1 Considerations for the Application of Thyristor Controlled Series Capacitors to Radial Power Distribution Circuits M. N. Moschakis, E. A. Leonidaki, Student Member, IEEE, N. D. Hatziargyriou, Senior Member, IEEE Abstract This paper deals with the application of Thyristor Controlled Series Capacitors (TCSCs) suitably rated for radial distribution circuits. The various problems existing on long distribution lines that can be alleviated by the connection of variable series compensation, as a TCSC provides, are discussed. An important feature is the capability of the TCSC to operate as a short circuit current limiter. Thus, sensitive loads connected to nearby substations will not experience any voltage sags caused by faults on the distribution line where the TCSC is connected. The benefits of the connection of a TCSC on a radial distribution system are verified by means of the ElectroMagnetic Transients for DC (EMTDC) simulation package. Index Terms Series compensated distribution lines, TCSC, fault current limiter, thyristor switches, power quality, power system simulation, sensitive loads, voltage sags. I. INTRODUCTION HYRISTOR controlled series capacitors (TCSCs) have T been used in transmission networks to control the equivalent impedance of transmissions lines, and therefore the power flow in the network [1], [2]. They have renewed the interest in transmission line series compensation because of their control system flexibility. Although mechanical switching could, in principle, be applied to achieve some flexibility, the fast electronic control and proven reliability of the thyristors lead to maximum controllability of the transmission system. As regards distribution systems, many different solutions have been proposed. Miske [3] and Souza et al [4] proposed the use of fixed and GTO-controlled series capacitors in radial distribution circuits, respectively. GTO or thyristor-controlled series capacitors have the advantage of self-regulating, continuous and instantaneous response. Some of the positive effects of series compensation on a long radial power distribution circuit when operating as a varying series capacitance are: M. N. Moschakis is with the Department of Electrical and Computer Engineering, National Technical University of Athens, Athens, Greece, (e-mail: mosxakis@power.ece.ntua.gr). E. A. Leonidaki is with the Department of Electrical and Computer Engineering, National Technical University of Athens, Athens, Greece, (e-mail: eleonid@central.ntua.gr). Increased power transmission capability by a decreased total circuit reactance and improved voltage profile along the circuit Decreased circuit losses Support during start of large asynchronous and synchronous motors Reduction of required reactive power input at the sending end of a radial circuit Reduced voltage fluctuations and voltage imbalance due to load variations In distribution systems, reactive power compensation is typically provided by shunt-connected components, such as SVC and STATCOM devices. Fixed series capacitors are clearly the most economical solution, but their optimal location is not easy to determine, when many loads are connected to the distribution system. An appropriately rated TCSC device can also be applied in radial power systems in order to keep load voltage within given limits for several load variations or to provide short circuit current limitation. More specifically, the TCSC appears as variable impedance that depends on thyristor firing angle. In this way the control system sends firing order to the thyristors according to basic scenarios in order to keep the load voltage between given limits. Furthermore, TCSC can operate in the inductive region to reduce short-circuit currents. TCSC capability to operate as a fault current limiter in transmission systems has already been proposed. Karady [5] proposed a new configuration of TCSC to obtain a combined short circuit current limiter, interrupter and series compensator in steps. The series compensation is controlled by the capacitors switching on and off, while the current limitation and interruption is achieved by the insertion of a resonant LC circuit in the transmission line. Godart et al [6] conducted a study about the feasibility of TCSC for distribution substations enhancement taking into consideration load expansion. The capability of TCSC to operate as short circuit current limiter when connected upstream the distribution substation, was confirmed. Moreover, Yamazaki et al [7] proposed and produced a prototype of a combined TCSC system with an additional series current limiting reactor. N. D. Hatziargyriou is with the Department of Electrical and Computer Engineering, National Technical University of Athens, Athens, Greece, (e-mail: nh@power.ece.ntua.gr).
