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Impact of Facts Devices on Distance Relay Naresh Patnana1, Kontham Hari Krishna2 Asst.professor, Department of EEE, Gitam University, 2Asst.professor, Department of EIE, Centurion University, Odisha, India 1 [email protected],[email protected] 1 Abstract In the power transmission system FACT controller is incorporated in the transmission line in order to increase power transfer capability as well as reactive power control. At the same time with the increase in the number of FACTS devices like TCSC (Thyristor Control Series Capacitor) and STATCOM (Static Synchronous Compensator) in transmission lines, operation of distance relays are affected. It is very important that the distance relays do not mal operate under system fault conditions, as this will result in the loss of stability or the security of the system, which defeats the main objective of installing a FACT device. In this context the impacts of FACT device on the performance of distance relay needs to be addressed and a suitable pattern classifier is necessary for proper classification of the fault in the transmission line as well. In the present work two types of FACT devices is considered i.e. TCSC and STATCOM which addresses the issues of adaptive protection of a transmission line where these devices are located at the middle of transmission line. Design and simulation of TCSC incorporated in the Transmission line is done with the occurrence of fault before as well as after TCSC when it is located in the middle of line. The dynamic operation of novel control scheme for STATCOM based on a model comprising a 48-pulse Gate Turn-Off thyristor voltage source converter for combined voltage stabilization and reactive power compensation is also addressed. The impact of STATCOM on the distance relay is described by means of impedance trajectory using S-transform with quadrilateral trip boundary and apparent impedance calculation is carried out in presence of STATCOM for LG fault case. In the present work the modular neural network is proposed and the design of classifier is subsequently done in presence of STATCOM and TCSC which is also another contribution in the above work. In this work STATCOM and TCSC are connected to the 230 kV interconnected system with 400 Km transmission line. Further the system model, control strategy, impedance trajectory and design of neural classifier are done in MATLAB/Simulink environment. KEYWORDS: Distance relay, Modular Neural Network, STATCOM, S-Transform, TCSC 1. Introduction The concept of flexible ac transmission systems (FACTS) in power systems envisages the use of power electronic devices for increasing the power transfer and providing the optimum utilization of system capability In principle, series controllers inject voltage in series with the line and the shunt controllers inject current into the system at the point of connection[1]. The presence of the TCSC/ STATCOM in fault loop not only affects the steady state components but also the transient components. Due to presence of these devices during fault condition the change in apparent impedance may lead to mal operation of a distance relay. It is very important that the distance relays do not mal operate under system fault conditions, as this will result in the loss of stability or the security of the system. Some research has been done to evaluate the performance of a distance relay for transmission systems with FACTS controllers. The work in [2,3] has presented some analytical results based on steady-state model of STATCOM, and the authors have studied the impact of FACTS on a distance relay at different load levels and its tripping boundaries, and both the parameters of FACTS controllers and their location in the line have an impact on the trip boundary. The work in [4] shows that thyristor-controlled series capacitor (TCSC) has a major influence on the mho characteristic and discussed about stable operating region. In this paper for observing impact of STATCOM /TCSC on distance relay, polygon tripping boundary characteristics are shown and the apparent impedance calculation is determined at the fault point using S-transformation. Further modular neural network is proposed for proper classification of the fault in the transmission line. 2.Modeling of FACT Devices : Fig. 1 Single-line diagram of Power system with FACTS device. 