EFFECTS OF SERIES COMPENSATION ON DISTANCE PROTECTION OF HIGH VOLTAGE TRANSMISSION LINES UNDER FAULT CONDITIONS

International Journal of Electrical Engineering & Technology (IJEET) Volume 9, Issue 6, November-December 218, pp. 57-66, Article ID: IJEET_9_6_6 Available online at http://www.iaeme.com/ijeet/issues.asp?jtype=ijeet&vtype=9&itype=6 ISSN Print: 976-6545 and ISSN Online: 976-6553 Journal Impact Factor (216): 8.1891 (Calculated by GISI) www.jifactor.com IAEME Publication EFFECTS OF SERIES COMPENSATION ON DISTANCE PROTECTION OF HIGH VOLTAGE TRANSMISSION LINES UNDER FAULT CONDITIONS M. Purohit Department of Electrical Engineering, MIT College of Engineering, Pune, India Dr. V. N. Gohokar Department of Electrical Engineering, Marathawada Mitramandal College of Engineering, Pune, India ABSTRACT Advent in power electronics enhanced the dynamic performance of modern power system and Flexible AC Transmission Systems (FACTS) provided vital solution for it. FACTS are basically thyristor-controlled devices which control the active and reactive power flow in the network and are equally adaptive to voltage magnitude control thus, utilize the existing power network optimally. But increase in complexity, configurations and total power transmitted lead to a number of problems in relation to transmission line protection. The most widely used protection system in case of high voltage and extra high voltage transmission lines is Distance protection. However, installation of series compensation system creates problems for protective relays and fault locators due to change in the magnitudes and directions of fault voltages and currents in the networks and causes malfunctioning of the distance relay. Since protection of transmission lines plays crucial role in improving the stability and reliability of electric power systems, this paper also throws light on the impact. MATLAB/SIMULINK is used effectively in modelling and simulation of distance relay as well as transmission line compensated with thyristor-controlled series compensator. The simulation results illustrate the effective use of MATLAB/SIMULINK in understanding the behaviour of distance relay in protecting the transmission lines with and without compensation. Keywords: Distance protection; Series compensation; FACTS; SSSC; TCSC; MATLAB/Simulink; Graphical User Interface Cite this Article: M. Purohit and Dr. V. N. Gohokar, Effects of Series Compensation on Distance Protection of High Voltage Transmission Lines under Fault Conditions, International Journal of Electrical Engineering & Technology, 9(6), 218, pp. 57 66. http://www.iaeme.com/ijeet/issues.asp?jtype=ijeet&vtype=9&itype=6 http://www.iaeme.com/ijeet/index.asp 57 editor@iaeme.com

M. Purohit and Dr. V. N. Gohokar 1. INTRODUCTION The advancement of transmission systems depends totally on the growing demand on electrical energy. Transmission lines and their protection plays vital role in improving power system reliability and utility [1]-[2]. Due to the restriction faced by power industry and deregulation of power industry regarding regulatory pressures, scarcity of natural resources, an ever-increasing energy demand and environmental concerns, the major task is to improve the power transfer capability and also to improve the system integrity of the given transmission system. These issues can be resolved by using series compensation. The series inductive reactance is one of the governing factors in controlling the amount of power transferred by the transmission lines. Hence by reducing net inductive reactance of transmission lines help to improve their steady state maximum power transfer capability. In modern power system, the series compensated transmission lines not only help to improve power system stability but also influences the power flow in a particular network segment which is resulted into reduction in active power losses and thereby prevents system - sub synchronous oscillations [4]-[7]. Optimum utilization of existing transmission lines in terms of high efficiency and without compromising on the quality and reliability of electrical power supply has thus to be found via alternative means. Thus, series compensation systems consisting of fixed or thyristor-controlled series capacitor help in improving the power quality and power transmission capability of existing transmission lines [9]-[1]. However, with the installation of series compensation system in transmission line creates problems for protective relays. Fast and accurate determination of a fault in electrical power system is a vital part in power restoration. Protection of transmission lines is very important from the point of view of power system reliability and restoration of power services utility. In Power system majority of the faults are happened to be single line to ground fault. Other important types of faults are LLG, LLL, LL, LLLG faults [11]-[12]. Distance protection is the most popular and widely used protection in power networks as the main and back up protection of transmission lines. Distance relays which are designed to work on principle of impedance measurement perform correctly on a resistive/ inductive system. When SCs are introduced, the normal voltage/current relationships are affected [12]. Hence distance relay mal operates due to introduction of series compensation system. Overreaching of distance relay due to series compensation is a major and critical problem. This paper presents analysis of distance relay performance under fault condition when connected in the transmission line provided with and without series compensation. Thyristor controlled series compensator (TCSC) is used for series compensation purpose. MATLAB/ Simulink is used for modelling of transmission line, distance relay and TCSC. 2. DISTANCE PROTECTION SCHEME FOR TRANSMISSION LINES Distance protection is the most widely used protection in power networks as the main and backup protection of transmission lines because of its suitability, simplicity, economy and reliability [1-3]. The measured impedance at the relaying point is the basis of distance protection operation. The operation of conventional distance relay consists of three measuring units, can be three separate units or one unit for the first and second zone with time delay and a second unit for the third zone. The first- and second-unit act as primary protection units whereas second and third zone provides remote back up protection for adjacent lines. The main target of these relay is to calculate impedance at the fundamental frequency between the relay point and the fault point. According to this impedance, it will determine if the fault is inside or outside the protection zones. This impedance is calculated from the measured voltage and current signals at the relay position. Thus, distance protection operation is based on measuring the impedance of the line. During normal conditions the impedance seen by the relay is very large. http://www.iaeme.com/ijeet/index.asp 58 editor@iaeme.com

