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 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 mal- operation 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. Fig. 1. Zones of protection for distance relay 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 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 Adaptive Mho Type Distance Relaying Scheme with Fault Resistance Compensation T 2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June 2013 208 978-1-4673-5074-7/13/$31.00 ©2013 IEEE