Address for Correspondence

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1 Research Paper COMPENSATION BY TCSC IN OPEN LOOP CONTROL SYSTEM 1* Sunita Tiwari, S.P. Shukla Address for Correspondence 1* Sr. Lecturer, Polytechnic,Durg Professor, Bhilai Institute of Technology, Durg ABSTRACT The FACTS controllers clearly enhance power system performance, improve quality of supply and also provide an optimal utilization of the existing resources. TCSC has been proposed to enhance the power transfer capability by changing the reactive power distribution in the power system. This paper discusses the TCSC s power enhancement capability. It has also discussed the effect of TCSC on steady state and transient stability. A transmission line model equipped with TCSC that is suitable for power transfer capability and transient stability analysis is proposed. This model is tested in a simple transmission system for open loop control system on MATLAB 7a software. Thyristor controlled series capacitors (TCSC) in closed loop system have been widely studied by many researchers but in this model the effect of TCSC in open loop control system is discussed. The simulation result shows that TCSC is capable of increasing power level and improving transient stability. KEYWORDS Transient stability, Power enhancement, FACT, TCSC I. INTRODUCTION An increasingly competitive market where economic and environmental pressures limit their scope to expand transmission facilities. The optimization of transmission corridors for power transfer has become a great importance. In this scenario, the FACTS technology is an attractive option for increasing system operation flexibility [1], New developments in high-current, high-power electronics are making it possible to control electronically the power flows on the high voltage side of the network during both steady state and transient operation. One important FACTS component is the TCSC which allows rapid and continuous changes of the transmission line impedance [ ]. Active power flows along the compensated transmission line can be maintained at a specified value under a range of operating conditions. Fig. 1 is a schematic representation of a TCSC module [], which consists of a series capacitor bank in parallel with a Thyristor Controlled Reactor (TCR). The controlling element is the thyristor controller, shown as a bidirectional thyristor valve. In this paper a short description of TCSC is given along with the simulation of transmission line using TCSC, a FACTS controller simulated in MATLAB- R7a. Analysis of the simulated transmission line (compensated with TCSC) model shows that TCSC can enhance power level of transmission line and has the similar functions as a physical one. The simulation of transmission line at different load conditions is done and the results show that the power transmitted through the line can be enhanced with the application of TCSC. Change in value of load affects the power level but, still, TCSC is capable of increasing power level of the system in all conditions. Controlled series compensation can be applied effectively to damp power oscillations. For damping power oscillations, it is necessary to optimize the applied compensation so as to counteract the accelerating and decelerating swings of the disturbed machine. To examine the transient stability of the system with and without TCSC, the same transmission line is subjected to transient disturbances i.e. a circuit breaker, with specified switching time, is connected in series with the transmission line and responses are observed. The main objective of this project is to demonstrate how a TCSC influence the power of the load connected to transmission line. The model developed in this project was verified by simulation studies for a series compensated system II.THYRISTOR CONTROLLED SERIES COMPENSATOR It is obvious by series compensation technique that power transfer between two station can be affected by adjusting the net series impedance of line. One such conventional and established method of increasing transmission line capability is to install a series capacitor, which reduces the net series impedance, thus allowing additional power to be transferred. Although this method is well known, slow switching times is the limitation of its use. Thyristor controllers, on the other hand, are able to rapidly and continuously control the line compensation over a continuous range with resulting flexibility. Controller used for series compensation is the Thyristor Controlled Series Compensator (TCSC). TCSC controllers use thyristor-controlled reactor (TCR) in parallel with capacitor segments of series capacitor bank (Figure 1). The combination of TCR and capacitor allow the capacitive reactance to be smoothly controlled over a wide range and switched upon command to a condition where the bidirectional thyristor pairs conduct continuously and insert an inductive reactance into the line. A TCSC is a series controlled capacitive reactance that can provide continuous control of power on the ac line over a wide range. The functioning of TCSC can be comprehended by analyzing the behavior of a variable inductor connected in series with a fixed capacitor, as shown in Figure 1.

