0885-8977 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPWRD.2015.2472638, IEEE Transactions on Power Delivery 1 Abstract--In this paper, a new double-end fault location technique is proposed for parallel transmission lines depending on unsynchronized measurements of both line ends without using line parameters. The proposed technique is based on profiling the correlation between the unknown synchronization angle and the fault distance to accurately estimate the fault point. The positive sequence network with lumped parameter line model is first utilized in order to facilitate pinpointing the required synchronizing angle between both line ends. The Least Square Error (LSE) method is used to solve the equivalent positive sequence network of the line using unsynchronized double end measurements. Then, an accurate estimation of the fault distance can be realized for all fault types depending on the positive sequence network of the faulty line. The performance of the proposed locator is thoroughly investigated using a detailed simulation of a selected 150 mile overhead parallel transmission system using the ATP/EMTP package. A wide variety of faulty conditions are covered in order to exploit the benefits of using double-end data even with very high fault resistance and untransposed transmission lines. Index Terms--Double Circuit transmission lines, Fault location, LSE, Line parameters, Unsynchronized measurements. I. INTRODUCTION AULT locators gained a very growing interest among protective devices in the recent few years. This is mainly due to their role in improving the reliability of high capacity power systems. Transmission lines usually experience a variety of faults resulting in disconnecting the power feeding to the loads. Hence, its restoration process can be expedited if the location of the fault is either known or can be estimated with a reasonable accuracy. Different benefits are gained by utilizing fault locators in power networks including reducing maintenance times, increasing the power availability, improving the power quality and avoiding future accidents. This can be interpreted as a cost reduction or a profit increasing which is an essential concept for competitive and de-regulated markets. Fault location algorithms can be categorized based on the extracted fault features, the number of measurement terminals and the line parameter availability. Impedance-based algorithm N. I. Elkalashy, T. A Kawady, W. M. Khater and A. I. Taalab are with the Electrical Engineering Department, Engineering Faculty, Minoufiya University, 32511, Shebin Elkom, Egypt (e-mails: nagy.elkalashy@sh- eng.menofia.edu.eg, t_kawady@ieee.org, taalab3@yahoo.com). depends on synchronized phasor measurement where the network is modeled in the faulty conditions and the model is solved in order to determine the fault distance as an unknown variable [1]. Utilizing transient signals or depending on the extracted travelling surges were proposed as reported in [2]- [6]. Regarding the utilized number of terminals, fault locators are categorized as one-terminal, two or multi-terminal fault location methods. Multi-terminal fault locators greatly eliminate the effects of fault resistance, remote feeding currents, line untransposition, weak sources and heavy loads [1]. Accordingly, double-end algorithms are interesting to locate the fault point in transmission systems accurately. Regarding line modeling, either lumped or distributed line parameters were used [7], [8]. Conventionally, accurate fault location can be realized when the line parameters are well- defined and the fundamental phasors are correctly measured and accurately synchronized. To reduce the requirements for data synchronization, fault locators with unsynchronized data were proposed [9-15]. Also, parameterless fault locators were proposed as addressed in [16]-[20]. Such methods were mathematically formulized considering the synchronization angle or the line parameters among the unknowns of the mathematical core of the fault location problem. Typically, these mathematical formulization with increased unknowns are solved iteratively using method such as Newton-Raphson approach [16]-[18]. Such algorithms were mostly based on the fault type and therefore a separate representation is required for each unsymmetrical fault. Hence, a correct identification of the fault type is required [20]. Parallel transmission lines are usually characterized with a significant increase in the mutual coupling effects. This raises remarkable errors for impedance-based protection equipment in particular. Owing to the mutual coupling, the total line impedance may significantly change resulting from the mutually reflected impedances from the other phases. Thus, the actual line parameters significantly deviate from those parameters that are adjusted for relay setting [21]. Regarding the fault location computation purposes in particular, remarkable errors can be then expected for parallel lines. As reported in the literature, different fault location algorithms were proposed particularly for parallel transmission lines using either single end [21]-[23] or double end measurements [24]- [28]. Negative-sequence voltage were utilized as seen in [29]. Distributed parameters line model was suggested as well [30]. However, these aforementioned algorithms assumed a correct Unsynchronized Fault Location Technique for Double-Circuit Transmission Systems Independent of Line Parameters Nagy I. Elkalashy, Tamer A. Kawady, MIEEE, Wagdy M. Khater and Abdel-Maksoud I. Taalab, SMIEEE 1 F