Electric Power Systems Research 56 (2000) 1 – 8 Dead zone phenomenon in distance relaying of overhead transmission lines  Dus ˇko D. Bekut*, Goran S. S  venda, Vladimir C. Strezoski Faculty of Engineering, Institute for Power and Electronic Engineering, Uniersity of Noi Sad, Fruskogorska 11, 21000 Noi Sad, Serbia, Yugoslaia Received 26 July 1998; received in revised form 3 January 1999; accepted 26 February 1999 Abstract This paper refers to the distance protection of overhead transmission line. The phenomenon of ‘dead zones’ in distance relay operation is established. This phenomenon consists of a segment of an overhead line, inside the distance relay reach, where the relay can not detect a fault. The nature of the dead zone phenomenon is the most complex when the distance protection of mutually coupled lines is considered. Thus, clarifying the nature and proposing measures for elimination of dead zones from distance protection are main objectives of this paper. A simple example is considered to get a qualitative insight into the dead zone appearance. Presented considerations are illustrated by example belonging to the actual power system of Yugoslavia. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Distance relaying; Distance relay dead zone; Mutually coupled lines www.elsevier.com/locate/epsr 1. Introduction The basic principle of the distance protection is based on the linear dependence between the value of the impedance measured by the distance relay (measured impedance 1 ) and the distance between locations of the relay and the fault [1,2]. Such a linearity is valid exclusively for solid faults which have occurred inside the first protective zone of a distance relay protecting a two-terminal line (the first protective zone belongs to the same line on which the corresponding relay is located). In other cases, this dependence is not linear. The most important cases in which such non-linearities occur are: 1. The fault at the first protective zone of a distance relay is accompanied by a resistance. If it occurs, the non-linearity culminates, when the line is double- side infeed and the short-circuit power in the part of the system behind the distance relay is ‘small’ and the short circuit power in the part of the system at the other side of the line is ‘large’. The non-linearity is more significant when lines are close and the coupling is partial. Very interesting and important situations are those when variations of the fault location at one line imply changes of both magni- tude and direction of currents in other mutually coupled lines. 2. Faults at three (multi) terminal lines are considered. 3. The fault and the corresponding distance relay do not belong to the same line. That is the case when higher protective zones (the second and the third one) of distance relays are considered. In such a situation, the value of the relay measured impedance significantly depends on the value of the infeed factor 2 . This value does not depend linearly on the fault location. Therefore, dead zones appear more frequently at the higher protective zones than at the first one.  The material presented in this paper is part of the investigations encompassed by the project for the EPS JP Elektrovojvodina Power Company, Novi Sad, Serbia, Yugoslavia, 1996. * Corresponding author. 1 ‘Measured impedance’ means the impedance magnitude, its resis- tance or reactance. 2 The ‘infeed factor’ is defined as the ratio of the faulted line current and the current of the line where the considered relay is located. 0378-7796/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0378-7796(00)00078-X