application of tcsc to enhance power quality.pdf

6
APPLICATION OF TCSC TO ENHANCE POWER QUALITY Mojtaba Khederzadeh (1) (1) Power & Water University of Technology, IRAN ABSTRACT TCSC as a dynamic system, besides its capability in increasing power transfer in transmission lines, can be used to enhance different power system issues. TCSC’s different advantages can be classified as steady-state and transient ones. During a fault, TCSC can enhance power quality by limiting the current and help to keep the voltage as high as possible. In this paper, the application of TCSC to enhance one of the important power quality issues, i.e., voltage sag is investigated. Different operating modes of TCSC have different impacts on the voltage of the bus that the line equipped with TCSC is connected to. Transferring to bypass mode upon occurrence of a fault is a salient feature of TCSC to improve voltage sag. The simulations on a sample network reveal these points. Keywords: TCSC, Voltage Sag, Power Quality, FACTS Devices. 1 INTRODUCTION The use of power electronics devices to improve the power transfer capability of long transmission lines forms the basis of the concept of FACTS. Thyristor Controlled Series Capacitor (TCSC) is one of the most popular devices of the FACTS [1]. Series capacitor compensation is an approach to improve stability limits and increase transfer capabilities. The transmitted power through a line is inversely proportional to the transfer impedance. For example, considering other parameters constant, 50% series compensation approximately doubles the steady-state transmitted power, whereas 75% series compensation would increase the transferred power to about four times the original value. TCSC may also be used for limiting fault current by adjusting its impedance dynamically to a large inductive value that depends upon the TCSC design. The use of TCSCs as fault current limiters has been investigated and reported in literature [2]. Power quality is an issue that is nowadays gaining significant interest to both electric utilities and end- users. The term power quality comprises a multitude of aspects, which is not new in essence, but has a growing concern due to the dissemination of sensitive loads (e.g. industrial plants) that use power electronics as a means of modernizing their manufacturing processes. Most of power quality complaints are concerned with voltage sags, which are mainly caused by short circuits on transmission and distribution systems. These sags can be mitigated by different means that include investments on the power systems through reinforcements; the use of power conditioners to protect the load against these sags; etc. Until a few years ago, the voltage sags due to faults were of little consequence. Use of adjustable speed drives and programmable logic-based process control in mining, pulp and paper, and electronic chip manufacturing plants has escalated over the last decade. It has made power quality problems associated with voltage sags an important design issue. Voltage sags of as little as 10% of the nominal voltage and of duration as short as 2 or 3 cycles can affect critical equipment and adversely impact the production processes. As an example, ac voltage is rectified and converted to pulsed dc when it is applied to an adjustable speed drive. This pulsing dc is stored in a capacitor, which in turn supplies smooth dc. Since the capacitor stores energy, it allows the system to ride through some sag. But if voltage sag is of sufficient depth and duration, the capacitor voltage will drop below a critical level (typically several cycles after the sag begins), at which point the drive may misoperate or simply shut down, resulting in process disruptions. Outages due to poor power quality can have as detrimental impact as sustained power interruptions [3]. There are a few papers investigating the effect of TCSC on the voltage sag [4], but they have mainly considered the TCSC in its steady-state operating conditions. Neglecting the dynamics of TCSC in disturbances prevent observing the transients and accurate system behavior. The main objective of this paper is to demonstrate in detail how the dynamics of a TCSC influence the voltage of the bus that the transmission line equipped with TCSC is connected to. It is shown if TCSC’s control system is well designed, it will help to mitigate the voltage sag consequences during emergencies. Another point is the oscillations created during and after TCSC’s transfer from one mode to the other. These oscillations could only be observed if TCSC is introduced by sophisticated models. TCSC’s operation as a fault current limiter to mitigate voltage sags is also investigated. UPEC 2007 - 607 Authorized licensed use limited to: IEEE Xplore. Downloaded on November 15, 2008 at 06:50 from IEEE Xplore. Restrictions apply.