2 In this paper, a TCSC model that operates both as a series compensator and fault current limiter is studied. The TCSC can be installed on a long distribution line, where the number of faults is expected to be high to decrease the short-circuit effect on neighboring loads. Thus, sensitive loads connected to nearby substations do not experience voltage sags caused by faults on the TCSC supported distribution line. II. TCSC STEADY-STATE OPERATION The basic circuit of TCSC is illustrated in Figure 1. It consists of a capacitor in parallel with a thyristor-controlled inductor. The control variable is the firing angle α of the thyristors, with reference to the capacitor voltage zero crossings. The thyristors are fired when the capacitor voltage and current are opposite in polarity. This is equivalent to thyristor firing angles between 90 and 180. Fig. 1. One-line diagram for TCSC circuit The TCSC can operate in three different modes. In the bypassed mode, the thyristor path is conducting continuously, the capacitor is bypassed and the apparent impedance becomes inductive. In the blocked mode, the thyristor path is blocked continuously, which is equivalent to the fixed capacitor reactance. Finally in the Vernier mode, the thyristor path is partially conducting resulting in a flow current circulating in the TCSC loop. Depending on the conduction time of the thyristors, this current may have the same or opposite direction with the internal capacitor current. In this way the TCSC appears as an apparent reactance that may be capacitive or inductive. The steady-state curve of the apparent reactance of the TCSC at fundamental frequency versus the firing angle is equal to: Fig. 2. Apparent impedance of TCSC. behavior for at least one angle is observed. This asymptote corresponds to the firing angle where resonance between inductor and the capacitor reactance occurs. This firing angle must be avoided, because it results in excessively high voltages and currents in TCSC circuit. By changing the reactor inductance, it is possible to increase the effective bandwidth of control and have a lower circulating current through the components [8][9]. Emphasis must be given on the rating of all parameters of a real TCSC not only to achieve the desired capacitive range, but also to select the appropriate components that are designed to withstand system contingencies that cause high current peaks without damage. The control system should take care to bypass the TCSC, in case these ratings are exceeded. Additional protection schemes are applied [2]. The thyristors have a Break-over Diode protection scheme for overvoltage protection. The capacitor is equipped with a gapless metaloxide varistor (MOV) surge arrester connected in parallel, which operates whenever the capacitor voltage overcomes a set limit. The behaviour of MOV across conventional capacitor banks is extensively described in [10]. Finally, a bypass breaker exists that operates whenever the device has to be removed from the system. Z TCSC = - X C + (X C +X LC )[(2σ+sin2σ)/ π ] - 4X 2 LCcos 2 σ [(ktankσ-tanσ)/π] /X L (1) X LC =X C. X L / (X C -X L ) σ = π-α w o 2 = 1/LC, w = 2πf, k=w o /w The apparent impedance of a typical TCSC versus the firing angle α is plotted in Fig. 2. In this figure an asymptotic Fig. 3.1 TCSC voltage and current waveforms in capacitive region. During normal operation, the TCSC generally operates in
3 the capacitive region. The capacitor voltage and current as well as the thyristor current for this operation are illustrated in Fig. 3.1. During system faults, it is desirable to operate in the inductive region to lower the fault current contribution (Fig. 3.2). Fig. 3.2 TCSC voltage and current waveforms in inductive region. Regarding the harmonic content of TCSC device, the harmonic content of thyristor current is equal to: 2A sin( n + 1) σ sin( n 1) σ + ( ) + 2Acosσ sin( k n) σ sin( k n) σ = + π n + 1 n 1 π coskσ k + n k n I T n where n=3,5,7,... (2) These harmonic currents circulate mainly inside the LC circuit, because the system impedance for harmonic frequencies is much greater than the capacitor impedance. This can be derived considering that capacitor and system impedance form a current divider for the thyristor current at harmonic frequencies. In this way the capacitor and system currents are given by the equations: connected through a long line. This load fluctuates from 9 to 15 MVA at a regular basis leading to voltage fluctuations and rapid voltage changes [13], which affect both this load and the loads connected to nearby substations. At feeder 