleAC Transmission System.

I. INTRODUCTION

T ODAY'S power systems are widely interconnectedamong different regions and countries for economic

reasons to reduce cost and improve reliability. But increasinglycomplex power systems can become less secure because ofinadequate power flow control, excessive reactive power andlarge dynamic swings, which become bottlenecks of fullyutilizing the potential of transmission interconnections[l][2].The FACTS technology is effective on alleviating these dif-ficulties. As an important kind of FACTS devices, SVC iswidely used in power systems for shunt reactive compensation.However, using TCR and TSC for reactive power generating,the thyristor controlled SVC brings harmonics and possibleharmonic resonance into system. Compared with SVCs, theVSC based STATCOM has better compensating capability,faster response, less harmonics and smaller physical size, andthus becomes a serious competitive alternative to conventionalSVCs[3].

Normally, self-commutated devices such as GTO, IGCTand IGBT are used in STATCOMs. To reduce switchinglosses, high-power STATCOMs usually use multi-leveledsquare-wave converters rather than PWM converters. Self-commutated devices as well as their firing, protection andcontrol are much more complex and expensive than those ofthyristors' and take up a great part of the total investmentin high-power STATCOMs. In order to take the advantage ofthyristor's low price and robust thyristor's can be used insteadof self-commutated devices in STATCOMs.

Song, Zanji WangTsinghua University, Beijing, 100084

Based on detailed analysis of the commutation process(turn-on/off sequence of self-commutated devices and diodes)in a normal STATCOM, this paper proposes a thyristor-basedSTATCOM, which can absorb reactive power. Simulation andprototype experimental results show that the thyristor-basedSTATCOM is applicable. Although cannot generate reactivepower, it has the same characteristics as normal STATCOMwhen absorbing reactive power. So thyristor-based STATCOMcan be used for power system reactive compensation.

II. PHASE COMMUTING PROCESS OF A NORMALSTATCOM

In order to explain the principles of thyristor-based STAT-COM, a conventional STATCOM based on square-wave volt-age source converter in Fig.1 is analyzed first. The converterconsists of 6 GTOs, 6 diodes and a capacitor at DC side. Itwas connect to system through a Y/Y transformer. System linevoltage is 380V 50Hz. Transformer is 1OkVA 380/400V, withleakage reactance of 0.2 p.u. and total loss of 0.04 p.u.. DCcapacitor is 2200,uF.

Fig. 1. Simulation circuit of 6 pulse STATCOM

Every GTO of the converter has a turn-on/off period of1800. PAM is used. Assuming R is equivalent phase resistance,L is equivalent phase reactance, d is converter fire angle, andsystem voltage is:

UA (t) = v2_Us sin (wt)UB (t) = vf2Us sin(wt - -)

UC (t) = vUs sin(wt + 2w3)According to mathematical model of STATCOM, the dy-

namic model of 6-pulse STATCOM in Fig. 1 can be describedas following:

diA 1=- [KUd, sin(wt + 6) -uA (t) - RiA]

dt L| diB 1 27r

)t KUd, sin(w)t + 6 - ) - uB (t) - RiB) dt L L3diC I 27r[KUd, sin(wt + 6 + ) - uC(t) - RiCdt 3d Ud - 27r 27r

C = -K iA sin(wt + 6) + iB sin(wt + 6 --) + iC sin(wt + 6 + )

1-4244-0111-9/06/$20.00 c2006 IEEE.

