17 Analysis And Imlemenation Of Thysristor Based Statcom

2006 International Conference on Power System Technology

I

Analysis

and

Implement

of

Thyristor-based

STATCOM Jianye Chen, Shan Song, Zanji Wang Department of Electrical Engineering, Tsinghua University, Beijing, 100084

Abstract- As an important member of FACTS family, STATCOM has got more and more widely application. However, conventional STATCOM is based on self-commutated devices, the price and unavailable homemade devices limit its application in China. Based on detailed analysis of commuting process of conventional STATCOM, the authors find that as STATCOM operated in the state of absorb reactive power, only turn-on signals is available, and turn-off signals have no effects. And when STATCOM generates reactive power, only turn-off signals are available. Hence, this paper proposed a new kind of STATCOM, where thyristors instead of self-commutated devices are used as switching devices. Simulation and experimental results show that such a thyristor-based VSC (Voltage source converter) STATCOM can absorb inductive reactive power by adjusting its fire angle and has the same characteristics as ordinary STATCOM within its inductive operating range, so the thyristorbased STATCOM can be used for static var compensation in power systems. Index Terms- STATCOM, Static Var Compensation, Flexible AC Transmission System. I. INTRODUCTION T ODAY'S power systems are widely interconnected

among different regions and countries for economic reasons to reduce cost and improve reliability. But increasingly complex power systems can become less secure because of inadequate power flow control, excessive reactive power and large dynamic swings, which become bottlenecks of fully utilizing the potential of transmission interconnections[l][2]. The FACTS technology is effective on alleviating these difficulties. As an important kind of FACTS devices, SVC is widely used in power systems for shunt reactive compensation. However, using TCR and TSC for reactive power generating, the thyristor controlled SVC brings harmonics and possible harmonic resonance into system. Compared with SVCs, the VSC based STATCOM has better compensating capability, faster response, less harmonics and smaller physical size, and thus becomes a serious competitive alternative to conventional

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-based STATCOM, which can absorb reactive power. Simulation and prototype experimental results show that the thyristor-based STATCOM is applicable. Although cannot generate reactive power, it has the same characteristics as normal STATCOM when absorbing reactive power. So thyristor-based STATCOM can be used for power system reactive compensation. II. PHASE COMMUTING PROCESS OF A NORMAL STATCOM In order to explain the principles of thyristor-based STATCOM, a conventional STATCOM based on square-wave voltage source converter in Fig.1 is analyzed first. The converter consists of 6 GTOs, 6 diodes and a capacitor at DC side. It was connect to system through a Y/Y transformer. System line voltage is 380V 50Hz. Transformer is 1OkVA 380/400V, with leakage reactance of 0.2 p.u. and total loss of 0.04 p.u.. DC capacitor is 2200,uF.

Fig. 1. Simulation circuit of 6 pulse STATCOM

Every GTO of the converter has a turn-on/off period of

1800. PAM is used. Assuming R is equivalent phase resistance,

L is equivalent phase reactance, d is converter fire angle, and system voltage is: UA (t)

=

v2_Us sin (wt)

UB (t) vf2Us sin(wt SVCs[3]. 2w) Normally, self-commutated devices such as GTO, IGCT UC (t) = vUs sin(wt + 3 and IGBT are used in STATCOMs. To reduce switching According to mathematical model of STATCOM, the dylosses, high-power STATCOMs usually use multi-leveled namic model of 6-pulse STATCOM in Fig. 1 can be described square-wave converters rather than PWM converters. Selfcommutated devices as well as their firing, protection and as following: 1 diA control are much more complex and expensive than those of RiA] =- [KUd, sin(wt + 6) -uA (t) dt L 1 27r | diB thyristors' and take up a great part of the total investment - ) uB (t) ) dt L L3KUd, sin(w)t + )t RiB in high-power STATCOMs. In order to take the advantage of I 27r diC 6 [KUd, sin(wt uC(t) dt 3 ) thyristor's low price and robust thyristor's can be used instead d Ud 27r 27r C of self-commutated devices in STATCOMs. ) = -K iA sin(wt + 6) + iB sin(wt + 6 --) + iC sin(wt + 6 + =

-

-

6

+

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

+

-

-

-

-

RiC

-)

The positive direction of line current is shown in Fig. 1. Take positive zero-crossing point of phase voltage as reference of 6, and leading zero point as positive direction of 6. K is the coefficient for fundamental frequency of converter output voltage. For 6-pulse bridge, K = 2/7. In steady state, 3 phase line currents and absorbed reactive power of the STATCOM can be calculated from the above model as following: ii~iA(t) A(t)

= N

~ ~ ~ ~ ~ Ot

UA

oL

3<

/ I1

U'i

4 iB(t) =X Us

sin(wt 61 2w 2 R 2 6 sin(wt + + 7r + 2w l~i c (t) = v \2Usin s 2 3 R 3U2 sin 26 2R