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Page 1: APPLICATION OF TCSC TO ENHANCE POWER QUALITY.pdf

APPLICATION OF TCSC TO ENHANCE POWER QUALITY Mojtaba Khederzadeh (1) (1) Power & Water University of Technology, IRAN

ABSTRACT TCSC as a dynamic system, besides its capability in increasing power transfer in transmission lines, can be used to enhance different power system issues. TCSC’s different advantages can be classified as steady-state and transient ones. During a fault, TCSC can enhance power quality by limiting the current and help to keep the voltage as high as possible. In this paper, the application of TCSC to enhance one of the important power quality issues, i.e., voltage sag is investigated. Different operating modes of TCSC have different impacts on the voltage of the bus that the line equipped with TCSC is connected to. Transferring to bypass mode upon occurrence of a fault is a salient feature of TCSC to improve voltage sag. The simulations on a sample network reveal these points. Keywords: TCSC, Voltage Sag, Power Quality, FACTS Devices.

1 INTRODUCTION The use of power electronics devices to improve the power transfer capability of long transmission lines forms the basis of the concept of FACTS. Thyristor Controlled Series Capacitor (TCSC) is one of the most popular devices of the FACTS [1]. Series capacitor compensation is an approach to improve stability limits and increase transfer capabilities. The transmitted power through a line is inversely proportional to the transfer impedance. For example, considering other parameters constant, 50% series compensation approximately doubles the steady-state transmitted power, whereas 75% series compensation would increase the transferred power to about four times the original value. TCSC may also be used for limiting fault current by adjusting its impedance dynamically to a large inductive value that depends upon the TCSC design. The use of TCSCs as fault current limiters has been investigated and reported in literature [2]. Power quality is an issue that is nowadays gaining significant interest to both electric utilities and end-users. The term power quality comprises a multitude of aspects, which is not new in essence, but has a growing concern due to the dissemination of sensitive loads (e.g. industrial plants) that use power electronics as a means of modernizing their manufacturing processes. Most of power quality complaints are concerned with voltage sags, which are mainly caused by short circuits on transmission and distribution systems. These sags can be mitigated by different means that include investments on the power systems through reinforcements; the use of power conditioners to protect the load against these sags; etc. Until a few years ago, the voltage sags due to faults were of little consequence. Use of adjustable speed

drives and programmable logic-based process control in mining, pulp and paper, and electronic chip manufacturing plants has escalated over the last decade. It has made power quality problems associated with voltage sags an important design issue. Voltage sags of as little as 10% of the nominal voltage and of duration as short as 2 or 3 cycles can affect critical equipment and adversely impact the production processes. As an example, ac voltage is rectified and converted to pulsed dc when it is applied to an adjustable speed drive. This pulsing dc is stored in a capacitor, which in turn supplies smooth dc. Since the capacitor stores energy, it allows the system to ride through some sag. But if voltage sag is of sufficient depth and duration, the capacitor voltage will drop below a critical level (typically several cycles after the sag begins), at which point the drive may misoperate or simply shut down, resulting in process disruptions. Outages due to poor power quality can have as detrimental impact as sustained power interruptions [3]. There are a few papers investigating the effect of TCSC on the voltage sag [4], but they have mainly considered the TCSC in its steady-state operating conditions. Neglecting the dynamics of TCSC in disturbances prevent observing the transients and accurate system behavior. The main objective of this paper is to demonstrate in detail how the dynamics of a TCSC influence the voltage of the bus that the transmission line equipped with TCSC is connected to. It is shown if TCSC’s control system is well designed, it will help to mitigate the voltage sag consequences during emergencies. Another point is the oscillations created during and after TCSC’s transfer from one mode to the other. These oscillations could only be observed if TCSC is introduced by sophisticated models. TCSC’s operation as a fault current limiter to mitigate voltage sags is also investigated.