2, a sensitive to voltage sags load is connected. The installation of this load contains protection systems that trip immediately after even shallow sags with only a short duration. The optimal position to connect a TCSC is at the sending end of feeder 1. In this location, it enables reactive power compensation and also reduces short-circuit currents introduced by faults that occur on feeder 1. The number of faults on feeder 1 is expected to be high, assumed proportionally related with the length of a line. As a basis for the specification of the design parameters of the TCSC model, the parameters of a real TCSC of 15 50 Ω at 400kV have been used. The scaling factor applied to all the constituent components is defined as the ratio of the model impedance base to the system impedance base at the same point of the circuit, as follows: Z b mod λ = (5) Z bsyst The resistors and reactors of the real system are multiplied by this scaling factor, while the capacitances are divided by it. Consequently, each phase of the TCSC model consists of a 4760 µf capacitor and a 52 mh inductor with quality factor equal to 110. The TCSC apparent impedance varies between - 0.668 Ω (α=180 ) and 0.1324 Ω (α=90 ). The apparent impedance of the applied TCSC versus the firing angle α is shown in Fig. 2. Attention has been paid, so that only one resonant point in the range of 90 to 180 exists, equal to 135.4. In addition, the reactor size was properly selected, in order to increase the effective control bandwidth in the inductive range. i C = i T. Z SYST /(Z SYST + Z C ) i ac = i T. Z C /(Z SYST + Z C ) (3) Assuming that the system impedance is infinite, the harmonic thyristor currents flow only through the capacitor, so the capacitor voltage at the n-harmonic is equal to: V C(n) = i T(n) X C(n) = i T(n) /(n.w.c) (4) The harmonic content and magnitude of circulating currents increases essentially as firing angle α gets nearer to the resonant point. It is shown in [11] that the Total Harmonic Distortion (THD) of transmission line current is below the current distortion limits recommended in IEEE Std 519-1992 [12]. III. STUDY CASE A. Power system and TCSC characteristics The test system consists of two feeders fed by 150/20 kv substations. At feeder 1 a nominal load of 10 MVA is Fig. 4. Single-line diagram of the test network B. TCSC as a varying series capacitance When TCSC operates in the capacitive region, it can be controlled so that the voltage at the terminals of load 1 and the voltage across the feeder 2 (assuming that other consumers are also connected to that feeder or will be connected in the future) are kept constant. Simulations were performed using
4 EMTDC to demonstrate the ability of TCSC to alleviate voltage fluctuations caused by load fluctuations. The simulation results are shown in Fig. 5. The load varies in steps from 9 to 15 MVA, as shown in the upper diagram of Fig. 5.1. The voltage at the terminals of load 1 with and without TCSC, are shown in the middle diagram. The instantaneous response of TCSC and the ability to control even rapid voltage changes can be easily seen. The lower diagram of Fig. 5.1 presents the variation of the control angle in order to keep the voltage at the load terminals constant. Furthermore, as voltage across the TCSC is proportional to the current, the higher current is flown on the feeder, the more benefits are gained by the connection of TCSC. Fig. 5.3 Capacitor voltage and current, line current and control angle following a load variation C. TCSC as fault current limiter Some of the positive effects of fault current limitation are: Fig. 5.1 Voltage with and without TCSC after extreme symmetrical load fluctuations In Fig 5.2, THD of the load phase voltage and the line current are presented. It can be seen that the harmonics injected in the voltage and current are acceptable, according to European Standard EN50160 [13]. Fig. 5.2 THD of phase voltage and line current. In Fig 5.3 the capacitor voltage and current inside the TCSC in the first 0.3 sec are shown. It should be noted that after the first load variation at 0.1 sec, the TCSC circuit gradually approaches steady state, which is reached when the half of the inductor current is exactly symmetrical about the zero crossings of capacitor voltage. Mitigation of voltage sags, swells and outages Longer life with higher reliability for nearby transformers Limited inrush current (soft start), even for capacitive loads Mitigation of the effects of distributed generation at lower voltages within Distribution Systems When TCSC operates in the inductive