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The positive direction of line current is shown in Fig. 1.Take positive zero-crossing point of phase voltage as referenceof 6, and leading zero point as positive direction of 6. K isthe coefficient for fundamental frequency of converter outputvoltage. For 6-pulse bridge, K = 2/7. In steady state, 3 phaseline currents and absorbed reactive power of the STATCOMcan be calculated from the above model as following:

~iA(t) 2Us sin 6iiA(t) = N R sin(wt + d +-)4 iB(t) =X Us sin(wt 61 2w

R 2 ~~3)\2Usin6 22wl~ic (t) = v s sin(wt + + 7r +

R 2 33U2 sin 26

2R

(1)

When d is +1.8 and -1.80, the steady-state phase currentand absorbed reactive power of STATCOM are in Table I

TABLE I

PHASE A CURRENT RMS VALUE AND ABSORBED REACTIVE POWER OF

SIMULATION SYSTEM

UL L=380Vd (degree) IA (A, RMS value) Q (kVar)

+1.8 12.0 (leading system voltage) +7.9-1.8 12.0 (lagging system voltage) -7.9

The results in Table I are consistent with equation (1). In(1), when d > 0, the fundamental frequency of phase currentleads system phase voltage by (7/2 + d). When d < 0, thefundamental frequency of phase current lags system phasevoltage by (7/2 -). Taking phase A for example, the relativephases of system phase voltage UA, inverter output phasevoltage UiA, GI fire signal gl, phase current iA and itsfundamental frequency iAl when d is +1.80 and -1.80 areshown in Fig.2 and Fig.3.

UA

oL 3< E z/I1 \.U'i ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Ot

o Wi~~~~~~~~~~~~~~~~~~~~~ogl IU2=

1I

foi K ~~~~~~~~~~~~~~~~~~I IoA

7 7 (I~~~~~~~~~~~~~~~~~~~~~

iDl 61G D4 G4 ~D1 GI :D4 G4~

Fig. 3. Relative phase of phase A voltage, current and fire signal when6 =-1.8

In Fig.2, iAl leads UA by 7/2 + 6, approximately 900.During the positive 1800 of gl, iA1 goes from the negativemaximum to the positive maximum. But GI can only conductcurrent when iA1 > 0. So during this time, GI turns onfirstly when iA1 > 0, then D1 turns on when iA1 < 0.Similarly, during the negative 180 of gl (positive 180 of G4gate signal g4), G4 turns on firstly when iA1 < 0, thenD4 turns on when iA1 > 0. The commutation process isD4-GIlDIlG4. Similarly, in Fig.3, when iAl lags iAl by7/2- 6, the commutation process is GIlD4-G4-DI. ThustAl can be divided by conducting range of every thyristor anddiode as following:

UA

_~~~~~~~~~

Ui

(Vt

7E/2 6

iA kA X 7K,

(~I i' I)XGI, DIIX 4i.D4 GI .DI (4 .D4

Fig. 2. Relative phase of phase A voltage, current and fire signal when6= +1.8

GID1

6 =1.80 G4D4D1

6 =-1.80 D4G4

wt[E[6, 6+ 2]wt E -6+]wt[E[6+r, 6+ 2 ]wtC[ 6+3~,2w 6]wt[E[-6, 6+ 2]wtCE [-d +2 76+]wtC[[6++ 6+ 2]wtE[ + 327 2- 6]

When considering harmonics, the zero-crossing points ofiA are little different from iAl, but the commutation processremains the same. Thus iA can be divide according to com-mutation process as shown in bottom of Fig.2 and Fig.3. Thecommutation process is illustrated in Fig.4 and Fig.5.When d = +1.80, the commutation sequence of phase A is

D4-GIlDIlG4, as shown in Fig.4(a)-(d). In Fig.4(a), D4is on. The turn-on signal of GI arrives at 1.80 leading UApositive zero-crossing point, as shown in Fig.2. GI turns onand iA flows through the leakage reactance during commutinguntil GI takes over iA from D4, as in Fig.4(b). After about1/4 cycle, GI naturally turns off when iA zero-crossing pointarrives, and DI turns on as in Fig.4(c). The commuting processfrom DI to G4 is the same as that from D4 to GI. Finally,G4 turns off when iA crosses 0 and D4 turns on, which goes

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from Fig.4(d) back to Fig.4(a).

AT. A

'

(4)

Fig. 4. Phase A commutation pro-cess when 6 = +1.8

,4L_TAA----I l-

(3)

Fig. 5. Phase A commutation pro-cess when 6 1.8

Simulation results of the thyristor-based STATCOM are inFig.7.

