power system stabilization by fault current limiter and thyristor controlled braking...

Power System Stabilization by Fault Current Limiter and Thyristor Controlled Braking Resistor

Masaki Yagami Member, IEEE

Hokkaido Institute of Technology 7-15 Maeda, Teine-ku,

Sapporo-shi, 006-8585, Japan [email protected]

Junji Tamura Senior Member, IEEE

Kitami Institute of Technology 165 koen-cho,

Kitami-shi, 090-8507, Japan [email protected]

Abstract -- This paper presents a power system stability enhancement scheme using combination of fault current limiter and thyristor controlled braking resistor. The fault current limiter operates for limiting of fault currents, enhancement of the power system stability and suppression of turbine shaft torsional oscillations. After that, the thyristor controlled braking resistor operates with the objective of fast control of generator disturbances. The effectiveness of both devices has been demonstrated by considering 3LG (three-lines-to-ground) fault in a two-machine infinite bus system. Also, temperature rise effect of both devices has been demonstrated. Simulation results indicate a significant power system stability enhancement and damping turbine shaft torsional oscillations as well as with allowable temperature rise.

Index Terms—Power system stability, Fault current limiter, Thyristor controlled braking resistor, Temperature rise effect

I. INTRODUCTION

The use of fault current limiter (FCL) is being evaluated as one element necessary to limit the fault currents and enhance the power system stability [1]-[3]. FCL is a device that limits the fault currents by generating an impedance when a fault occurs. In addition, the limiting impedance generated to limit fault currents proves helpful in increasing generator output degraded by a fault, thus providing stabilization of power system. However, as FCL installed in series with transmission line can be just operated during the period from fault occurrence to fault clearing, FCL cannot control the generator disturbances after the clearing of fault.

The braking resistor is also known as a very effective device for power system stability control. Besides, with the recent development of power electronics technology, replacing the circuit breaker with the semiconductor device is becoming feasible. Several thyristor-based control techniques have been proposed in the literature for the switching of braking resistor [4]-[6]. Since the thyristor controlled braking resistor (TCBR) can control an accelerating power in generator with flexibility, the power system stability may be enhanced more than that of the use of braking resistor controlled by mechanical device. However, as TCBR is installed in parallel with the transmission line, TCBR cannot be operated before the clearing of faults.

From these viewpoints, we have proposed the power

system enhancement scheme using combination of FCL and TCBR for the purpose of a significant power system stability enhancement and damping turbine shaft torsional oscillations. If both devices-FCL and TCBR operate at the same bus, the stabilization control scheme can be carried out continuously and with flexibility. Namely, FCL operates from the fault occurrence instance to the fault clearing, and then TCBR operates dynamically until the generator disturbance becomes small. Through the simulation analysis, the effectiveness of combination of FCL and TCBR on power system stability enhancement is demonstrated.

Moreover, this paper presents the results of analyses about temperature rise effect of resistance materials of both devices. FCL and TCBR dissipate accelerating energy of the generator in the form of heat. Therefore, temperature of the resistor material rises above ambient temperature. In particular, TCBR may become high-temperature state because it operates for a long duration. Higher temperature may cause the resistor material to melt away and due to higher resistance value, the system response may be somewhat affected. In order to confirm the temperature rise effect, both devices with various resistance values are tested in the simulation. These analyses are performed using EMTP.

II. MODEL SYSTEM

A. Power system

The two-machine infinite bus system model used for the simulation is shown in Fig. 1. It consists of two generators, an infinite bus, transformers, FCL, TCBR and double-circuit transmission lines. The line constants in the diagram are given in terms of R + jX (jB/2) per circuit. Turbine shaft model has six masses namely high-pressure (HP) turbine, an intermediate-pressure (IP) turbine, two low-pressure turbines (LPA, LPB), the generator (GEN) and exciter (EXC) as shown in diagram. Table I and Table II show the generator parameters [7] and turbine shaft parameters [8] respectively. The models of AVR and governor are shown in Fig. 2.