~~3)

(1)

UL L=380V +1.8 -1.8

IA (A, RMS value) 12.0 (leading system voltage) 12.0 (lagging system voltage)

+7.9 -7.9

_~~~~~~~~~

iDl

G

61

=1.80

6 =-1.80 (Vt

GI, DIIX

6

7K,

i' I)X 4i.D4

GI .DI

(4

G4

~D1

GI :D4

G4~

In Fig.2, iAl leads UA by 7/2 + 6, approximately 900. During the positive 1800 of gl, iA1 goes from the negative maximum to the positive maximum. But GI can only conduct current when iA1 > 0. So during this time, GI turns on firstly when iA1 > 0, then D1 turns on when iA1 < 0. Similarly, during the negative 180 of gl (positive 180 of G4 gate signal g4), G4 turns on firstly when iA1 < 0, then D4 turns on when iA1 > 0. The commutation process is D4-GIlDIlG4. Similarly, in Fig.3, when iAl lags iAl by 7/2- 6, the commutation process is GIlD4-G4-DI. Thus tAl can be divided by conducting range of every thyristor and diode as following:

Ui

(~I

D4

(I~~~~~~~~~~~~~~~~~~~~

Fig. 3. Relative phase of phase A voltage, current and fire signal when 6 =-1.8°

6

kA X

I

7

7

UA

iA

f

(kVar)

