2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June 2013 Adaptive Mho Type Distance Relaying Scheme with Fault Resistance Compensation Muhd Hafizi Idris, Member, IEEE, Mohd Saufi Ahmad, Ahmad Zaidi Abdullah, Surya Hardi School of Electrical System Engineering Universiti Malaysia Perlis, Arau, Perlis, Malaysia. Abstract-- This paper describes an adaptive distance relaying scheme which can eliminate the effect of fault resistance on distance relay zone reach. Distance relay is commonly used as main protection to protect transmission line from any type of fault. For a stand-alone distance relay, fault resistance can make Mho type distance relay to be under reached and thus the fault will be isolated at a longer time. In this scheme, the relay detects the fault location using a two-terminal algorithm. By knowing fault location, fault voltage at the fault point can be calculated by using equivalent sequence network connection as seen from local terminal. Then, fault resistance is calculated by using simple equation considering contribution from remote terminal current. Finally, the compensation of fault resistance is done onto calculated apparent resistance as seen at relaying point. The modeling and simulation was carried out using Matlab/Simulink software. Several cases were carried out and the results show the validity of the scheme. Index Terms—Distance relay, Fault location, Fault resistance, Matlab/Simulink, Mho type, Single line to ground I. INTRODUCTION T ransmission line is one of the main components in High Voltage (HV) and Extra High Voltage (EHV) power system. The protection of transmission line from any type of fault is very important because any mis-operation or maloperation of protection relays can give a great effect on the stability of power system entirely. One of the main protection used to protect transmission line is distance or impedance relay. It uses the impedance measurement technique to measure the apparent impedance as seen by the relay at the relaying point. The inputs for distance relay are three phase current and voltage phasors during fault occurrence. Transmission line is segregated into several zones of protection normally zone 1, zone 2 and zone 3 as shown in Fig. 1 for relay at substation A [1]. Distance relay acts as main protection for faults within zone 1 while for zone 2 and zone 3, it acts as backup protection for adjacent line. Zone 1 reach is normally set only up to 80% - 90% of the protected line. It is not set 100% of the protected line to avoid relay from under reached or over reached due to current and voltage measurement errors, transient effect and inaccuracy in transmission line parameters. If a fault occurred within this zone where distance relay acts as main protection, the relay will instantaneously send trip signal to open the circuit breaker. To ensure full coverage of the protected line by considering errors and other effects, zone 2 is set at minimum 120% of the protected line. It is a common practice to set zone 2 reach equal to 100% of the protected line plus 50% of the shortest adjacent line. For faults within zone 2 reach, tripping signal will be sent at a delayed time where the relay acts as backup for main protection at adjacent line. The tripping time for zone 2 normally set at several hundred miliseconds. Backup protection for entire adjacent line is covered by zone 3 reach. It is normally set at least 1.2 times the impedance of protected adjacent line. The set tripping time for zone 3 reach is typically several seconds. An accurate apparent impedance measurement during fault occurrence by distance relay is very important because false measurement might result in delayed tripping signal sent by distance relay. There are several factors which can lead to inaccurate apparent impedance measurement such as high fault resistance, mutual inductance of parallel line, line charging capacitance and transient effects due to switching of Flexible Alternating Current Transmission System (FACTS) devices [2]-[6]. In this paper, focus is given to compensate the effect of fault resistance on the accuracy of Mho type distance relay. Fault resistance can be high or low depending on the nature of fault. Even a small fault resistance value can make the relay to be under reached when it is used to protect short transmission line. The relay also might be under reached when the fault is near to remote substation terminal. Delayed tripping of circuit breaker due to under reached of distance relay will make the power system in stress for a longer time. II. THEORIES OF THE PROPOSED SCHEME Fig. 1. Zones of protection for distance relay 978-1-4673-5074-7/13/$31.00 ©2013 IEEE This section presents the methodologies used to compensate the effect of fault resistance on the accuracy of apparent impedance measurement. The process began with determining the fault location during the occurrence of fault. There are many techniques currently and previously used to 