2 X International Journal of Advanced Engineering Technology E-ISSN Fig.1. Thyristor Controlled Series Capacitor (TCSC) III.POWER SYSTEM STABILITY Power system stability may be broadly defined as the ability of a power system to remain in a state of operating equillibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance.[3] Stability of power system has been a major concern in system operation. The stability of a system determines whether the system can settle down to the original or close to the steady state after the transients disappear. In general, power system stability is the ability to respond to a disturbance from its normal operation by returning to a condition where the operation is again normal. [3] A power system is said to be steady state stable for a particular operating condition if, following any small disturbance, it reaches a steady state operating condition which is identical or close to the predisturbance operating condition. [3] Transient stability is defined as the ability of the power system to maintain synchronism when subjected to a severe transient disturbance. A system is transiently stable if it can survive the initial disturbance but it is transiently unstable if it cannot survive. For the transiently stable system, a large disturbance suddenly occurs, the system angle spread starts to increase but reaches a peak and then starts to decline, making the system transiently stable. The resulting system response involves large excursions of generator rotor angles. Transient stability is sometimes called first swing stability as the instability often occurs during the first angle swing.[3] IV.FUNDAMENTAL REACTANCE OF TCSC The effective reactance of TCSC is given by equations (1) and (). [4] Equation (1) assumes that the capacitor voltage is free from harmonics and considers the only the TCR current harmonics. ( X 1 ) TCR X C X 1 = TCSC j ( X ) (1) 1TCR X C σ cos λ = + σ σ σ sin j 1 λ tan λ tan π λ 1 λ () Where π ( ) X1 TCR = j ω L σ sinσ (3) TCSC X C ω = and λ ω N ω = 1 L C On the other hand equation () gives a more accurate representation of reactance of TCSC by considering ( σ) the harmonics of both the capacitor voltage and the TCR current. Intuitively, in the case of equation () the extra charge injected into the capacitor during the capacitive vernier mode increases the fundamental component of voltage, increasing the effective TCSC capacitive reactance as seen by the power system. As a result equation () results in higher value of TCSC reactance for a given value of conduction angle when compared to equation (1), in addition be presenting a more complete representation. In the above equations, σ is the conduction angle, L is the inductance of the TCR inductor, C is the capacitance of the fixed capacitor, ω N is power system frequency in radians per second and ω is the resonant frequency of the TCSC circuit. Fig. [5] shows the effective reactance of TCSC. Fig.- Reactance Characteristics of TCSC Simulation results match more closely to characteristics drawn using equation (4.). The negative and positive portions of the characteristics represent capacitive and inductive vernier modes of operation. V. MODES OF OPERATION IN STEADY STATE By controlling the firing angle of the thyristors the effective reactance of the TCR can be varied. This variable TCR reactance in parallel with a fixed capacitor allows the TCSC to operate in four different modes; blocking mode; bypass mode; capacitive boost mode; and inductive boost mode. [4 ] [5] [6] Blocking Mode: 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. Bypass Mode: 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 X L /X C ratio between.1 to.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. Capacitive Boost Mode:

3 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. Inductive Boost Mode 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 for increasing the line loadability by decreasing the line impedance. Meanwhile, this mode is useful during short circuits to decrease the fault current. This mode is normally used as a current-limiting system, helping to reduce the voltage sag during the faults. V. TCSC MODELING USING SIMULINK uncompensated line (ii) line equipped with TCSC (at three different firing angle). In second condition, the load is changed, making it more inductive and the results are identified in both the condition i.e. when line is compensated (at one particular firing angle) and when line is not compensated. In third condition, load is again changed, making it more resistive, the results are identified in both the condition, when line is compensated (at one particular firing angle) and when line is not compensated. For analyzing the effect of TCSC on transient stability of transmission system, transient disturbance is applied on line in both the conditions, when it is uncompensated, and when it is compensated with TCSC (at three different firing angle) is observed and results are compared. Condition-I (When load is P = 1 KW and Q L = 1KVar ) CASE-1 Single phase transmission system Fig. 4 Active, Reactive Power (without compensation) Table -1 Active/Reactive power output CASE- Simulation of tr. Line with TCSC: (i)when firing angle is 15 Table- Active/Reactive power output Figure 3. Model of SMIB system using TCSC The complete system has been represented in terms of SIMULINK blocks in a single integral model. SIMULINK is a software tool associated with MATLAB, used for modeling, simulating and analyzing dynamical systems. Single Machine Infinite Bus (SMIB) system with all the required components is modeled and is described. Simulink model of SMIB system with TCSC has been shown in Figure 3. VII. SIMULATION RESULTS For analyzing the effect of TCSC on transmission system, three conditions of line is taken. In first condition, at particular load, power transfer capability of line is noted and the results are compared for (i) Fig. 5 Active, Reactive Power at α=15 o