1 To study the impact of FACT devices on distance relay a 230 kV,50Hz interconnected power system is considered in which STATCOM is installed at mid point of transmission line as shown in the Fig.1 also the same power system is considered with TCSC located at different locations in transmission line for fault identification , classification and localization. Fig. 3 Switching charcterstics of STATCO 2.2 48 pulse conveter 2.1 Modeling of STATCOM : Fig.4 48- pulse converter Fig.2 Transmission line with STATCOM The static compensator (STATCOM) as shown in Fig.2 is a device used to regulate voltage and improve dynamic stability of power systems. GTObased STATCOM are multilevel line-commuted voltage-sourced inverters that are shunt-connected to a power system bus through a set of transformers. By varying the amplitude of the output voltage E, the reactive power exchange between the converter and the AC system can be controlled. If E is the STATCOM voltage and V is the System voltage, then E > V Capacitive Mode, Q generated E<V Inductive Mode, Q delivered. The switching characteristics of STATCOM are shown in Fig.3. k 0 ,1, 2 2.3.Control Circuit Transient rating VT Transient rating The STATCOM shown in Fig.4 is built with four 3-phase 3-level inverters coupled with four phase shifting transformers introducing phase shift of +/- 7.5°. This transformer arrangement neutralizes all odd harmonics up to the 45th harmonic, except for the 23rd and 25th harmonics (for a perfectly balanced network). Those two harmonics are minimized choosing an appropriate conduction angle for the three-level inverters (σ= 172.5°). The line-to-neutral 48-pulse ac output voltage from the STATCOM model is expressed as: 8 Vab48 (t ) Vabn sin( nt 18.5n 18.75i) ------- (1) 3 n 48k 1 1.0 pu 0.75 pu 0.5 pu Fig. 5 GTO firing pulse generator 0.25 pu IC IC max IL max Capacitive Inductive IL Control circuit is used to operate the voltage source inverter to inject or absorb reactive power to regulate the connecting point voltage to the setting value Vref. (The dead angle of STATCOM is kept fixed at γ=π/48). The process of generating firing pulses by voltage source inverter shown in Fig. 5 2 Where is the firing angle, XL is the reactance of the inductor. The parameters considered are shown below Voltage Regulator gain Current Regulator Gain 2.5 Impact of FACTS device on Distance Relay Kp 15 6 Ki 3000 40 Vshunt = Three phase voltages at the connecting point I shunt = Three phase currents of STATCOM 2.4. Modeling of TCSC The TCSC is one of the main FACTS devices, which improves the stability and power transfer capability of the existing transmission system which not only affects the steady state components but also the transient components. Simulink model of open loop TCSC device connected in series with the interconnceted three phase transmission system is shown in Fig.6. The circuit thus has an equivalent impedance ( ) under steady state conditions which is represented by The operation of distance relay depends on apparent impedance measurement which is mainly influenced by the type of FACTS device, location, control parameters of the device and fault resistance. From equation (2) X net varies due to changes in firing angle and during fault condition it effects the apparent seen by the relay leading to its maloperation. Apparent impedance during Lg fault in presence of STATCOM. For the analysis associated with the operation of a distance relay, the power system shown in Fig. 3 is used, the relay is installed on the before the STATCOM. The apparent impedance is calculated from symmetrical component transformation at relay point [7]. When Lg fault occurs at the right side of STATCOM and the distance is n*L from the relay point, the apparent impedance is given by [6] Z= -------------------- (2) Where X c capacitive reactance of the parallel capacitor and X1 is the reactance of the ---------------- (4) are quantities at relay point the relaying current On simplification, the impedance seen by this relay can be expressed as: Z nz1 Where I sh I relay (n 0.5) z1 If I relay R f ------ (5) R f = fault resistance, z1 =positive sequence impedance, n=distance in PU The second part of equation (5) shows impact of STATCOM on the apparent impedance and results from the shunt current I sh injected by the STATCOM. 2.6 S-Transform Fig. 6 Simulink diagram of TCSC Variable reactor which is given by ------------------- (3) S-transform is an invertible time-frequency spectral localization technique that combines the elements of wavelet transform and short Fourier transform. In this application S-transform used for calculating amplitude and phase of the faulted current and voltage signals. The impedance to the fault point is calculated by using the phasor information from stransform. The impedance trajectories(R-X plot) for different operating conditions are found out and the circuit breaker tripped when the trajectory enters in to the relay operating zone and thus protects the line. 3 2.7 Classification scheme by Modular neural network Table 2 Test result for fault location for AG fault Cases Fig. 7 Modular structure The modular concept to neural network is borrowed from the principle of divide and conquers. Such a strategy in neural network is applied to directional relay for a line in [5].In the approach any task is divided into number of possible subtasks where each one is accomplished by an individual neural network. Finally all network outputs are integrated to achieve the overall task. In this work such a strategy is applied to identification of fault section using probabilistic neural network. When