Effects of Series Compensation on Distance Protection of High Voltage Transmission Lines under Fault Conditions With the measured current and voltage, the relay calculates the resulting resistance and reactance. If these values are smaller than the known and stored data of the line, the fault has to be in the protected area and the relay operates. Regardless of the type of fault involved, the voltage and current used to energize the distance relay. 3. PURPOSE OF COMPENSATION In modern power system, the series compensated transmission lines are widely used because of many advantages such as system stability improvement, increased transmittable power, reduction in transmission losses, and enhancement in voltage control, damping low frequency oscillation during grid disturbance and increased loading capacity of the line. Series compensated transmission systems have an effective control over transmitted power which can be both active and reactive power. In the conditions that voltage is under the level then the reactive generation model of compensation techniques is required while when the voltage is above its limitation, the reactive absorption models are required. Based on different models used, different designs are possible. The capacitors provide reactive power in the systems while inductors absorb the reactive power flowing in the system. Thus, for improving the power quality and to increase the power transmission capability transmission lines are provided with fixed or thyristor-controlled series capacitor. FACTS devices improve the controllability, stability and power quality of power system. They can be connected in series, in shunt and in some cases both in series and shunt. FACTS devices are mostly static devices and hence they have very low level of power losses on the system. Their main characteristic is that they are controlled by power electronic equipment such as thyristors and are compensating the level of voltage or power to a better condition based on the firing angle of thyristors involved. Shunt connected FACTS devices help to maintain the voltage level of the system at the desired level by injecting/absorbing the reactive power. But due to the introduction of compensation devices the conventional distance characteristics are greatly subjected to maloperation in both the forms of over- reaching or under- reaching the fault point. Series compensation adds certain complexities in the operation of existing control and monitoring systems. So, it is necessary to analyze the performance of distance relay under fault condition for compensated lines [14]. 4. THYRISTOR CONTROLLED SERIES COMPENSATOR A series capacitor in the transmission line increases transmission line power transfer capability by reducing its net series impedance. Even though this method is well known it has limitations due to slow switching times. Hence, Thyristor Control Series Compensator (TCSC) plays vital role in providing solutions for increasing power demand nowadays. Thyristor controllers provide rapid, flexible and continuous control over wide range [15]. It also helps to increase damping in large interconnected electrical systems and overcomes the problem of Sub Synchronous Resonance (SSR). TCSC acts as a capacitive series compensator which is composed of a fixed series capacitor bank shunted by a thyristor-controlled reactor (TCR) which helps to provide a smoothly varying series capacitive reactance [14-17]. Thus, no interfacing equipment like e.g. high voltage transformers is required. This makes TCSC much more economic than some other competing FACTS technologies. It makes TCSC simple and easy to understand the operation. The switching element of the TCR consists of two antiparallel thyristors, which alternate their switching at the supply frequency. The system is controlled by varying the firing angle of the thyristor firing pulses relative to the zero crossing of some reference waveform. The effect of such variation results into the variation in the value of the capacitive, inductive reactance at the fundamental frequency. Fig. 1 shows schematic representation of single phase TCSC. http://www.iaeme.com/ijeet/index.asp 59 editor@iaeme.com