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2 STRUCTURE OF A TCSC The basic structure of TCSC is shown in Fig.1. It is mainly constituted by four parts: series compensating capacitor C, bypass inductance L, bi-direction thyristor SCR and zinc oxide voltage limiter MOV. The degree of TCSC basic compensation is controlled by the capacity size of capacitor C. The main function of bypass inductance L is to reduce the short circuit current and the energy absorbed by MOV. Bi-direction thyristor SCR is used to transform the equivalent impedance of TCSC which fulfill the needs in all kinds of power system condition, such as improving the stability, increasing the transmission capability, restraining hypo-synchronization resonance and so on [5]. By controlling the trigger pulse, TCSC can transform the trigger angle of thyristor. Subsequently, the current value of inductance sub-circuit which controlled by TCSC can be transformed, and then the total equivalent impedance will be changed continuously. Generally, when the trigger angle is 55

o~90

o, measuring from the

peak of capacitor voltage, the equivalent impedance of TCSC is appeared as capacitance. When the trigger angle is 0

o~50

o, the equivalent impedance of TCSC is

appeared as inductance as which characteristic can restrict short circuit current in system failure.

Figure 1 Configuration of TCSC

3 TCSC’S DIFFERENT MODES OF OPERATION

In normal operating conditions, there are four modes of operation; blocking mode; bypass mode; capacitive boost mode; and inductive boost mode [6]-[10]. When the thyristor valve is not triggered and the thyristors are kept in non-conducting state, the TCSC is operating in blocking mode. In this mode, the TCSC performs like a fixed series capacitor. In bypass mode the thyristor valve is triggered continuously and the valve stays conducting all the

time; so the TCSC behaves like a parallel connection of the series capacitor with the inductor, Ls, in the thyristor valve branch. In this mode, the resulting voltage in the steady state across the TCSC is inductive and the valve current is somewhat bigger than the line current due to the current generation in the capacitor bank. For practical TCSCs with XL/XC ratio between 0.1 to 0.3 range, the capacitor voltage at a given line current is much lower in bypass than in blocking mode. Therefore, the bypass mode is utilized as a means to reduce the capacitor stress during faults. In capacitive boost mode a trigger pulse is supplied to the thyristor having forward voltage just before the capacitor voltage crosses the zero line, so a capacitor discharge current pulse will circulate through the parallel inductive branch. The discharge current pulse adds to the line current through the capacitor and causes a capacitor voltage that adds to the voltage caused by the line current. The capacitor peak voltage thus will be increased in proportion to the charge that passes through the thyristor branch. The fundamental voltage also increases almost proportionally to the charge. From the system point of view, this mode inserts capacitors to the line up to nearly three times the fixed capacitor. This is the normal operating mode of TCSC. In inductive boost mode the circulating current in the TCSC thyristor branch is bigger than the line current. In this mode, large thyristor currents result and further the capacitor voltage waveform is very much distorted from its sinusoidal shape. The peak voltage appears close to the turn on. The poor waveform and the high valve stress make the inductive boost mode less attractive for steady state operation. This mode increases the inductance of the line, so it is in contrast to the advantages associated with the application of TCSC.

4 SIMULATION RESULTS In order to show the effect of TCSC on the voltage of the buses with sensitive loads, a sample network as Figure 2 is used. This network is composed of two sources connected by two transmission lines. The transmission line A1-B1 is compensated by a TCSC. Sensitive load is connected to bus B1. The TCSC consists of a fixed capacitor and a parallel Thyristor Controlled Reactor (TCR) in each phase. The nominal compensation is 75%, i.e. using only the capacitors (firing angle of 90°).

Figure 2 Sample network used for simulation

Ld C.B.

T1

T2

MOV

Ls

GAP

C

A1B1G ∞

Sensitive Load

TCSC

Fault

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The natural oscillatory frequency of the TCSC is 163Hz, which is 2.7 times the fundamental frequency. The impedance of line A1-B1 is 6.0852+j163Ω. The left source voltage is 539 kV and the right is 477.8kV. System frequency is 60Hz. The ratio of TCSC, i.e., Xl/Xc is 0.1343; C=21.977µF and L=0.043H. This sample system is described in [11]. Without the TCSC the power transfer is around 110MW, as seen during the first 0.5s of the simulation when the TCSC is out of service by the bypass C.B. operation, as seen in Figure 3. The TCSC can operate in capacitive or inductive mode, although the latter is rarely used in practice. Since the resonance for this TCSC is around 58° firing angle, the operation is prohibited in firing angle range 49° - 69°. Note that the resonance for the overall system (when the line impedance is included) is around 67°. The capacitive mode is achieved with firing angles 69°-90°. The