the fault is beyond the TCSC from relay side the network should provide 1 else 0. After the net work is trained it is tested at different section within or beyond TCSC to identify the section of fault. Test result for identification for fault section for different types of fault given Table 1 Table 1 Identification of fault Cases Fault at 15% AG.fault ( before TCSC) Fault at25% (before TCSC) BG fault Fault at 35%( before TCSC) AG fault with fault inception angle 900 Fault at 65% (after TCSC) AB fault Fault at 75% (after TCSC) BCfault Fault 55% (after TCSC) CG.fault with change in frequency 52 Hz Network output 0 Network output % Error Fault at 17% before TCSC 0.1734 2 Fault at 27% before TCSC 0.2684 0.59 Fault at 32% before TCSC 0.3206 0.19 Fault at 41% before TCSC 0.4123 0.56 Fault at 63% before TCSC 0.6277 0.0037 Fault at 77% before TCSC 0.7704 0.05 Fault at 88% before TCSC 0.8841 0.47 Accuracy of distance estimation for different values of fault resistance as given in Table 3 Table 3 Accuracy Estimation Distance (km) 51 81 96 123 231 264 Error (%) (Rf=0.1Ω) 3.65 1.41 1.61 2.54 1.43 2.11 Error(%) (Rf=10.Ω) 1.81 2.52 1.31 1.98 1.26 2.24 3. Results and Discussion: 0 0 1 1 1 The table 2 indicates a test result for a-g fault at different location of the transmission line using standard back propagation neural network. Fig. 8 Impedance characteristics of TCSC From Fig. 8 it is clear that resonance occurs at a firing angle of / 2 and on left side of this 4 point the TCSC operates in inductive mode whereas on the right it is capacitive mode of operation. Fig.9 48-pulse converter output voltage 1349.38 1349.34 1349.30 1349.26 1349.22 137.02 137.01 137.01 137.01 137.01 1361.05 1361.01 1360.97 1360.92 1360.88 170.63 170.58 170.52 170.45 170.37 The impact of STATCOM on distance relay during fault (LL fault, phase ‘a’) condition is shown in Fig. 11 by the quadrilateral trip boundary characteristics. It clearly shows the chance of mal operation of distance relay with and without STATCOM during fault condition. The characteristic with statcom during fault condition is observed to be not entering into the tripping zone which may lead to mal operation of distance relay. The reactive power injected by STATCOM when its settings are 0.88, 1 and 1.12 respectively is shown in Fig.10. Mho relay and polygon shape charecteristics 100 50 X Plane 0 -50 -100 -150 -200 0 100 200 300 400 500 600 700 800 900 R Plane Fig. 11 Quadrilateral trip boundary characteristics of distance relay with STATCOM (dotted line) & without STATCOM (solid line) Fig.10 Reactive power control in STATCOM In the system shown in the Fig.2, an LL fault occurs on the right side of STATCOM and the fault distance to relay point is 200km; the setting value in terms of the desired voltage for STATCOM is 1.0pu. The change of apparent impedance during fault conditions with STATCOM and without STATCOM is shown in Table 4. Fault is created after 20 km from STATCOM having Simlary for quadrilatral trip boundry charctarstics for phase C is shown in Fig.12. 250 200 R f =10ohms and inception angle 150 0 of 18 . Table 4 R,X vaules at Fault Condition XPlane 100 50 0 Without STATCOM With STATCOM -50 R( ) 1349.52 1349.49 1349.45 1349.42 X( ) 137.17 Rs( ) 1361.22 Xs( ) 170.78 137.05 1361.18 137.04 1361.14 137.03 1361.09 170.68 170.67 170.65 -100 -150 0 100 200 300 400 R Plane 500 600 700 800 Fig. 12 phase C Characteristics 5 4. CONCLUSION This paper firstly presents a detailed model of a Transmission system employing STATCOM and TCSC. In the present work the modular neural network is used for fault section identification and fault localization in presence of TCSC.which is observed to be less time consuming and efficient technique. The simulation result shows clearly the impact of STATCOM on distance relay performance is done by the help of quadrilateral trip boundary using S-transformation. 5. References: [1] Hingorani N. G. and Gyugyi L., Understanding FACTS Concepts and Technology of Flexible AC Transmission Systems. New York: IEEE Press, 2000. [2] El-Arroudi K., Joos G., and McGillis D. T., “Operation of impedance protection relays with the STATCOM,” IEEE Trans. Power Del., vol.17, no. 2, pp. 381–387, Apr. 2002. [3] Dash P. K., Pradhan A. K., Panda G., and Liew A. C., “Adaptive relay setting for flexible AC transmission systems (FACTS),” IEEE Trans. Power Del., vol. 15, no. 1, pp. 38–43, Jan. 2000. [4] Wang W. G., Yin X. G., Yu J., Duan X. Z., and Chen D. S., “The impactof TCSC on distance protection relay,” in Proc. Int. Conf. Power SystemTechnology (POWERCON ’98), vol. 1, Aug. 1998, pp. 18–21. [5] Lahiri U., Pradhan A. K,Mukhopadhyaya S., “Modular neuralnetwork-based directional relay for transmission line protection,” IEEE Trans. on Power System, vol. 20, no. 4, pp. 2154-2155, 2005. [6] Xiaoyao ZhouU, Haifeng Wang R. K. Aggarwal and Phil Beaumont ”Performance Evaluation of a Distance Relay as Applied to a Transmission System With UPFC”IEEE Trans. on Power Delivery vol 21,no.3 1137-1147,2006 6