208 2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June 2013 locate the fault point. The most and commonly used technique to locate fault at transmission line is impedance based technique [7]. It measures the impedance by dividing the fundamental frequency of voltage and current phasors at the relaying point. It is widely used because of its simplicity and easy to be adapted to electronic devices. However, this technique is severely affected by fault resistance value where the inaccuracy of fault location estimation increases with the increase in fault resistance. Available impedance based techniques also can be classified into one-terminal and two-terminal algorithms. Because of limitation in gaining measurement parameters, one-terminal algorithm uses many assumptions in fault location estimation which may lead to inaccurate result. Twoterminal algorithm is more accurate than one terminal algorithm because it uses the voltage and current measurements from both substation terminals. The data are sent using low speed communication channel available at the substation. This algorithm is expected to replace one-terminal algorithm in the future because of the increasing use of intelligent electronic devices (IED). Two-terminal algorithm as was proposed by the author from the previous research is used to estimate the fault location [8]. The algorithm does not need source impedance parameters and estimated fault location is not influenced by fault resistance value. Equation (1) represents the algorithm for fault location calculation for Single Line to Ground (SLG) fault while Fig. 2 shows the SLG fault condition at a phase line. m (1) the fault point can be calculated using (2). The value of fault resistance, RF then directly calculated using (3). During fault condition, current from both substations will flow into fault point and return back to their sources. Hence, the contribution of current from remote substation must be included into fault resistance calculation. Fig. 3. Sequence network connection as seen from local substation terminal Where; m (2) IA0 = Zero sequence component of phase current from local substation. RF (3) Where; RF = Fault resistance VF = Fault voltage between fault point and ground IA = Phase current from local substation IB = Phase current from remote substation After the relay calculated the fault resistance, next step is to compensate the effect of fault resistance on Mho type distance relay. This is done first by measuring the apparent impedance at the relaying point. The measurement of apparent impedance is done by using (4) [9]. The apparent resistance, R and reactance, X are the real and imaginary values of (4) respectively. Where; m = fault location estimation (in per unit) Z1 = Positive sequence impedance Z2 = Negative sequence impedance Z0 = Zero sequence impedance VA = Phase to ground voltage for local substation VB = Phase to ground voltage for remote substation IA = Phase current from local substation IB = Phase current from remote substation (4) Where; k0 = residual compensation factor and k0 = (Z0-Z1)/kZ1. I0 = Zero sequence component of neutral current. After that, the apparent resistance will be subtracted with fault resistance as shown in (5). Rcompensated = R – RF Fig. 2. Single line to ground fault Fig. 3 shows the equivalent sequence network connection for SLG fault as seen from local substation terminal. From the result of fault location calculated using (1), fault voltage, VF at Where; Rcompensated = Compensated apparent resistance R = Measured apparent resistance. 209 (5) 2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June 2013 The proposed scheme was modeled using Matlab/Simulink software. Table I lists the parameters used for transmission line model. Fig. 5 shows the overall model for distance relay with fault resistance compensation. The function of Analog Low Pass Filter block is to filter any harmonic components which may add to the fundamental frequency component of voltage and current phasors. Next, the filtered voltage and current phasors are transferred to Fourier Analysis block. The function of this block is to extract the magnitude and phase angle of every voltage and current phasors. Then, the magnitudes and phase angles are used inside Fault Location Calculation block. The output of this block is fault location in kilometer. After that, from the value of fault location, it is used inside Fault Resistance Calculation block to calculate fault voltage and fault resistance. Apparent impedance seen by the relay is calculated inside Apparent Impedance Measurement block. The outputs of this block are apparent resistance and apparent reactance at the relaying point. Finally, the compensation is done by subtracting the fault resistance from apparent resistance inside Compensated Apparent Resistance block. Start V & I phasors from both substation terminals Calculate fault location, m Calculate fault voltage, VF Calculate fault