impedance is lowest at 90°, and therefore power transfer increases as the firing angle is reduced. In capacitive mode the range for impedance values is approximately 120-136Ω. This range corresponds to approximately 490-830MW power transfer range (100%-110% compensation). Comparing with the power transfer of 110 MW with an uncompensated line, TCSC enables significant improvement in power transfer level. The inductive mode corresponds to the firing angles 0°-49°, and the lowest impedance is at 0°. In the inductive operating mode, the range of impedances is 19-60Ω, which corresponds to 100-85 MW range of power transfer level. As can be seen from figure, the voltage of bus B1 is constant and during the first second of simulation time.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

200

400V_B1 [RMS in kV]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

500

1000Power [MW]

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

100

200TCSC impedance (reference and measured)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

50

100

Time

alpha [deg]

Figure 3 Sample system initialization from t=0 to t=1 Sec.

As Figure 4 shows, at t=1.0 Sec. a three phase fault occurs on line A1-B1 at 75% from the beginning of the line. TCSC transfers to bypass mode to save the capacitor from overvoltage. As can be seen from the figure, the voltage of bus B1 with sensitive loads does not drop considerably; voltage sag is at a permissible level. At t= 1.1 Sec., TCSC transfers to C.B. bypass operation in order to save itself from heavy current flow for long time. Voltage sag is higher than the TCSC bypass mode. This transfer is only for demonstration, because in real cases, the fault is cleared much faster, and rarely it is necessary to operate bypass C.B.. Between t=0 to t=1.1, the firing angle α is zero, and

after t=1.2 Sec. firing angle will remain 90° for easy transfer of TCSC to capacitive boost mode. Figure 5 shows the operation of TCSC for the same fault, but without C.B. bypass operation. Voltage sag of bus B1 is at its lowest value, but the same figure shows oscillations before reaching to a stable situation. Figure 6 shows the voltage sag of B1 without the presence of TCSC on line A1-B1. As can be deduced from this figure, the voltage sag is significant and would disturb the sensitive loads connected to bus B1. TCSC can transfer to fault current limiting at t=1.0 Sec. instead of bypass operation. This case can be seen in Figure 7 for α=49°. As can be deduced from this figure,

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voltage sag is the smallest and this mode can be considered for conservative conditions. Figure 8 shows the instantaneous values of voltage and current of bus B1. This figure indicates the necessity of considering the dynamics of TCSC during transition from a mode to another. If TCSC dynamics is neglected,

the resulting phasor model uses the equivalent impedances at the fundamental frequency, and therefore it is not as accurate as the thyristor transient model. Nevertheless, the phasor model is much simpler and the speed of simulation is increased.

0 0.5 1 1.5 2 2.5 30

200

400V_B1 [RMS in kV]

0 0.5 1 1.5 2 2.5 3-1

0

1x 10

4 Power [MW]

0 0.5 1 1.5 2 2.5 30

50

100

Time

alpha [deg]

Figure 4 Sample system behaviour for TCSC bypass operation followed by bypass C.B. closing

0 0.5 1 1.5 2 2.5 30

200

400V_B1 [RMS in kV]

0 0.5 1 1.5 2 2.5 3-1

0

1x 10

4 Power [MW]

0 0.5 1 1.5 2 2.5 30

100

200TCSC impedance (reference and measured)

0 0.5 1 1.5 2 2.5 30

50

100

Time

alpha [deg]

Figure 5 Sample system behaviour for TCSC bypass operation without bypass C.B. closing

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0 0.5 1 1.5 2 2.5 30

100

200

300

400V_B1 [RMS in kV]

0 0.5 1 1.5 2 2.5 3-200

0

200

400

600

Time

Power [MW]

Figure 6 Voltage sag of bus B1 without the presence of TCSC

0 0.5 1 1.5 2 2.5 30

500

1000V_B1 [RMS in kV]