FCL is installed on Y-side of the transformer, Tr.1. In general, FCL should be installed wherever fault current is extremely strong. However, FCL is installed only at the generator terminal because this study is focused mainly on

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the stabilization of the generator. Commutation-type FCL which consists of a shunt resistor with RFCL value and a variable resistor is used.

TCBR is installed in parallel at the same bus. Resistor with RTCBR value is connected to the grid through the thyristor switching circuit. Thyristor switch is controlled by firing-angle α which is calculated based on the generator speed deviation ωΔ .

In the simulation study, it has been considered that the three-lines-to-ground (3LG) fault occurs near the generator G1 at 0.1 (sec). The circuit breakers on the faulted line are opened at 0.2 (sec) and reclosed at 1.0 (sec). It is assumed that the circuit breakers clear the line when the current through it crosses the zero level.

B. FCL

Commutation-type FCL which consists of the shunt resistor with RFCL value and the variable resistor is used. The value of the variable resistor is 0 (pu) during a normal condition. When the fault currents exceed a critical value which is set to 2.5 (pu), the resistance value of the variable resistor increases linearly to 25 (pu) within 1 (ms) (such a characteristic can be achieved by a superconducting coil [9]).

(pu) (pu) (sec) (sec) (sec) (sec) (sec)

X’’q

X0

T’d0 T’q0 T’’d0 T’’q0 H

200

(pu) (pu) (pu) (pu) (pu) (pu) (pu)

G1

ra

Xl

Xd

Xq

X’d

X’q

X’’d

0.003 0.102 1.651

1.59

0.232

0.38

0.171

0.1710.13

5.90.5350.0330.078

9.0

Rating (MVA)

(pu) (pu) (sec) (sec) (sec) (sec) (sec)

X’’q

X0

T’d0 T’q0 T’’d0 T’’q0 H

130

(pu) (pu) (pu) (pu) (pu) (pu) (pu)

G2

ra

Xl

Xd

Xq

X’d

X’q

X’’d

0.004 0.078

1.22

1.16

0.174

0.25

0.134

0.1340.138.97

1.50.0330.141

6.0

Rating (MVA)

TABLE I GENERATOR PARAMETERS

19.303 34.929 52.038 70.858

2.82

HP Turbine LP Turbine LPA Turbine LPB Turbine Generator Exciter

Inertia constant (s) Spring constant (pu T/rad)

0.288890 0.483849 2.670287 2.749726 2.700840 0.106406

G1

19.303 34.929 52.038 70.858

2.82

HP Turbine LP Turbine LPA Turbine LPB Turbine Generator Exciter

Inertia constant (s) Spring constant (pu T/rad)

0.192593 0.322566 1.780191 1.833151 1.800560 0.070937

G2

TABLE II TURBINE SHAFT PARAMETERS

Fig. 1. Power system model

Infinite bus 1.04 0.0∠

3LG

Tr.1

(P/Q=1.9/0.1)

(P/Q= 1.25/0.5)

(P/Q=1.0/0.35)

0.0238+j0.2016 (j0.0523)

G1

j0.0625

G2

(P/V=1.2/1.02) 0.017+j0.144 (j0.0373)

(P/Q= 0.9/0.3)

0.020+j0.170 (j0.044) 0.034+j0.184

(j0.0395)

0.064+j0.322 (j0.0765)

0.078+j0.340 (j0.0895)

j0.0586

j0.0576

Tr.2

Tr.3

FCL

TCBRRTCBR

ωΔ

HP IP LPA LPB GEN EXC

*Unit is pu value on system base (100 MVA)

Turbine Shaft System outR

FCLR

PI

HP: IP: LPA, LPB: GEN: EXC:

High-Pressure Turbine Intermediate-Pressure Turbine Low-Pressure Turbine Generator Exciter

)(α

Vso

Vs +

- 25

1+0.2S

Efdo + +

4.0

-4.0 (a) AVR

1+2.0S

20 + + 1.05

0.0

Po

- +

ωm0

ωm

(b) Governor

Fig. 2. AVR and governor models

Efd

P

549


Therefore, majority of the current flows into the shunt resistor, and then limited. To avoid an excess load, we have assumed that the resistance value of the variable resistor returns to 0 (pu) at the same time when the circuit breakers are opened at 0.2 (sec).

