Steady State and Dynamic Performance of STATic synchronous COMpensator

Nitus Voraphonpiput Assist. Prof. Dr.Somchai Chatratana System Control and Operation Division Electrical Engineering Department Phone: 024362164 Fax.: 024368913 King Mongkut's Institute of Technology North Bangkok nitus@it.egat.or.th scn@kmitnb.ac.th Abstract

The static synchronous compensator or STATCOM is classified as a shunt compensator type of Flexible AC Transmission System (FACTS) which can increase voltage and angle stability of power system. This paper describes the steady state and dynamic performance of STATCOM in the function of voltage regulation. The simple radial system was used as an example case for the study. It showed that the STATCOM could perform as a reactive power generator which could absorb or deliver reactive power from the power system, and result in regulated voltage at the receiving end. To achieve the purpose, the PI controller has to be designed, and the modulus optimum (MO) was used for tuning. The performance of STATCOM under MO design was illustrated in this paper and all simulation was done on ATP-EMTP. The simulation results showed that the example case could reject all disturbances from switching in and switching out of difference load types such as RL-type and RC-type within three cycles (60 ms). 1. Introduction

The power system always has disturbances from load, generators and transmission lines, so they affect variation of bus voltage at any connection points. Sometimes, the disturbances may cause unacceptable voltage level to be over or under the standard limit. The shunt compensator was a good practice for recovering bus voltage to the acceptable level. The high attention has been focused on FACTS devices to improve power system stability (voltage and angle stability) by using fast action of power electronic devices and static controller. The STATCOM is a kind of FACTS devices which is classified as a shunt compensator. One basic purpose in control engineering is to implement STATCOM as a voltage regulator of the power system at the bus where STATCOM is installed. The AC voltage regulation is regulated by controlling the reactive power exchanged between STATCOM and the power system. STATCOM was installed in many utilities such as the Tennessee Volley Authority (TVA), and the National Grid Company (NGC), because they used it to prevent voltage collapse. [1-2]

This paper described the principles of STATCOM and their configurations. The steady state and dynamic performance under voltage regulation function are also discussed. To achieve the AC voltage regulation, A classical control theory was used for controller design. This paper applied the modulus optimum (MO) which is well practical, and selects a 230 kV radial system that consists of single three-phase sources, transmission lines and 500 MVA load. The 300 MVAr six-pulse STATCOM was installed at the receiving end for generating or absorbing reactive power. This case was simulated on ATP-EMTP and the Transient Analysis of Control System (TACS) was used in the simulation for controlling the STATCOM model 2. Principle and configuration

Considering the two-bus system as shown in figure 1, the reactive power flow from bus 1 to bus 2 depends on the bus voltage level. Namely, The reactive power flows from the higher voltage level to the lower voltage level. This phenomenon can be explained by using the phasor

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diagram in Figure 1. The current flow (I) from bus 1 to bus 2 must always lag voltage drop on the inductor with the 90 degree angle, and this voltage is the difference between voltage Bus 1 and Bus 2. consequently if the voltage U2 is higher than U1, the current will act as a leading current, and if the voltage U2 is less than U1, the current will act as a lagging current. The voltage at Bus 2 can control the reactive power in this system.

Figure 1. The simple two-bus power system Considering Figure 2, The three-phase circuit where the STATCOM is installed at Bus 2.

The STATCOM circuit consists of voltage source converter (VSC), and operates as an inverter and a capacitor with the direct current in its capacitance side. The voltage source converter might be a GTO or an other high power electronic switch. In this figure, the voltage source converter is a six-pulse converter which has 180-degree conduction period in each switching.

Figure 2. The six-pulse STATCOM Circuit

The Bus 2 voltage (U2) varies with the capacitor voltage (Udc) which can be controlled by

the capacitor current (Idc). Neglecting line resistance, The capacitor current can be approximated by this equation [3].

