Published in IET Generation, Transmission & Distribution Received on 25th October 2010 Revised on 4th February 2011 doi: 10.1049/iet-gtd.2010.0707 ISSN 1751-8687 New adaptive digital distance relaying scheme for double infeed parallel transmission line during inter-circuit faults V.H. Makwana 1 B. Bhalja 2 1 Department of Electrical Engineering, G H Patel College of Engineering and Technology, Vallabh Vidyanagar 388 120, Gujarat, India 2 Department of Electrical Engineering, A D Patel Institute of Technology, New Vallabh Vidyanagar 388 121, Gujarat, India E-mail: bhaveshbhalja@gmail.com Abstract: Presence of various types of inter-circuit faults between parallel transmission lines combined with the effect of mutual coupling and fault-resistance makes the protection of parallel transmission lines quite complicated. The authors have proposed a new adaptive digital distance relaying scheme which takes care of all the abnormalities of the conventional ground distance relays and measures the correct value of impedance during phase-to-phase and phase-to-phase-to-ground inter-circuit faults. The impact of fault-resistance and mutual coupling phenomena has been considered during such types of inter-circuit faults. The proposed adaptive scheme is based on the derivation of the compensated value of impedance using symmetrical component theory without the need of remote end data. To validate the proposed scheme, numerous computer simulations have been carried out on an existing 400 kV parallel transmission line network using MATLAB/SIMULINK software. The simulation results demonstrate the effectiveness of the proposed scheme giving an average percentage error of +0.579%. The results indicate that the proposed technique is highly accurate and robust for a wide variation in system and fault conditions. 1 Introduction In regions where large blocks of power are being transferred over double-circuit lines, the occurrence of an inter-circuit fault because of conductor geometry, could initiate serious system instability [1–3]. These faults include unearthed inter-circuit faults and earthed inter-circuit faults. The former faults occur between two different phases of the parallel transmission lines on the same transmission tower through an arc, whereas the later faults occur between two different phases of the parallel transmission lines and ground on the same transmission tower [4]. The probability of occurrence of these types of faults is very high due to bush fires under the transmission lines, lightning stroke, man-caused accident or catastrophic accident. These faults result in unusual voltage and current distributions owing to which, distance calculation performed by the conventional distance relay is incorrect which leads to the suboptimal performance of a distance relay [4–9]. Further, these faults create a serious problem of loss of phase selectivity for single-pole tripping schemes because of the presence of zero-sequence currents [10]. Performance of the conventional distance relays is also affected by the mutual coupling between parallel lines. The positive and negative sequence coupling between the two feeders is usually less than 5–7% and, hence, has negligible effect on protection. The zero-sequence coupling, on the other hand, can be strong and its effect cannot be ignored. The mutual impedance can be as high as 50–70% of the self-impedance. Mutual coupling can cause incorrect tripping of distance relays. The problem is compounded by the composite voltage circuits and by the remote source infeed to the fault branch; these factors can significantly modify the apparent impedance measured by the relay at the local end. The aforementioned problems are particularly endemic when there is an inter-circuit fault on parallel transmission lines. The situation could become even worse when the impact of fault-resistance is considered, which may under certain conditions to cause the protected line’s middle section to lose the first zone coverage altogether [11, 12]. Very few papers have been published by researchers to analyse the problems of inter-circuit faults on parallel transmission lines using various fault analysis methods, such as sequence-domain method and phase-domain method [13–17]. But none of the papers have presented the complete solution to measure the correct value of impedance of the faulted portion of transmission line during inter-circuit faults between parallel transmission lines considering the effect of mutual coupling, remote infeed/ outfeed and fault-resistance. In this paper, the authors have proposed a new adaptive protection scheme to solve the problems of two types of inter-circuit faults on parallel transmission lines. The proposed scheme is based on digital computation of the compensated value of impedance using symmetrical IET Gener. Transm. Distrib., 2011, Vol. 5, Iss. 6, pp. 667–673 667 doi: 10.1049/iet-gtd.2010.0707 & The Institution of Engineering and Technology 2011 www.ietdl.org