region, it represents a varying inductance, which can reach values much higher than the nominal inductor value, as the control angle increases towards the resonance value (Fig. 2). In cases of excessive currents flowing in feeder 1, significant damping can be effected by regulating accordingly the TCSC angle. Thus, although operation near the resonance area should be avoided, such operation of TCSC during a short circuit, lasting essentially no longer than a few periods, e.g. 5 periods (100 ms for a 50 Hz system) is very beneficial. This time period corresponds also to the required time for the normal operation of a distribution protection system. This strategy is used in the following simulations investigating the effect of fault current limitation via TCSC. It is assumed that the TCSC model is equipped with a MOV appropriate selected for overcurrent and overvoltage protection of the device components. The fault detection strategy used is based on the rate of current increase. In this way, a fault is detected within a few µs and the fault current limitation begins even before the first
5 peak is reached. The detection strategy is shown in Fig. 6.1. The rate of the line current is compared with the instantaneous line current. When this rate is above a preset limit (set to 200 pu/s for this case), the output of the rate limiter is presented in Fig. 6.2. Thus, a signal is issued almost instantaneously and after the first zero crossing of the thyristor current the operation of the TCSC in the current limiting mode is initiated. The two middle diagrams show the line current with and without a TCSC. The lower diagram shows harmonic injection, when TCSC operates in the current limiting mode. It can be seen that the THD is about 5%, well below the limits set in EN50160, verifying the fact that TCSC does not inject significant harmonics into the system. Fig. 6.1 Fault detection strategy Fig.7. TCSC operation sequence in current limiting mode The TCSC can also operate independently at each phase, hence, it can deal with asymmetrical faults. This means that the faulted phase will operate in the current limiting mode while the healthy phases will operate in the capacitive region. To demonstrate the ability of TCSC to deal with asymmetrical faults, the following simulations were performed. 1) Single- phase fault Fig. 6.2 Fault detection logic in a three-phase fault In Fig.7, the repetitive operation of TCSC in a three-phase fault is shown. In the beginning, TCSC operates in the capacitive region, a fault immediately after the TCSC (which is the worst case) occurs at t=0.1 sec and it lasts until t=0.8 sec. The TCSC operates for 100 ms and the protection breaker opens. The breaker recloses after 300 ms, the fault is still present and TCSC operates again for 100 ms limiting the fault current until the breaker opens for the second time. The breaker recloses again after 300 ms, the fault has been cleared and the TCSC returns in the capacitive region. In the upper diagram in Fig. 7, the voltage at the terminals of the sensitive load 2 with and without a TCSC is shown. It is clearly shown that the voltage sag experienced by the sensitive load 2 is significantly lower when TCSC is present. The simulation results when a single-phase fault occurs immediately after the TCSC are shown in Figures 8.1 and 8.2. The fault occurs at t=0.025 sec and is cleared after 100 ms. The load 1 at pre-fault time is 10 MVA. In Fig. 8.1 the load 2 voltage with and without TCSC is presented. In Fig. 8.2 it is observed that the phase c is also affected but continues to operate in capacitive region. The voltage sag magnitude with the TCSC reaches a value of 72.2% and lasts for 25.66 ms. Furthermore, the THD of load 2 voltage is again within acceptable limits.
6 Fig. 8.1 Load 2 phase voltages with and without TCSC Fig. 9.1 Load 2 phase voltages with and without TCSC Fig. 8.2 Load 2 phase a and c line currents, THD of voltage Va 2) Phase-to-phase fault Phase-to-phase faults (between phase -a and -b) are also examined. The pre-fault conditions are the same as in the previous case. Fault occurs at t=0.03 sec and is cleared after 100 ms. The voltage sag magnitude experienced by the load is 62.9% and its duration is 69 ms. The simulation results are shown in Figures 9.1 and 9.2. Fig. 9.2 Load 2 phase a and b line currents, THD of voltage Va