Q

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

7E/2

~~~~~~~~~~~~~~~~~~I

oA

TABLE I PHASE A CURRENT RMS VALUE AND ABSORBED REACTIVE POWER OF SIMULATION SYSTEM

(degree)

o Wi~ ~ ~ ~ ~ ~ ~ o

1I

oi K

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

d

\. E z

IU2=

gl

2Us Rsin 6 sin(wt + d +-)

2

.D4

Fig. 2. Relative phase of phase A voltage, current and fire signal when 6= +1.8°

GI D1 G4

wt[ E[6, 6±+ 2] wt E

-6+]

wt[ E[6+r, 6+ 2 ] D4 wtC[ 6+3~,2w 6] D1 wt[ E[-6, 6± + 2] wtCE [-d +±2 76+] D4 wtC[[6++ 6+ 2] + 327 2- 6] G4 wtE[

When considering harmonics, the zero-crossing points of iA are little different from iAl, but the commutation process remains the same. Thus iA can be divide according to commutation process as shown in bottom of Fig.2 and Fig.3. The commutation 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), D4 is on. The turn-on signal of GI arrives at 1.80 leading UA positive zero-crossing point, as shown in Fig.2. GI turns on and iA flows through the leakage reactance during commuting until GI takes over iA from D4, as in Fig.4(b). After about 1/4 cycle, GI naturally turns off when iA zero-crossing point arrives, and DI turns on as in Fig.4(c). The commuting process from 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

3

Simulation results of the thyristor-based STATCOM are in

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

CV .Tl

Fig.7.

.

<-

i,4,

VI I

Al--

Cot

0

'11.

4

4

0

1) EL

Vill,

O)t

`.Uy

-

AT.

500

A

.

0~~

-

-500 0.4

0.405

0.41

0.415

0.42

0.425

0.43

0.435

0.44

0.425

0.43

0.435

0.44

0.43

0.435

(s) 20

<.-,,

-20 t

GI

V

ik,,.I-A

0.4

iAl.

')

"'A

cot

----I

,4L_TAA

l-

(3)

-I

(3)

_

0.405

0.41

0.415

i10

A/

0.42

(s)

XTl DJ\Tti 0.425

,, 0.44

30

10

(4) 0.4

Fig. 4. Phase A commutation process when 6 = +1.8°

0.42

20

C,4.1

4w

0.415

a20 .J7, -0

'

0.41

(s)

0.4

v

0.405

30

Fig. 5. Phase A commutation process when

6

1.8

iDl II

iDl 0.405

0.41

0.415

0.42

(s)

0.425

Fig. 7. Phase A current waveforms when 6

=

I

I

0.43

T4

0.435

0.44

+1.8'

When d = -1.80, the commutation process of phase A In Fig.7, when d = +1.8°, the waveform of line current is is GIlD4-G4-DI, as shown in Fig.5(a) (d). GI is on in the same as in Fig.2. The commutation process of phase A Fig.5(a). As in Fig.3, the turn-off signal of GI arrives at 1.8° is D4-TIlDIlT4, the same process as in Fig.2 and Fig.4. lags UA negative zero-crossing point and GI is forced to turn Thyristors take over line currents from diodes when fire signals off. The leakage reactance keeps iA during this commuting arrive. process until iA goes from GI into D4. D4 is on as in Fig.5(b). According to simulation, when d= +1.8°, the STATCOM After about 1/4 cycle, iA crosses 0 and D4 naturally turns in Fig.7 has an absorbed reactive power of 7.9kVA. The line off. At this time, G4 turn-on signal has already arrived, so current RMS value is 12.OA. These results are the same as G4 naturally turns on as in Fig.5(c). The commuting process calculated using equation (1). from G4 to DI is the same as that from GI to D4. Finally, D4 The simulation shows that thyristor-based STATCOM benaturally turns off and GI turns on when iA crosses 0, which haves just the same as normal STATCOM when d > 0 and goes from Fig.5(d) back to Fig.5(a). line currents are continuous. They have same phase comThe above analysis reveals the essential features of a mutation process and can be described with same equations. STATCOM: when d > 0 and STATCOM inputs reactive So thyristor-based STATCOM can also be used for reactive power, GTOs takes over line currents from diodes when turn- power compensation. Since thyristors cannot be turned off, on signals arrive, and turn-off signals have no effects. When the thyristor-based STATCOM can only input reactive power. d < 0 and STATCOM generates reactive power, line currents It must be used together with capacitors to generate reactive in GTOs are forced into diodes when turn-off signals arrive, power. and turn-on signals have no effects. Thus when d > 0, GTOs behave just the same as thyristors. It is possible to replace selfIV. PROTOTYPE VERIFICATION OF THYRISTOR-BASED commutated devices in a normal STATCOM with thyristors to STATCOM get a thyristor-based STATCOM, which can input adjustable reactive power when d > 0. A. Structure of 12-pulse Prototype System A 12-pulse prototype of thyristor-based STATCOM was III. SIMULATION OF THYRISTOR-BASED STATCOM built to verify the theoretical and simulation analysis. The The thyristor-based STATCOM is shown in Fig.6. Replace structure of prototype system is shown in Fig.8. The main the GTOs GIG6 in Fig.1 with thyristors TIT6. Other circuit consists of a Y/YID three-winding transformer (D windings lag Y by 300), 2 six-pulse bridges of thyristors and parameters remain the same. Set fire angle d = +1.8°. diodes (thyristors TYITY6, diodes DYIDY6 in Y bridge, thyristors TD1-TD6, diodes DD1-DD6 in D bridge), and a DC side capacitor. The operation of main circuit is controlled by digital controllers. As in Fig.8, the 12-pulse thyristor STATCOM has an open-loop controller based on FPGA (Xilinx XC3S200) and a close-loop controller based on DSP (TI TMS320F2812). The open-loop controller obtains synchronized signals from 3-phase system voltage, produces 12 fire signals according to Fig. 6. Simulation circuit of 6 pulse thyristor-based STATCOM required fire angle, and amplifies fire signals to drive gates

4

C

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