C. TCBR

As shown in Fig. 1, the generator speed deviation ωΔ and the desired resistance value outR are selected as the signals of input and output respectively. Following a fault in the power system, ωΔ of the generator is measured, and then outR is determined by PI controller device which is tuned by trial and error method. Firing-angle α for the thyristor switch is calculated from the output of the PI controller (i.e., outR ). The desired power consumption determined by outR and the real power consumption determined by TCBRR are equal and hence α can be calculated from this relationship. When ωΔ becomes less than 0.001 (pu), TCBR is removed from the grid.

D. Resistor Material

In this work, we have used an austenitic stainless steel [10] as resistor materials of both devices. For simplicity, the same values of material parameters (i.e., mass, surface area, heat exchange coefficient and specific heat) are used for both devices. The material parameters are shown in Table III.

When a current, I, flows into the resistor (i.e., the device is in ‘ON’ condition), temperature of the resistor can be calculated from the following equation [12]:

(1) where R is resistance of the resistor, tON is ‘ON’ duration time of the device and TON0 is initial temperature of the resistor.

On the other hand, when the device is not in the circuit (i.e., the device is in ‘OFF’ condition), it gets cooled down by dissipating heat into the air. Therefore, temperature of the resistor decreases exponentially. The temperature decrease can be calculated from the following Newton’s law of cooling:

(2) where TOFF is temperature of the resistor in ‘OFF’ condition, Ta is ambient temperature (20 °C), TOFF0 is initial temperature of the resistor, n is positive constant (0.01 for this work) and tOFF is ‘OFF’ duration time of the device.

When temperature of the resistor changes, resistance value of the resistor also changes according to the following equation [13]:

(3) where R is resistance after the temperature change, R0 is initial resistance in the ambient temperature, γ is

temperature coefficient (0.0012°C-1 [13]).

III. SIMULATION RESULTS

A. Power system stability

Fig. 3 shows the load angle responses of each generator in cases of “no devices”, “only FCL with RFCL=1.1 (pu)”, “only TCBR with RTCBR=0.6 (pu)” and “both devices-FCL and TCBR with RFCL=1.1 (pu) and RTCBR=2.5 (pu)”. Because of the absorption of real power by FCL and TCBR, the load angle oscillations are restrained in the case of “with devices”. In particular, the load angle oscillations in the case of “both devices” are restrained effectively for the overall duration of simulation. In “both devices”, FCL dissipates majority of the accelerating energy of the generator, and then TCBR dissipates the accelerating energy of the remainder. As a result, the use of both devices makes the system stabile quickly more than the use of “one-unit”.

KSCM

tRITT ON

ONON ++=

2

0

OFFntaOFFaOFF eTTTT −−+= )( 0

})(1{ /0 aOFFON TTRR −+= γ

0 1 2 3 4 5 6 72 0

4 0

6 0

8 0

1 0 0

Loa

d an

gle

(d

eg

)

T im e (s )

N o d e v ic e s F C L T C B R F C L & T C B R

0 1 2 3 4 5 6 72 0

4 0

6 0

8 0

1 0 0

N o d e v ic e s F C L T C B R F C L & T C B R

Lo

ad

an

gle

(d

eg

)

T im e (s )

(a) G1

Fig. 3. Load angles of each generator

(b) G2

520 (J/(kg·°C)) [11]

300 (kg) 16.5 (W/(m2·°C)) [11] 8 (m2)