U1 U2 U1

U2

UL UL

I

I

I

U1

U2 UL

U1A

U1B

U1C

R L

R L

R L C

GTO D

Udc

Idc U2A

U2B

U2C

+

-

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δπ

sinfL2

kUI 1

dc = (1)

Where π2k = , U1 is Bus 1voltage, U2 is Bus 2 voltage, Udc is capacitor voltage [V], L is

transformer leakage reactance [H], δ is difference angle between two buses, f is frequency [Hz]. The capacitor voltage is related with the integral of its current divided by the capacitance

value. It can be seen that the capacitor current can be controlled by the difference angle between two buses. It might be said that capacitor voltage can be controlled by δ, and δ is directly related with the lagging or leading angle of voltage source converter when comparing with the voltage at Bus 1. Namely, if the firing angle is delayed, the capacitor voltage will be high. Conversely, If the firing angle is advanced, the capacitor voltage will be low. This phenomenon can be illustrated in the phasor diagram in Figure 3, and the waveform of voltage and current at steady state are shown in Figure 4.

Figure 3. Phasor Diagram of two bus power system with STATCOM

(a) (b) Figure 4. The waveform of voltage and current (phase A) at steady state of the system

δ δ U1

U2

jXL I

I

R I

U1

U2

jXL I

I

R I

U1 U2 jXLR

I

Leading power factor Lagging power factor

(f ile STATCOMdesign.pl4; x-var t) c:XXS1 -XXU1 v:XXSA-v:XXSB v:XXU1 -XXU2 v:XXSA

0.160 0.165 0.170 0.175 0.180 0.185 0.190 0.195 0.200[s]-600

-400

-200

0

200

400

600

(file STATCOMdesign.pl4; x-var t) c:XXS1 -XXU1 v:XXU1 -XXU2 v:XXSA-v:XXSB v:XXSA

0.160 0.165 0.170 0.175 0.180 0.185 0.190 0.195 0.200[s]-600

-400

-200

0

200

400

600

U1ABU2AB

U1A

I

U1ABU2AB

U1A

I

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3. Voltage regulation performances Considering the power system in Figure 5, it contains a 230 kV power system, an

equivalent circuit, and loads. The step-down transformer is connected between the equivalent circuit and the six-pulse STATCOM. This system has 20.08 kA, three-phase short circuit current. The size of STATCOM capacitor is 4800 µF.

Figure 5. The power system voltage regulation case study

This power system is considered as the controlled system and its transfer function is needed. This system can be identified by a step response test. The basic transfer function should be in the form of equation below. [1]

sT1eA

)s()s(U

)s(F1

sTm

d

+==

−

δ (2)

This equation is confirmed by the step response test in Figure 6. The delayed time of the

power system is equal to 28 ms, the measuring delayed time is equal to 6.66 ms, and the system gain is 1.66 [1/rad]. The delayed time of the voltage source converter is approximately 1.7 ms and it can be neglected because of very small delayed time, so this plant is approximate as two time delayed elements.

Figure 6. The step response and approximated block diagram of plant

(f ile STATCOM_perf ormance4T.pl4; x-v ar t) factors:offsets:

10

t: V 1-0.993

t: ALPHA 0.010

0.48 0.52 0.56 0.60 0.64 0.68[s]0

5

10

15

20

25

30

35*10-3

100%

28 ms 63%

1.7 ms

0.0288

1 6.66 ms 1.66 28 ms U2(s)δ(s)

6-pulse STATCOM

U1 U2 jXL R

I

Rload

Xload

Firing Circuit

H300 MVA,230/16 kV

%uk =20%

-

+

uref

um

PI controller δmax

δmin

6

230 kV Isc =20.08 kA

85 deg

C US

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Applying modulus optimum technique, the PI controller is suggested. [4] The controller parameters are as follows: Kp = 1.27 [rad] and Ti = 28 ms, the performance of voltage regulation under modulus optimum is presented in figure 7 and figure 8.