500 .

0~~

-5000.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44

(s)

20

-20 t

0.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44(s)

30

a20 _ A/i10 XTl DJ\Tti

.J7, ,,-00.4 0.405 0.41 0.415 0.42 0.425 0.43 0.435 0.44

(s)

30

20

10 iDl

0.4 0.405

Fig. 7. Phase A current waveforms when 6 = +1.8'

When d = -1.80, the commutation process of phase Ais GIlD4-G4-DI, as shown in Fig.5(a) (d). GI is on inFig.5(a). As in Fig.3, the turn-off signal of GI arrives at 1.8lags UA negative zero-crossing point and GI is forced to turnoff. The leakage reactance keeps iA during this commutingprocess until iA goes from GI into D4. D4 is on as in Fig.5(b).After about 1/4 cycle, iA crosses 0 and D4 naturally turnsoff. At this time, G4 turn-on signal has already arrived, soG4 naturally turns on as in Fig.5(c). The commuting processfrom G4 to DI is the same as that from GI to D4. Finally, D4naturally turns off and GI turns on when iA crosses 0, whichgoes from Fig.5(d) back to Fig.5(a).The above analysis reveals the essential features of a

STATCOM: when d > 0 and STATCOM inputs reactivepower, GTOs takes over line currents from diodes when turn-on signals arrive, and turn-off signals have no effects. Whend < 0 and STATCOM generates reactive power, line currentsin GTOs are forced into diodes when turn-off signals arrive,and turn-on signals have no effects. Thus when d > 0, GTOsbehave just the same as thyristors. It is possible to replace self-commutated devices in a normal STATCOM with thyristors toget a thyristor-based STATCOM, which can input adjustablereactive power when d > 0.

III. SIMULATION OF THYRISTOR-BASED STATCOM

The thyristor-based STATCOM is shown in Fig.6. Replacethe GTOs GIG6 in Fig.1 with thyristors TIT6. Otherparameters remain the same. Set fire angle d = +1.8.

Fig. 6. Simulation circuit of 6 pulse thyristor-based STATCOM

In Fig.7, when d = +1.8, the waveform of line current isthe same as in Fig.2. The commutation process of phase Ais D4-TIlDIlT4, the same process as in Fig.2 and Fig.4.Thyristors take over line currents from diodes when fire signalsarrive.

According to simulation, when d= +1.8, the STATCOMin Fig.7 has an absorbed reactive power of 7.9kVA. The linecurrent RMS value is 12.OA. These results are the same ascalculated using equation (1).The simulation shows that thyristor-based STATCOM be-

haves just the same as normal STATCOM when d > 0 andline currents are continuous. They have same phase com-mutation process and can be described with same equations.So thyristor-based STATCOM can also be used for reactivepower compensation. Since thyristors cannot be turned off,the thyristor-based STATCOM can only input reactive power.It must be used together with capacitors to generate reactivepower.

IV. PROTOTYPE VERIFICATION OF THYRISTOR-BASEDSTATCOM

A. Structure of 12-pulse Prototype SystemA 12-pulse prototype of thyristor-based STATCOM was

built to verify the theoretical and simulation analysis. Thestructure of prototype system is shown in Fig.8. The maincircuit consists of a Y/YID three-winding transformer (Dwindings lag Y by 300), 2 six-pulse bridges of thyristors anddiodes (thyristors TYITY6, diodes DYIDY6 in Y bridge,thyristors TD1-TD6, diodes DD1-DD6 in D bridge), and aDC side capacitor.The operation of main circuit is controlled by digital con-

trollers. As in Fig.8, the 12-pulse thyristor STATCOM has anopen-loop controller based on FPGA (Xilinx XC3S200) anda close-loop controller based on DSP (TI TMS320F2812).The open-loop controller obtains synchronized signals from3-phase system voltage, produces 12 fire signals according torequired fire angle, and amplifies fire signals to drive gates