of thyristors. The fire angle in open-loop controller can be calculated in close-loop controller according to voltage and current signals and transfer to open-loop controller through 16bit data bus. In prototype experiments, fire angle is set to different fixed values to verify the absorbed reactive power under different fire angles. The phase shifting and pulse generating in the open-loop controller are implemented as in Fig.9. Taking phase A for example, 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 edge of voltage synchronize signal arrives. The counter value is compared to fire angle register value in a comparator. The comparator generates start signals for TYI, TY4, TD1, TD4 at the right moment. Since TYI and TY4 (TD1 and TD4) are on the same leg, and their fire signals cannot overlap each other, same pulse-width counter and pulse-modulator can be used for TYI and TY4 (TD1 and TD4). The pulse-width counter gives a square-wave signal according to thyristor start signal and required pulse width. The square-wave is modulated by pulse modulator into a pulse group. The pulse group is amplified and 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 thyristor phase-shifting and firing circuits and ICs cannot provide such a precision. In the FPGA open-loop controller, the delay between rising edge of the square wave from pulse-width counter and the first rising edge of the pulse group from pulse modulator is limited into 2 clock cycles by special designs in the counter and modulator in Fig.9. According to field tests, the total error of fire angles from voltage zero-crossing detection, FPGA phase shifting and delay in thyristor fire circuits is no more than 0.10, which meets the requirement of STATCOM. B. Experimental Results of 12-pulse Prototype In experiments, the prototype device is 5kvar, system phase voltage is 190V/5OHz. The equivalent phase reactance wL 3.5Q, resistance R 1.3Q. DC capacitor is 2200,uF. 1) Steady-state Current Waveforms: Set the fire angle of 12-pulse prototype to 1.80 and 3.6°(leading system phase voltage by 100 and 200us). Fig.10 is the phase A waveforms. The channel 1 of waveforms is system phase voltage UA. The positive direction of iA, tAY, 'AD is marked in Fig.8.

(a) 6= 1.8°, iA

(b) 6

=

3.6°, iA

U

U .

:1)Chl1

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

10O0Vlt: loinl

....... . . . .....

2)Ch2

As 1O

-i

)C h l1: 2)Ch2:

()6 1.8' iAY

(d)

A

6

lo0InS lO ms

...

3.6°, iAY

U OH.l-z('

Fig. 9. Phase shifting and pulse generating in FPGA

The operating range of fire angle in a STATCOM is usually several degrees(e.g., about 100 in the prototype device). To adjust fire angle within this range and control absorbed/generated reactive power accurately, the precision of fire signals must be high enough. In reference [4][5]and[6] mentions a STATCOM controller providing pulse signals at a precision of 0.10. Fire signal precision of the STATCOM controller in reference [7]

2

1)Chl

2) Ch2

100l Volt 10ins A 1O ms

(e) 6

=

J

1.8, iAD

Fig. 10. Phase A current waveforms when 6

(f) d = 3.6°, iAD =

1.8° and 3.6°

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

+<>r~ Fieanlg

experimental results, when d is 1.80 and 3.60, the output line current leads phase voltage by 90° (input line current lags phase voltage by 900), indicating that the STATCOM absorbs reactive power. As d increases, IA and Q increase. Experimental results have verified the theory analysis that thyristor-based STATCOM is applicable. 2) d Q curve of 12-pulse prototype device: Let 0 < < 11.70. In 0 4.50, fix dat every 0.90(50,us). In 4.50 11.70, fix d at every 1.80(100,us). Measure the steady-state absorbed reactive power under every fixed 6, a d - Q curve can be drawn as in Fig.lI. In Fig.lI, when d > 1.8°, Q is in

d

U

5 4 3 2 _S

C~ ~ ~

0

Fig. 11.

6

-

Q

2

curve

~

4

~~

6 8 6 (degree)

10

12

of 12-pulse prototype device

proportion to 6, which is consistent with equation (1). When d < 1.80, a non-adjustable region exists in which Q does not change much as d decreases. This is because the phase current of thyristor-based STATCOM is not continuous when d is relatively small. This non-adjustable region can be reduced by increasing equivalent reactance. 3) Experiment of Close-loop Control: A algorithm consists of feed forward and feed back control is used in the DSP close-loop controller, as shown in Fig. 12. F-ed-back

co---tntrol reactive power

maeanx

+

fI

A

Feed f6-ard

control

Fig. 12.

Close-loop control algorithm

The feed forward loop calculate 60 directly from control objective Qref using coefficient KFD, and the feed back loop calculate A6 from Q,f - Q through a PI controller. To simplify the calculation, p.u. value is used in the algorithm, the measured reactive power Q is converted to p.u. value by coefficient 1/Qbase. The p.u. value of d is calculated from control loop and converted to real value by coefficient 6base When using p.u. value, the feed forward coefficient KFD = 1. To test the close-loop control algorithm, the step response of input 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 of Q is shown in Fig.13. The curve data of Q is calculated in DSP and acquired by emulator. In Fig.13, the total transient time is

5 0.32l

ll

.3

0.226

b--

a0.24 _

-L;; 0

Fig. 13.

10

20

30

40

-X

50 t (ms)

60

70

Close-loop step response of Q when Kp

80

---

90

6.0, KI

1 00

=

3.0

about 30ms(1.5 cycle). The result of step response indicates that by choosing appropriate coefficients, the algorithm in Fig.12 can provide a fast-response control for thyristor-based STATCOM. V. CONCLUSION Based on the analysis of commutation process in a normal STATCOM, this paper presents a thyristor-based STATCOM which can input adjustable reactive power when fire angle > 0. Simulation results show that it behaves the same as normal STATCOM within its operating range. Detailed implementation commentary for the 12-pulse prototype system and its digital controller is provided. Experimental results show good agreement to the theoretical and simulation analysis, indicating thyristor-based STATCOM has the same characteristics as normal STATCOM when inputting reactive power. So in power systems, the thyristor-based STATCOM has the potential for shunt reactive compensation when used together with shunt capacitors. REFERENCES [1] B. Zhang and Q. Ding, "The development of facts and its control," in Proceedings 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 reactive compensation," Power Engineering Journal), vol. 16, no. 3, pp. 151-160, 2002. [3] N. G. Hingorani and L. Gyugyi, Understanding FACTS: Concepts and Technology 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 system for 20 MVar STATCOM," in Industrial Electronics Society, 26th Annual Conference of the IEEE, vol. 2, 2000, pp. 1347-1352. [6] L. Xiu, Q. Wang, and D. Shen, "Study on high accurate asvg digital pulse generator," 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 architecture for a cascaded STATCOM with energy storage," in Applied Power Electronics Conference and Exposition, vol. 3, 2004, pp. 1757-1763.