(a) (b)

Figure 7. Performance of Voltage regulation during energizing 500 MVA load

(a) (b)

Figure 8. Voltage regulation performance during tripping 250 MVA load And switching a 200 MVAr capacitor

Figure 7(a) presents the voltage at Bus 2, and it compares the system performance

between the system without compensator and the system with STATCOM under the MO design. The simulation result demonstrates that the bus voltage can recover to the pre-disturbance level within three cycles after disturbance from 500 MVA load occurred. Figure 7(b) demonstrated that the phase A of STATCOM current responses, when the leading current is increased after this disturbance.

STATCOM_perf ormance5without.pl4: t: V STATCOM_PERFORMANCE5M.pl4: t: V

0.48 0.52 0.56 0.60 0.64 0.68 0.72 0.76[s]0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

1.02

(f ile STATCOM_PERFORMANCE5M.pl4; x-v ar t) factors:offsets:

10

v :U1A 10

c:U2A -UPA 1000

0.48 0.52 0.56 0.60 0.64 0.68 0.72 0.76[s]-200

-150

-100

-50

0

50

100

150

200*103

3 cycle With MO design

Without STATCOM

U [pu] U1A ISTATCOM

(f ile STATCOM_perf ormance5M.pl4; x-v ar t) factors:offsets:

10

v :U1A 10

c:U2A -UPA 1000

0.96 1.00 1.04 1.08 1.12 1.16 1.20[s]-200

-150

-100

-50

0

50

100

150

200*103

(f ile STATCOM_perf ormance5M.pl4; x-v ar t) t: V 0.96 1.00 1.04 1.08 1.12 1.16 1.20[s]

0.96

0.97

0.98

0.99

1.00

1.01

1.02

1.03

1.04

3 cycle

Measured voltage

U [pu] U1A ISTATCOMA(la

gging) ISTATCOMA(le

ading)

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The STATCOM response during changing mode was shown in Figure 8. In this case, the voltage reference is set to 1.0 p.u, 250 MVA load is rejected from the power system, and a 200 MVAr capacitor load is switched in at the same time, Figure 8(a) shows that the controlled bus voltage can also recovered within three cycles. Figure 8(b) shows that the phase A, STATCOM current responses during changing mode from generating reactive power to absorbing reactive power, and the current was changed from a leading current to a lagging current within a cycle.

4. Conclusion

This work is focused on steady state and dynamic of STATCOM. This device can perform as a reactive power source which can generate or absorb reactive power in the power system. The STATCOM capacitor is used for exchanging the reactive power and maintaining the dc voltage level at the dc side. The reactive power can flow into STATCOM by reducing voltage at a capacitor and it can be controlled by making the leading δ angle, conversely, the STATCOM can generate reactive power by making the lagging δ angle, both conditions are introduced in this paper. Furthermore, it also suggests the simple way for rough system identification by using the step response test, and the test indicates the dominant pole of a plant. The other dominant pole come from the measuring system, thus the STATCOM and the power system can be estimated by two delay elements.

The AC voltage regulation can be achieved by using closed loop control. The modulus optimum (MO) is applied for PI controller design. The simulation was done on ATP-EMTP, indicates that the controlled bus voltage can be recovered within three cycles after disturbance. The simulation result also demonstrates the STATCOM current during a disturbance, and the leading current can be changed to the lagging current within a cycle.

Because of its fast response and less reactance, STATCOM is an attractive shunt compensator in the power system and the greatest benefit of a STATCOM is applied for automatic AC voltage regulation. This may be a new coming in Thailand, understanding STATCOM performance might be a good preparation for EGAT in the near future. References [1] Narain G. Hingorani, Laszlo Gyugyi "Understanding FACTS concepts and Technology of Flexible AC Transmission Systems" IEEE Press 2001 [2] GE Power Systems "Flexible AC Transmission System (FACTS) Technologies on the TVA Transmission" EPRI TR-106462 May 1996 [3] Philip Moore, Peter Ashmole “Flexible AC Transmission systems part 4 Advanced FACTS controllers” Power Engineering Journal, April 1998 [4] Friedrich Fröhr, Fritz Orttenburger “Introduction to Electronic Control Engineer” SIEMENS AKTIENGESELLSCHAFT 1982

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