IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 23, NO. 3, JULY 2008 1319 Ground Distance Relaying With Fault-Resistance Compensation for Unbalanced Systems André Darós Filomena, Student Member, IEEE, Rodrigo Hartstein Salim, Student Member, IEEE, Mariana Resener, Student Member, IEEE, and Arturo Suman Bretas, Member, IEEE Abstract—Fault resistance is a critical variable in distance relaying. If not considered due to underreaching phenomenon, it may cause the misoperation of ground distance relays for internal faults. Still, as a consequence of the overreaching phenomenon, the unbalanced nature of loads and asymmetry of lines can affect the distance protection operation efficiency. Mainly due to these aspects, there is low precision in protection zone limits of ground distance relays. In this paper, a new algorithm is proposed to increase the precision of these limits, improving efficiency in the distance protection process. The proposed method is based in phase coordinates and uses a fault resistance estimate to develop the trip decision procedure. The results show that the algorithm is suitable for online applications, and that it has an independent performance from the fault resistance magnitude, the fault loca- tion, and the line asymmetry. Index Terms—Fault diagnosis, fault resistance, power system faults, power system protection, protective relaying. I. INTRODUCTION F AULTS are common disturbances in power systems. This phenomenon is associated with different causes, such as insulators breakdown, lightning, equipment failure, and even trees or animals in contact with electrical equipment. Due to its stochastic nature, faults are also hardly avoidable, leaving to protection engineers the task of designing protection schemes that prevent severe system damages, eliminating the fault as fast, secure, and reliable as possible. In the history of electric power systems, protection engineers have developed such de- signs based on different approaches, such as reliability, security, selectivity, and coordination. These designs make use of dif- ferent protection equipment, such as reclosers, circuit breakers, fuses, sectionalizers, and relays [1]. Distance relaying, either phase or ground type, is frequently applied as the main protection of important transmission lines. Distance relays perform a comparison between the positive-se- quence apparent impedance measured from one terminal of the line and the relay operation characteristic to decide between line Manuscript received January 29, 2007; revised May 15, 2007. This work was supported in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq ) and in part by the Companhia Estadual de Distribuição de Energia Elétrica do Rio Grande do Sul (CEEE-D). Paper no. TPWRD-00033- 2007. A. D. Filomena is with the Federal University of Rio Grande do Sul, Porto Alegre RS 90035-190, Brazil and also with Companhia Estadual de Geração e Transmissão de Energia Elétrica (CEEE-GT), Porto Alegre RS 91410-400, Brazil (e-mail: afilomena@ece.ufrgs.br). R. H. Salim, M. Resener, and A. S. Bretas are with the Federal University of Rio Grande do Sul, Porto Alegre RS 90035-190, Brazil (e-mail: salim@ece. ufrgs.br; mariana@ece.ufrgs.br; abretas@ece.ufrgs.br). Digital Object Identifier 10.1109/TPWRD.2007.909210 Fig. 1. Underreaching in distance relaying due to fault resistance. tripping or not. This procedure is carried out after the fault detec- tion, and is based on the line impedance for the trip decision [1]. During a low resistance fault, it is possible to achieve reasonable accuracy using this method, since the effective impedance be- tween the relay and ground is close to the apparent impedance measured by the relay. Traditional distance relaying is designed to operate as primary protection (first zone, Zone 1) for a lim- ited line impedance value. For faults outside this zone, distance relays can be used as backup protection, in time-delay coordi- nated stages (second and third zones) [2]. Fault resistance introduces an error in the distance estimation obtained with traditional distance relays, since in resistive faults, the distance between the relay and the fault point is not neces- sarily proportional to the impedance seen by the relay [1], [3]. The error introduced by the fault resistance for a symmetrical fault [1] is given by (1) (1) where is the measured apparent impedance, and are the voltage and current fundamental phasors calculated with data from the relay point, is the actual line impedance between the relay and the fault point, is the fault resistance, and and are, respectively, the current phasors from the remote and the sending end of the line. The fault resistance causes an underreaching phenomenon in distance relaying, as shown in Fig. 1. A way to overcome this problem is to compensate the fault re- sistance. Traditional distance relays achieve this compensation using a quadrilateral characteristic that depends on the angle be- tween and [3]. By applying this technique, it is possible to obtain a better fault resistance coverage and arc compensation, without problems associated with overload misoperation of the distance relay. Other shapes of distance relay trip zones are also possible [3]–[6]. However, the fault resistance compensation is 0885-8977/$25.00 © 2007 IEEE