M. Purohit and Dr. V. N. Gohokar Figure 1 TCSC circuit diagram Thus, TCSC works on the principle of controlling the transmission line impedance by varying its own reactance XTCSC. The equivalent reactance of TCSC depends on the firing angle of TCR. Hence by varying firing angles of thyristors TCSC reactance can be varied. Thus, steady state impedance of the TCSC is that of a parallel LC circuit, consisting of fixed capacitive impedance XC and variable inductive impedance XL (α). Thus, TCSC reactance is given by the equation, XTCSC XCXL α XL αxc TCSC can be controlled to work either in the capacitive or inductive modes. Fig. 2 shows the impedance characteristics of TCSC. Figure 2 Impedance characteristics of TCSC 5. SIMULATION AND RESULTS Fig. 3 and 4 represents the model developed using MATLAB/SIMULINK for calculation of a single phase to ground fault. http://www.iaeme.com/ijeet/index.asp 6 editor@iaeme.com

Effects of Series Compensation on Distance Protection of High Voltage Transmission Lines under Fault Conditions Figure 3 Transmission line model with TCSC Figure 4 Performance parameters under observation during the simulation run Table 1 System Specifications Sr. No. Equipment Ratings 1 Three phase source 1 13.8 kv, 5 Hz 2 Three phase source 2 4 kv, 5 Hz 3 Two winding transformer 1MVA, 5 Hz, 13.8 kv (Delta)/ 4 kv (Star grounded) 4 TCSC TCR inductance: 6.37 mh TCSC capacitance: 43.212 µf Quality factor: 5 5 Three phase PI section long transmission line model 6 Fault Length:3 km Frequency: 5 Hz R 1 =.1165 Ω/km R =.2676 Ω/km L 1 =.8679 mh/km L = 3.8 mh/km C 1 = 12.74 nf/km C = 7.751 nf/km Type: Three phase to ground fault at 1.2 sec with fault resistance.1 Ω 7 Simulation sampling time 5 µsec Three phase to ground fault at various locations is considered for simulation purpose on a transmission line with and without series compensation. The result of impedance trajectories diagrams with red, pink and blue colour are respectively of zone 3, zone 2 and zone 1 for the purpose of clearance. Various results are given in Fig. 5 to Fig.12. http://www.iaeme.com/ijeet/index.asp 61 editor@iaeme.com

Effects of Series Compensation on Distance Protection of High Voltage Transmission Lines under Fault Conditions 1.5 Current at Bus B1 versus Time 1 Current at Bus B1 in pu.5 -.5-1 -1.5.4.5.6.7.8.9 1 1.1 1.2 1.3 Time (sec) Figure 8 Current at bus B1 in per unit 3 x 18 2 Active and Reactive Power Versus Time at TCSC location. P (W) Q (VAR) Active (W) and Reactive (VAR) Power 1-1 -2-3 -4-5.5 1 1.5 2 2.5 Time (sec) Figure 9 Active (P, W) and reactive power (Q, VAR) flow 2 TCSC Current (A) Versus Time 18 16 TCSC Current in Amperes 14 12 1 8 6 4 2.2.4.6.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.5 Time (sec) Figure 1 TCSC current in ampere http://www.iaeme.com/ijeet/index.asp 63 editor@iaeme.com

M. Purohit and Dr. V. N. Gohokar 1 9 Resistance-Reactance diagram for LLLG fault on the protected line without TCSC Impedance trajectory during fault 8 7 6 Reactance X Ohms 5 4 3 2 1-1 -2-1 -8-6 -4-2 2 4 6 8 1 Resistance R Ohms Figure 11 R-X diagram for a fault at 7 km without TCSC 1 9 8 Resistance-Reactance diagram for fault of the protected line Impedance trajectory during fault 7 6 Reactance X Ohms 5 4 3 2 1-1 -2-1 -8-6 -4-2 2 4 6 8 1 Resistance R Ohms Figure 12 R-X diagram for a fault at 7 km with TCSC Impedance of TCSC is given by ZTCSC=VTCSC/ITCSC. In the absence of TCSC, impedance seen by the distance relay is the impedance of the line section between the relaying and fault point, ZL and in the presence of TCSC, the impedance seen by relay is ZLTCSC=ZL+ZTCSC (Inductive mode) and ZLTCSC=ZL-ZTCSC (Capacitive mode). If the fault occurs between the relaying point and the TCSC location, then TCSC is not present in the fault loop and the impedance measured by the distance relay is equal to the actual impedance of the line section between the relaying and fault point. On the other hand, when TCSC is involved in the fault loop, even in the case of zero fault resistance, the measured impedance would be deviated from its actual value. 7. CONCLUSION This paper presented effect of series compensation inserted in high voltage transmission line on the performance of distance relay under fault conditions. MATLAB/SIMULINK is used for simulating the transmission line with series compensation. 4 kv, 3 km transmission line with TCSC as a series FACTS device was used for this purpose. Mho characteristics was considered for the analysis of distance relay performance under different fault locations. It was http://www.iaeme.com/ijeet/index.asp 64 editor@iaeme.com

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