C M K S

specific heat mass heat exchange coefficient surface area

TABLE III RESISTOR MATERIAL PARAMETERS OF BOTH DEVICES

550


Fig. 4 shows the firing-angle responses of the thyristor switch for phase ‘a’ in the cases of “only TCBR” and “both devices”. The firing-angle α varies from 0 to 180 (deg) according to the value of ωΔ . When ωΔ becomes less than 0.001 (pu), α is compulsorily set to 180 (deg) (i.e., TCBR is removed from the grid). As can be seen in Fig. 4(a), α in the case of “only TCBR” varies until about 2.7 (sec). On the other hand, α in the case of “both devices” has a constant value of 180 (deg) from the early time. This is because majority of the accelerating energy are dissipated by FCL before TCBR operates. This indicates that the temperature rise of TCBR can be limited as shown in section III-C.

To evaluate numerically the power system stability, we have used a stability index Wc given by

(4) where Wtotal is a summation of kinetic energy of each generator and T is a simulation time selected to 7 (sec). By using Wtotal, we can evaluate the stability of overall power system, taking each generator power rating into account. Wc is the integrated variation of the energy exchanged between the generator rotor and the system; thus, the lower the value of the parameter, the smaller the transient swing, and the better the overall system stability.

Fig. 5 shows the stability index Wc for the resistance values of each device (i.e., RFCL and RTCBR) varied in the range from 0.6 to 2.6 (pu) with a step of 0.1 (pu). In “both devices”, RFCL is set to the constant value of 1.1 (pu). From the results in the cases of “one-unit”, the most effective value of FCL (RFCL) and TCBR (RTCBR) are 1.1 (pu) and 0.6 (pu) respectively. Also, it can be seen that the resistance which can dissipate large energy is effective. In the case of “both devices”, although all results indicate less than the most effective value of “one-unit”, in particular, large value is effective (minimum point is 2.5 pu). This is because TCBR does not need to dissipate the accelerating energy of the generator so much.

B. Turbine shaft torsional oscillations

The rotor of generator has a very complex mechanical structure consisting of several predominant masses (such as rotors of turbine sections, generator rotor, couplings, and exciter rotor) connected by shafts of finite stiffness. Therefore, when the generator is perturbed, torsional oscillations occur between different sections of the turbine-generator rotor. The certain electrical system disturbance can significantly reduce the life expectancy of turbine shafts [14]. Therefore, sufficient damping is needed to reduce the turbine shaft torsional oscillations. To analysis the torsional oscillations of the turbine shaft, we have used the structure of a typical lumped-mass of the generator driven by a tandem turbine as shown in Fig. 1. It consists of six torsional masses. Fig. 6 shows the turbine shaft torque responses of generator G1. In the case of “both devices”, the torques of turbine shafts between each mass are effectively restrained.

Fig. 7 shows the integrated variation of the turbine shaft torque in the range of 0 to 7 (sec). The lower value improves the life expectancy of turbine shaft. As seen, the use of FCL is effective in comparison with the use of TCBR on the damping of shaft torque oscillations. In the case of “both devices”, all results indicate less than the most effective value of “one-unit” for all shafts.

=T

totalc dtWdt

dW

0(sec) / system base power

0 1 2 3 4 5 6 7

0

60

120

180

Firi

ng-a

ngle

(de

g)

T im e (s )

0 1 2 3 4 5 6 7

0

60

120

180

Firi

ng-a

ngle

(de

g)

T im e (s )

Fig. 4. Firing-angle of TCBR (phase ‘a’)

(a) TCBR

(b) FCL and TCBR

0 .5 1 .0 1 .5 2 .0 2 .51 .0

1 .5

2 .0

2 .5

3 .0

FC L & T C B R

T C B R

Wc (

s)

R e s is ta n ce (p u )