resistance, RF Calculate apparent resistance, R and reactance, X Subtract fault resistance, RF from apparent resistance, R Plot apparent reactance, X versus compensated apparent resistance, Rcompensated End TABLE I TRANSMISSION LINE PARAMETERS Fig. 4. Proposed fault resistance compensation scheme Fig. 4 shows the overall steps for the proposed fault resistance compensation scheme. The final step is to plot the apparent reactance versus the compensated apparent resistance on R-X diagram. III. MODELING THE PROPOSED SCHEME Line Parameters Line length Nominal frequency Phase to phase voltage Positive sequence resistance Zero sequence resistance Positive sequence resistance Zero sequence resistance thetaIa Out3 Ia Out4 Va Out1 Value 47 50 132,000 0.045531917 0.151489359 0.000617657 0.001533983 Unit km Hz Volt Ω / km Ω / km H / km H / km magnitude Z angle Z R Apparent resistance Out2 thetaVa (pi /180 )*u Continuous Apparent Impedance Measurement radian 1 powergui X Apparent reactance R Out1 Va fcn In1 Out2 Rcompensated Rf thetaVa Rcompensated (pi /180 )*u Iabc Local In1 Out1 Vabc local In2 Out4 Compensated apparent resistance Out7 radian 2 Ia Rcompensated In4 thetaIa Out8 Iabc remote In3 Out7 In4 Out10 To Workspace1 fcn Fl _km SLG Fault Vb Out13 Vabc remote Out14 Transmission Line Fault Location (km) thetaVb (pi /180 )*u In7 radian 3 Ib Analog Low Pass Filter Out19 thetaIb In10 Fault initiation RF Out20 Fourier Analysis (pi /180 )*u Fault Location Calculation Va radian 4 thetaVa Ia thetaIa fcn Rf Ib thetaIb m Fault Resistance Calculation Fig. 5. Overall model of distance relay with fault resistance compensation. 210 Fault Resistance (ohm ) 2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June 2013 IV. SIMULATION RESULTS This section presents the simulation results of Mho type distance relay when subjected to fault with a resistive value. The effectiveness and adaptability of the proposed scheme is discussed. There are four cases simulated with different fault locations and fault resistances. Below is the fault data for each case. Fig. 7 shows the result of apparent impedance locus for case 2 during fault occurrence with the same fault location as in case 1 but with different fault resistance. It can be seen that for uncompensated apparent impedance locus, fault resistance of 2 Ω has deviated away the final point of locus from zone 1. By using the proposed scheme, the locus of apparent impedance stop within the correct zone thus prevent distance relay from under reached zone 1. The calculated fault location and fault resistance are 34.55 km and 1.971 Ω respectively. Case 1 1. Actual fault location = 35 km 2. Actual fault resistance = 0.01 Ω Uncompensated Compensated Case 2 1. Actual Fault location = 35 km 2. Actual fault resistance = 2 Ω Case 3 1. Actual Fault location = 45 km 2. Actual fault resistance = 0.01 Ω Fig. 7. Apparent impedance locus for case 2 Case 4 1. Actual Fault location = 45 km 2. Actual fault resistance = 2 Ω The selected zone reach settings for Mho type distance relay at the local substation except Zone 3 are as follow; Zone 1 = 80 % of the transmission line (equal to 37.6 km) Zone 2 = 100 % + 20 % of the adjacent line (equal to 56.4 km) Fig. 8 shows the result of apparent impedance locus for case 3 during fault occurrence at 45 km. It can be seen that both locus for compensated and uncompensated apparent impedance stop within the same zone and location. This is because the fault resistance is very small compared to line resistance and the effect is hardly seen. The calculated fault location and fault resistance are 44.31 km and 0.1968 Ω respectively. Fig. 6 shows the result of apparent impedance locus for case 1 during fault occurrence at 35 km which is near the reach setting of zone 1. It can be seen that both locus for compensated and uncompensated apparent impedance stop within zone 1 at the same location. This is because the fault resistance is very small compared to line resistance. The calculated fault location and fault resistance are 34.65 km and 0.06976 Ω respectively. Compensated Uncompensated Compensated Fig. 8. Apparent impedance locus for case 3 Uncompensated Fig. 6. Apparent impedance locus for case 1 Fig. 9 shows the result of apparent impedance locus for case 4 during fault occurrence with the same fault location as in case 3 but with different fault resistance. It can be seen that for uncompensated apparent impedance locus, fault resistance of 2 Ω has deviated away the final point of locus from zone 2. By using the proposed scheme, the locus of apparent impedance stop within the correct zone thus prevent distance relay from under reached zone 2. The calculated fault location and fault resistance are 44.17 km and 1.933 Ω respectively. 211 2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June 2013 VI. REFERENCES Compensated [1] [2] Uncompensated [3] [4] [5] Fig. 9. Apparent impedance locus for case 4 [6] V. 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