IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 31, NO. 3, JUNE 2016 999 A Three-Terminal Line Protection Scheme Immune to Power Swing Paresh Kumar Nayak, Ashok Kumar Pradhan, Senior Member, IEEE, and Prabodh Bajpai, Member, IEEE Abstract—Communication-assisted distance protection schemes are widely used for three-terminal line protection and are suscep- tible to power swings. The available differential protection scheme is also considered to be suited to protect three-terminal lines and is free from problems associated with the power swing. However, the performance of the current differential protection scheme may be affected if an outfeed condition occurs during an internal fault. This paper presents a new technique for protecting three-terminal transmission lines immune to power swing. The technique uses synchronized voltage and current measurements from three terminals for calculating voltage at the tee-point. The maximum magnitude of the superimposed component of positive-sequence voltage estimated at the tee-point is compared with the maximum magnitude of superimposed componentas of the positive-sequence voltage measured at three terminals of the line to discriminate a fault from power swing. The method is accurate even for an outfeed condition during the internal fault, current-transformer saturation during severe internal or external faults, large load rejection, and different prefault loading conditions occur during the power swing. The proposed algorithm is verified using data simulated through EMTDC/PSCAD for a 400-kV double-circuit three-terminal system. Index Terms—Communication-assisted distance protection schemes, differential protection, fault detection, line fault, power swing, three-terminal lines. I. INTRODUCTION M ULTITERMINAL lines are often used both at the transmission and subtransmission levels to exploit economic, technical, and environmental benefits over two-ter- minal lines. Among different multiterminal configurations, the most used one is the three-terminal line. The use of the three-terminal line imposes severe protection challenges to different protection schemes, including distance relays [1]. The presence of a third source-terminal causes the distance relay to underreach for line faults beyond the tap point. The underreach problem can be overcome by extending the relay reach which limits the load-carrying capability and increases the possibility of unwanted operation during stable power swings. The NERC blackout report [2] points out that most of the lines that tripped just before the cascade tripping were three-ter- Manuscript received May 14, 2014; revised September 12, 2014 and November 15, 2014; accepted December 22, 2014. Date of publication January 13, 2015; date of current version May 20, 2016. Paper no. TPWRD-00553-2014. P. K. Nayak is with the Department of Electrical Engineering, Indian School of Mines, Dhanbad 826004, India (e-mail: pareshkumar.nayak@gmail.com). A. K. Pradhan and P. Bajpai are with the Department of Electrical Engi- neering, Indian Institute of Technology, Kharagpur 721302, India (e-mail: akpradhan@ee.iitkgp.ernet.in; pbajpai@ee.iitkgp.ernet.in). Digital Object Identifier 10.1109/TPWRD.2014.2387873 minal lines. The distance relay also has an overreaching problem while protecting three-terminal lines during the outfeed condi- tion. The limitation of stepped-distance relaying for the pro- tection of the three-terminal line is overcome by using com- munication-assisted distance protection or the unit protection scheme [3], which provides 100% protection to such lines. The communication-assisted distance protection schemes for such lines are direct underreaching transfer trip (DUTT), permissive overreaching transfer trip (POTT), and directional comparison blocking (DCB). Out of these, POTT and DCB schemes are set with zone-2 coverage for protecting the full line which needs to be extended to account for the infeed effect. This may cause the apparent impedance to appear within the protected zone at all terminals during the stable power swing, resulting in unwanted protection operation [4]. To enhance the security of operation during the power swing, the power-swing blocking (PSB) func- tion is utilized with such schemes to block them from malop- eration [5]. However, if a fault occurs during the power swing, it must be detected correctly to unblock the scheme. The detec- tion of the symmetrical fault during the power swing is a chal- lenging issue as both phenomena are balanced in nature. Dif- ferent techniques have been proposed on this issue using local signals [6]–[10] and are applied to two-terminal lines only. The current differential scheme is also used for the protection of three-terminal lines. This scheme has faster response and is free from problems associated with the power swing [11]–[13]. However, the scheme may fail if the outfeed condition arises due to an internal fault the during power swing, or CT satura- tion occurs due to severe external faults in a three-terminal line during the power swing [4], [14]. Thus, there is a scope for im- provement in the protection for such lines. In this paper, a superimposed component of the voltage-based technique is proposed for three-terminal line protection with a target to improve the protection function even during the power swing. The scheme detects a fault in such a line by comparing the maximum magnitude of the superimposed component of the positive-sequence estimated voltage at the tee-point with the maximum magnitude of the superimposed component of the positive-sequence voltage at three terminals of the line. The pos- itive-sequence voltage at the tee-point is estimated by using syn- chronized positive-sequence voltage and current signals from each end of the three terminals. The performance of the algo- rithm is evaluated for various fault and nonfault events during the power swing and is found to be accurate. II. PROPOSED ALGORITHM Protective relays, today with global positioning system (GPS) technology, have synchronized measurement capabilities 0885-8977 © 2014 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.