F C L

Fig. 5. Stability index Wc for resistance of each device

551


C. Temperature

Fig. 8 shows the responses of temperature and resistance of the resistor in the case of “both devices”. RFCL and RTCBR are set to 1.1 and 2.5 (pu) at 20 (°C) respectively. As mentioned earlier, FCL operates from the fault occurrence instance (0.1 sec) to the fault clearing (0.2 sec), and then TCBR operates dynamically until the generator disturbance becomes small. In the case of “both devices”, as shown in Fig. 4, TCBR only operates from about 0.3 to 0.7 (sec). As seen, temperature and resistance of FCL rise up to about 65 (°C) and 1.16 (pu) respectively. Those of TCBR rise up to about 70 (°C) and 2.65 (pu) respectively. As TCBR operates for a long time in comparison with FCL, temperature of TCBR rises more than that of FCL. However, the temperature rise of TCBR is not so large, and the resistance rise of TCBR is also small.

Fig. 9 shows the maximum temperature for each resistance value. In “both devices”, RFCL is set to the constant value of 1.1 (pu). As can be seen, the maximum temperature of RFCL is in inverse proportion to the resistance value. On the other hand, RTCBR is not always in inverse proportion to the resistance value due to the dynamic operation of TCBR. Japanese Industrial Standards (JIS) [15] indicates that an upper temperature limit of the austenitic stainless steel is 300 (°C). As seen, the maximum temperatures of all cases are below the upper temperature limit. However, majority of the

0 1 2 3-0 .5

0 .0

0 .5

1 .0

1 .5

2 .0

2 .5

Tu

rbin

e sh

aft

torq

eue

(pu

)

T im e (s )

H P -IP IP -L P A L P A -L P B L P B -G E N

0 1 2 3-0 .5

0 .0

0 .5

1 .0

1 .5

2 .0

2 .5 H P -IP IP -L P A L P A -L P B L P B -G E N

Tur

bine

sha

ft to

rque

(pu

)

T im e (s )

Fig. 6. Turbine shaft torque of G1 (b) FCL and TCBR

(a) No devices

0 .5 1.0 1 .5 2 .0 2 .50.0

0.6

1.2

1.8

Inte

gra

ted

val

ue

R es is tance (pu )

T C B R

FC L

F C L & T C B R

0 .5 1 .0 1 .5 2 .0 2 .50 .0

0 .6

1 .2

1 .8

Inte

gra

ted

val

ue

R e s is ta n c e (p u )

T C B R

F C L

F C L & T C B R

0 .5 1 .0 1 .5 2 .0 2 .50 .0

0 .6

1 .2

1 .8

Inte

gra

ted

val

ue


T C B R

F C L

F C L & T C B R

0 .5 1 .0 1 .5 2 .0 2 .50 .0

0 .6

1 .2

1 .8

Inte

gra

ted

va

lue


F C L

T C B R

F C L & T C B R

(a) HP-IP

(b) IP-LPA

(c) LPA- LPB

(d) LPB- GEN

Fig. 7. Integrated values of the turbine shaft torque responses

552


maximum temperatures in the case of “only TCBR” exceed 200 (°C). Therefore, if a large disturbance such as an unsuccessful reclosing occurs, the temperature of RTCBR may exceed the limit value of 300 (°C). On the other hand, the maximum temperature in the case of “both devices” can be decreased to about 70 (°C).

IV. CONCLUSION

In order to enhance the power system stability and damp the turbine shaft torsional oscillations, the use of combination of the fault current limiter and the thyristor controlled braking resistor is proposed in this paper. Furthermore, the temperature rise effect of both devices is included in the simulation. The simulation results indicate a significant power system stability enhancement and damping turbine shaft torsional oscillations as well as with allowable temperature rise.

REFERENCES [1] Y. Shirai, K. Furushiba, Y. Shouno, M. Shiotsu, and T. Nitta,

"Improvement of power system stability by use of superconducting fault current limiter with ZnO device and resistor in parallel," IEEE Trans. Applied Superconductivity, vol. 18, pp. 680-683, June. 2008.