CV.Tl.4

Al-- Vill,

4

C

Fig. 8. Prototype system of 12 pulse thyristor-based STATCOM

of thyristors. The fire angle in open-loop controller can becalculated in close-loop controller according to voltage andcurrent signals and transfer to open-loop controller through16bit data bus. In prototype experiments, fire angle is set todifferent fixed values to verify the absorbed reactive powerunder different fire angles.The phase shifting and pulse generating in the open-loop

controller are implemented as in Fig.9. Taking phase A forexample, the 4 thyristors in leg A of Y and D bridges (TYI,TY4, TD1, TD4) are synchronized to phase A system voltage.The phase shifting counter starts when rising/falling edgeof voltage synchronize signal arrives. The counter value iscompared to fire angle register value in a comparator. Thecomparator generates start signals for TYI, TY4, TD1, TD4 atthe right moment. Since TYI and TY4 (TD1 and TD4) are onthe same leg, and their fire signals cannot overlap each other,same pulse-width counter and pulse-modulator can be used forTYI and TY4 (TD1 and TD4). The pulse-width counter givesa square-wave signal according to thyristor start signal andrequired pulse width. The square-wave is modulated by pulsemodulator into a pulse group. The pulse group is amplifiedand goes through pulse transformer to drive thyristor gates.

is 8us (z 0.140). In prototype of thyristor based STATCOM,the same precision is needed. But most of existing thyristorphase-shifting and firing circuits and ICs cannot provide sucha precision. In the FPGA open-loop controller, the delaybetween rising edge of the square wave from pulse-widthcounter and the first rising edge of the pulse group from pulsemodulator is limited into 2 clock cycles by special designsin the counter and modulator in Fig.9. According to fieldtests, the total error of fire angles from voltage zero-crossingdetection, FPGA phase shifting and delay in thyristor firecircuits is no more than 0.10, which meets the requirementof STATCOM.

B. Experimental Results of 12-pulse Prototype

In experiments, the prototype device is 5kvar, system phasevoltage is 190V/5OHz. The equivalent phase reactance wL3.5Q, resistance R 1.3Q. DC capacitor is 2200,uF.

1) Steady-state Current Waveforms: Set the fire angle of12-pulse prototype to 1.80 and 3.6(leading system phasevoltage by 100 and 200us). Fig.10 is the phase A waveforms.The channel 1 of waveforms is system phase voltage UA. Thepositive direction of iA, tAY, 'AD is marked in Fig.8.

(a) 6= 1.8, iA

U

U

Fig. 9. Phase shifting and pulse generating in FPGA

The operating range of fire angle in a STATCOM is usuallyseveral degrees(e.g., about 100 in the prototype device). To ad-just fire angle within this range and control absorbed/generatedreactive power accurately, the precision of fire signals must behigh enough. In reference [4][5]and[6] mentions a STATCOMcontroller providing pulse signals at a precision of 0.10. Firesignal precision of the STATCOM controller in reference [7]

. ....~~~~~~~~~~~.....

:1)Chl1 10O0Vlt: loinl2)Ch2 As 1O -i

()6 1.8' iAY................... ........ .... ..............

1)Chl 100l Volt 10ins2) Ch2 A 1O ms

(e) 6 = 1.8, iAD

U

(b) 6 = 3.6, iA

)Ch l1: lo0InS2)Ch2: A lO ms ...

(d) 6 3.6, iAY

(f) d = 3.6, iAD

Fig. 10. Phase A current waveforms when 6 = 1.8 and 3.6

In Fig. 10, the output direction is the positive directionof line currents, just the same as in Fig.8. According to

J

OH.l-z('

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experimental results, when d is 1.80 and 3.60, the outputline current leads phase voltage by 90 (input line currentlags phase voltage by 900), indicating that the STATCOMabsorbs reactive power. As d increases, IA and Q increase.Experimental results have verified the theory analysis thatthyristor-based STATCOM is applicable.