4 (ii). When firing angle, α = 16 Table-3 Active/Reactive power output in series with the transmission line and responses are observed. Case-1.When line is uncompensated: (iii)when firing angle, α = 173 Table -4 Active/Reactive power output Condition-II (When load is P = 1 KW and Q L =1 KVar) Table-5 Active/Reactive power output Fig.-6 Power oscillation diagram The amplitudes of oscillations are 1st positive Peak = above 74 MW 1 st negative Peak = below 64 MW nd positive Peak = above 719 MW nd negative Peak = below 67 MW 3rd positive Peak = above 69 MW 3rd negative Peak = above 68 MW Case-.When line is compensated with TCSC: (ii)when firing angle is 15 : Condition III (When load is P = 1 KW and Q L= 1 Var) Table- 6 Active/Reactive power output It is clear from above simulations, for all the cases of transmission line and all the conditions of load when line is series compensated by TCSC, the transmission capacity of line gets increased. It is also concluded that transmission capacity can be controlled by operating the model at different firing angle. Moreover TCSC can be operated in capacitive mode as well as inductive mode whenever it is required. VIII. TRANSIENT STABILITY IMPROVEMENT BY TCSC After the application of transient disturbances, if power oscillations persist for longer period and the amplitude of oscillation is also high, then the system is called unstable. To improve stability of the system it is required that oscillations should damp fast. Controlled series compensation can be applied effectively to damp power oscillations. For damping power oscillations it is necessary to optimize the applied compensation so as to counteract the accelerating and decelerating swings of the disturbed machine. To examine the transient stability of the system with and without TCSC, the same transmission line is subjected to transient disturbances i.e. a circuit breaker, with specified switching time, is connected Fig.-7 Power oscillation diagram at 15 The amplitudes of oscillations are-: 1st positive Peak = above 158 MW 1 st negative Peak = below 149 MW nd positive Peak = above 155 MW nd negative Peak = below 153 MW (ii)when firing angle is 16 : Fig.-8 Power oscillation diagram at 16 The amplitudes of oscillations: 1st positive Peak = above 16 MW 1 st negative Peak = below 15 MW nd positive Peak = above 157 MW nd negative Peak = below 154 MW (iii)when firing angle is 173 Fig.-9 Power oscillation diagram at 173

5 The amplitudes of oscillations are- 1st positive Peak = above 155 MW 1 st negative Peak = below 145 MW nd positive Peak = above 151 MW nd negative Peak = below 154 MW Comparison and discussion for stability: Table-7 Oscillation Time Seconds.4. Uncompe146 (with159 (with17 (with Seconds Seconds Fig.1 Stability diagram for transmission line By comparing the four cases of power oscillations, it is observed that oscillations damp faster when the firing angle of thyristor is 17 o, taking only. second and the amplitude of oscillations is comparatively low. But, when the line is uncompensated, oscillations damp after.8 seconds and the amplitude of oscillations is high as compared to compensated line. It is proven that TCSC improves transient stability of the system. X. CONCLUSIONS: This paper analyzes the effect of TCSC on the power flow through the buses with resistive and inductive loads. The simulation results show one of the salient features of TCSC, i.e., enhancement of power by operating TCSC in capacitive region power as it is the important issues of power transmission system. Table 8 Comparison of power (MW) uncompensated line. Moreover the amplitude of oscillations is lower in case of compensated line. Now it is well proven that TCSC improves stability of system. REFERENCE 1. N. H. Hingorani, "Flexible AC transmission systems,"ieee Spectrum p. 445, Apr N. Cbristl, R. Hcdin, K. Sadck, P. Llitzelberger, p. E. Krduse, S. M. McKcnna, A. H. Maiitaya, and D. Togerson, "Advanced series compensiuion (ASC) with thyristor controllcd impedance," Paper14/37/ MATLAB Based Simulation of TCSC FACTS Controller Preeti Singh, Mrs.Lini Mathew, Prof. S. Chatterji,N.I.T.T.T.R. Chandigarh. RIMT-IET, Mandi Gobindgarh. March 9, The Impact of FACTS Devices on Digital Multi-functional Protective Relays Mojtaba Khederzadeh,. 5. Identification of Thyristor Controlled Series Capacitor (TCSC) Erivelton G. Nepomuceno1, Ricardo H. C. Takahashi1, Luis A. Aguirre1, Oriane M. Neto. 6. Power Quality Enhancement by TCSC Application to Mitigate the Impact of Transformer Inrush Current Mojtaba Khederzadeh, Senior Member, IEEE 8 IEEE. 7 Selection of TCSC parameter:inductor and Capacitor IEEE 11, S. Meikandasivam, Rajesh Kumar Nema, and Shailendra Kumar Jain. Paper presented (i) Sunita Tiwari and S.P.Shukla Implementation of TCSC on a Transmission Line model to analyze the variation in Power Transfer capability, BITCON, National conference,nov.8. (ii) Sunita Tiwari and S.P. Shukla Thyristor-Controlled Series Capacitor and its application on Transmission System to improve Transient stability AICON, National conference, CSIT, Durg, Feb.9. It is observed from the table 8 that without compensation, the transmission line transfers 685 MW. When this transmission line is series compensated with fixed capacitor, line transfers 19 MW. With TCSC operated in capacitive region, the power transfer capability of line increases and it becomes 155 MW which is.3 times more (16%) than the power when line is not compensated (i.e..685mw). By comparing the four cases of power oscillations, it is observed that oscillations damp faster in compensated line as compared to