7 IV. CONCLUSIONS To date, TCSC systems have been proposed as an effective means to increase power transfer capability of transmission lines and rapid controllability for power swing damping and stability in transmission systems. In addition, the ability of TCSC to operate as a fault current limiter has been mentioned. In this paper the application of an appropriately rated TCSC system in distribution networks is proposed. A suitable TCSC model is applied in a distribution test system and its effectiveness in the mitigation of several power quality problems is investigated. It is shown that TCSC operating as a varying series capacitance can effectively alleviate load voltage sags due to load fluctuations. In addition, it is shown that TCSC operating in the inductive region contributes to fault current limitation and maintenance of voltage at sensitive loads in neighboring feeders close to its nominal value. In general, it is believed that TCSC systems can provide an important control tool to distribution system engineers. Their ability to reduce short-circuit currents can provide solutions to many operational and power quality problems. However, because of its high cost, the viability of such a device should be further investigated. VI. BIOGRAPHIES M. N. Moschakis received his Diploma in Electrical Engineering from the National Technical University of Athens (NTUA), Greece, in 1998. Currently he is a Ph.D student in NTUA. His scientific interests mainly concern Custom Power Device Modeling and Evaluation, and Voltage Sag Stochastic Assessment. E. A. Leonidaki received her Electrical Engineering degree in 1992 from NTUA. She is now with Public Power Corporation of Greece and she works towards a PhD degree in Electrical Engineering Department of NTUA. Her interests include FACTS modeling and control for power system dynamic analysis and transient phenomena. She is student member of IEEE, member of CIGRE and member of Technical Chamber of Greece. N. D. Hatziargyriou received a Diploma in Electrical and Mechanical Engineering from NTUA, and an Msc and PhD degree from UMIST,UK. He is currently Professor at Power Division of the Electrical Engineering Department of NTUA. His research interests include Modeling and Digital Techniques for Power System Analysis and Control. He is Senior member of IEEE and secretary of IEEE Greek section, member of CIGRE SC C6 and of Technical Chamber of Greece. V. REFERENCES [1] E. Larsen, C. Bowler, B. Damsky, S. Nilsson, Benefits of Thyristor- Controlled Series Compensation, CIGRE Paper 14/37/38-04, Paris 1992. [2] N. Christl, R. Hedin, et al. Advanced Series Compensation (ASC) with Thyristor Controlled Impedance, CIGRE Session 1992, Paper 14/37/38-05, Paris, 1992. [3] S. Miske, Considerations for the Application of Series Capacitors to Radial Power Distribution Circuits, IEEE Trans. Power Delivery, vol. 16, No.2, pp. 306-318, April 2001. [4] L. Souza, E. Watanabe, M. Aredes, A GTO Controlled Series Capacitor for Distribution Lines, CIGRE Paper 14-201, Paris 1998. [5] G. Karady, Concept of a Combined Short Circuit Limiter and Series Compensator, IEEE Trans. Power Delivery, vol. 6, No.3, pp. 1031-1037,, July 1991. [6] T. Godart, A. Imece, J. McIver, E. Chebli, Feasibility of Thyristor Controlled Series Capacitor for Distribution Substation Enhancements, IEEE Trans. Power Delivery, vol. 10, No.1, pp. 203-209, January 1995. [7] Y. Yamazaki, S. Sugimoto, S. Ogawa, H. Konishi and A. Kikuchi, Development of TCSC application to Fault Current Limiters, Electrical Engineering in Japan, vol. 140, No. 3, 2002, Translated from Denki Gakkai Ronbunshi, vol. 121-B, No. 4, April 2001, pp. 514-519. [8] S. Helbing, G. Karady, Investigation of an Advanced Form of Series Compensation, IEEE Trans. Power Delivery, vol. 9, No.2, pp. 939-947, April 1994. [9] E. Larsen, K. Clark, S. Miske, J. Urbanek, Characteristics and Rating Considerations of Thyristor Controlled Series Compensation, IEEE Trans. Power Delivery, vol. 9, No.2, pp. 992-1000, April 1994. [10] G.J. Georgantzis, N.D.Hatziargyriou, E.A. Leonidaki, Transient Simulation of Series Compensated EHV Transmission Lines for Shortcircuit Studies, IEEE MELECON 96, p. 1584-1587, Bari, Italy, May 1996. [11] E.A Leonidaki., N.D.Hatziargyriou, B.C.Papadias, G.J.Georgantzis, "Investigation of Power System Harmonics and SSR Phenomena Related to Thyristor Controlled Series Capacitors", Proceedings of the 8th International Conference on Harmonics and Quality of Power (ICHQP 98), Athens, Greece, October 1998, pp. 848-852. [12] IEEE Std. 519-1992, "IEEE Recommended Practices and Requirements for harmonic Control in Electrical Power systems", 1992. [13] European Standard EN 50160, Voltage characteristics of electricity supplied by public electricity distribution networks, November 1994.