2) d Q curve of 12-pulse prototype device: Let 0 1.8, Q is in

_S

U

5

4

3

2

C~~~ ~ ~~

0 2 4 66 (degree)

8 10 12

Fig. 11. 6 - Q curve of 12-pulse prototype device

proportion to 6, which is consistent with equation (1). Whend < 1.80, a non-adjustable region exists in which Q doesnot change much as d decreases. This is because the phasecurrent of thyristor-based STATCOM is not continuous whend is relatively small. This non-adjustable region can be reducedby increasing equivalent reactance.

3) Experiment of Close-loop Control: A algorithm consistsof feed forward and feed back control is used in the DSPclose-loop controller, as shown in Fig. 12.

F-ed-backco---tntrol

reactive power maeanx+ fI A+ < +> r ~~~~~~~~~~~Fire anlgle

Feed f6-ardcontrol

Fig. 12. Close-loop control algorithm

The feed forward loop calculate 60 directly from controlobjective Qref using coefficient KFD, and the feed backloop calculate A6 from Q,f - Q through a PI controller. Tosimplify the calculation, p.u. value is used in the algorithm,the measured reactive power Q is converted to p.u. value bycoefficient 1/Qbase. The p.u. value of d is calculated fromcontrol loop and converted to real value by coefficient 6baseWhen using p.u. value, the feed forward coefficient KFD = 1.

To test the close-loop control algorithm, the step response ofinput reactive power Q is measured. Let KP = 6.0, KI = 3.0,and Qref jumps from 0.2p.u. to 0.3p.u.. The step response ofQ is shown in Fig.13. The curve data of Q is calculated in DSPand acquired by emulator. In Fig.13, the total transient time is

0.32l ll

.3

0.226 -b- -X ---

a0.24 _ -L;;

0 1 0 20 30 40 50 60 70 80 90 1 00t (ms)

Fig. 13. Close-loop step response of Q when Kp 6.0, KI = 3.0

about 30ms(1.5 cycle). The result of step response indicatesthat by choosing appropriate coefficients, the algorithm inFig.12 can provide a fast-response control for thyristor-basedSTATCOM.

V. CONCLUSION

Based on the analysis of commutation process in a normalSTATCOM, this paper presents a thyristor-based STATCOMwhich can input adjustable reactive power when fire angle > 0.Simulation results show that it behaves the same as normalSTATCOM within its operating range. Detailed implementa-tion commentary for the 12-pulse prototype system and itsdigital controller is provided. Experimental results show goodagreement to the theoretical and simulation analysis, indicatingthyristor-based STATCOM has the same characteristics asnormal STATCOM when inputting reactive power. So in powersystems, the thyristor-based STATCOM has the potential forshunt reactive compensation when used together with shuntcapacitors.

REFERENCES

[1] B. Zhang and Q. Ding, "The development of facts and its control," inProceedings of the 4th International Conference in Power System Control,Operation and Management, 1997, pp. 49-53.

[2] D. J. Hanson and M. L. Woodhouse, "Statcom, a new era of reactivecompensation," Power Engineering Journal), vol. 16, no. 3, pp. 151-160,2002.

[3] N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts andTechnology of Flexible AC Transmission Systems. IEEE Press, 2000.

[4] Q. Jiang, "Modeling and controlling of advanced static var generator,"Ph.D. dissertation, Tsinghua University, 1999.

[5] C. Li, Q. Jiang, and W. Liu, "Field test of a DSP-based control systemfor 20 MVar STATCOM," in Industrial Electronics Society, 26th AnnualConference of the IEEE, vol. 2, 2000, pp. 1347-1352.

[6] L. Xiu, Q. Wang, and D. Shen, "Study on high accurate asvg digital pulsegenerator," Journal of Tsinghua University (Sci and Tech), vol. 37, no. 7,pp. 35-38, 1997.

[7] Q. Chang and L. C. Mariesa, "A multi-processor control system architec-ture for a cascaded STATCOM with energy storage," in Applied PowerElectronics Conference and Exposition, vol. 3, 2004, pp. 1757-1763.

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