0 0.5 1 1.5 2 2.5 3-2

0

2x 10

4 Power [MW]

0 0.5 1 1.5 2 2.5 30

100

200TCSC impedance (reference and measured)

0 0.5 1 1.5 2 2.5 30

50

100

Time

alpha [deg]

Figure 7 Fault Current Limiting with firing angle 49°

4 CONCLUSIONS This paper analyzes the TCSC's effect on the voltage sag of the buses with sensitive loads. The simulation results show one of the salient features of TCSC, i.e., enhancement of voltage sag during disturbances, as one

of the important issues of power quality. This is achieved by transferring of TCSC to appropriate modes during disturbances. For investigating the voltage condition of the desired buses during system disturbances, a detailed dynamic model of the TCSC is required. Otherwise, the analysis does not show the transients occurring during TCSC's mode transfer.

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0.96 0.98 1 1.02 1.04 1.06 1.08 1.1 1.12

-500

0

500

V_B1 in kV

0.96 0.98 1 1.02 1.04 1.06 1.08 1.1 1.12-0.03

-0.02

-0.01

0

0.01

0.02

Time

I_B1 in kA

Figure 1 Instantaneous Voltage and Current of Three phase fault

5 REFERENCES 1. Hingorani, N. G. and Gyugy, L., "Understanding

FACTS", Concepts and Technology of Flexible AC Transmission System. New York: Inst. Elect. Electron. Eng., Inc., 2000.

2. Tenorio, A. R. M., Jenkins, N., "Investigation of the TCSC as a Fault Current Limiter," Proc. of International Conference on Power System Transients (IPST'97), pp. 345-350, Seattle, USA, 1997.

3. Nagpal, M., Martinich, T. G., Moshref, A., Morison, K., and Kundur, P., "Assessing and Limiting Impact of Transformer Inrush Current on Power Quality," IEEE Trans. On Power Delivery, vol. 21, no. 2, April 2006, pp.890-896.

4. Tenorio, A. R. M. and Daconti, J. R. "Voltage Sag Mitigation by Thyristor Controlled Series Capacitors", 8th International Conference on Harmonics and Quality of Power ICHQP'98, IEEE/PES, Athens, Greece, October 14-16, 1998, pp. 572-576.

5. Jalali, S. G., Hedin, R. A., Pereira, M. and Sadek, K., “A stability model for the advanced series compensator (ASC),” IEEE Trans. Power Del., vol. 11, no. 2, pp. 1128–1137, Apr. 1996.

6. Fuerte-Esquivel, C. R., Acha, E., and Ambriz-Perez, H.,“A thyristor controlled series compensator model for the power flow solution of practical power networks,” IEEE Trans. Power Syst., vol. 9, no. 15, pp. 58–64, Feb. 2000.

7. Jalali, S. G., Lasseter, R. H., and Dobson, I., “Dynamic response of a thyristor controlled switched capacitor,” IEEE Trans. Power Del., vol. 9, no. 3, pp. 1609–1615, Jul. 1994.

8. Ghosh A., and Ledwich, G., “Modeling and control of thyristor controlled series compensators,” in Proc. Inst. Elect. Eng., Gen. Transm., Distrib., vol. 142, May 1995, pp. 297–304.

9. Mattavelli, P., Verghese, G. C., and Stankovic, A. M., “Phasor dynamics of thyristor controlled series capacitor systems,” IEEE Trans. Power Syst., vol. 12, no. 3, pp. 1259–1267, Aug. 1997.

10. Mathur, R. M., Varma, R. K., “Thyristor-Based FACTS Controllers for Electrical Transmission Systems,” IEEE Press, 2002.

11. Jovcic, D., Pillai, G. N., "Analytical Modeling of TCSC Dynamics" IEEE Transactions on Power Delivery, vol 20, Issue 2, April 2005, pp. 1097-1104

AUTHOR'S ADDRESS Dr. Mojtaba Khederzadeh can be contacted at Electrical Engineering Department, Power & Water University of Technology, Tehranpars, Vafadar Bldv., P. O. Box: 16765-1719, Tehran, IRAN. Email: [email protected]

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