[2] E.Leung, "Superconducting fault current limiters," IEEE Power Engineering Review, pp. 15-30, Aug. 2000.

[3] M. Sjostrom, R. Cherkaoui and B.Dutoit, "Enhancement of Power System Transient Stability Using Superconducting Fault Current Limiters," IEEE Trans. Applied Superconductivity, vol. 9, pp. 1328-1330, Jun. 1999.

[4] M. H. Ali, T. Murata, and J. Tamura, "Effect of coordination of optimal reclosing and fuzzy controlled braking resistor on transient stability during unsuccessful reclosing," IEEE Trans. Power Systems, vol. 21, pp. 1321-1330, Aug. 2006.

[5] A. Rubaai, A. R. Ofoli, D. Cobbinah, and M. D. Kankam, "Two-layer supervisory controller-based thyristor-controlled braking resistor for transient stability crisis," IEEE Trans. Industry Applications, vol. 41, pp. 1539-1547, Nov-Dec. 2005.

[6] H. Jiang, J. Dorsey, and T. Habetler, "A cost effective generator brake for improved generator transient response," IEEE Trans. Power Systems, vol. 9, pp. 1840-1846, 1994.

[7] Y. Sekine, “Transient analysis theory of power system,” Ohm Press, 1989 (in Japanese).

[8] IEEE subsynchronous resonance task force of the dynamic system performance working group power system committee, "First benchmark model for computer simulation of subsynchronous resonance," IEEE Trans. Power Apparatus System, vol. PAS-96, pp.1565-1572, 1977.

[9] R. F. Giese, and M. Runde, "Assessment study of superconducting fault-current limiters operating at 77 K," IEEE Trans. Power Delivery, vol. 8, pp. 1138-1147, July. 1993.

[10] Takahashi, "Metallic Materials," Morikita Press, 1990 (in Japanese). [11] National Astronomical Observatory, "Chronological Scientific Tables

(Rika Nenpyo),” Maruzen Press, 1995 (in Japanese). [12] M. H.Ali, T. Murata, and J. Tamura, "The effect of temperature rise of

the fuzzy logic-controlled braking resistors on transient stability," IEEE Trans. Power Systems, vol.19, pp.1085-1095, May. 2004.

[13] Ieda, "Handbook of Electrical and Electronic Materials," Asakura Press, 1987 (in Japanese).

[14] P.Kundur, "Power system stability and control," McGraw-Hill, 1994. [15] Japanese Industrial Standards Committee, [Online]. Available:

http://www.jisc.go.jp/eng/index.html

0.0 0.2 0.4 0.6 0.8 1.0

1.10

1.15

1.20

1.25

1.30

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

Re

sist

ance

(pu

)

resistance

Tem

pera

ture

(de

g C

)

T im e (s)

A phase B phase C phase

tem perature

0.0 0.2 0.4 0.6 0.8 1.0

2.5

2.6

2.7

2.8

2.9

0.0 0.2 0.4 0.6 0.8 1.00

20

40

60

80

resistance

tem perature

Re

sist

ance

(pu

)

T im e (s)

Te

mpe

ratu

re (

deg

C)

A phase B phase C phase

Fig. 8. Temperature and resistance in the case of “both devices”

(a) FCL

(b) TCBR

0 .5 1 .0 1 .5 2 .0 2 .5

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

1 6 0

Te

mpe

ratu

re (

deg

C)


A p h a s e B p h a s e C p h a s e

0 .0 0 .5 1 .0 1 .5 2 .0 2 .50

5 0

10 0

15 0

20 0

25 0

30 0u p p e r te m p e ra tu re lim it

a m b ie n t te m p e ra tu re

T C B R

Tem

pera

ture

(de

g C

)


A p h a se B p h a se C p h a se

F C L & T C B R

2 0

(a) FCL

(b) TCBR, and FCL and TCBR

Fig. 9. Maximum temperature for each resistance

553


power system stabilization by fault current limiter and thyristor controlled braking...

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