International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 5, May 2013) 225 Microprocessor Based Differential Protection Of A Feeder Depending On Phase Comparison Of Current Chandrani Das 1 1 Department of Electrical Engineering, B.P.Poddar Institute of Management & Technology, Kolkata, West Bengal Abstract— Aim of this paper is to protect a transmission line section/feeder from internal faults by means of differential protection through a microprocessor. The conventional differential relay operates on the circulating current principle, which involves a direct comparison of the magnitude and phase of the currents entering and leaving the protected equipment or line section. The present scheme is based on the difference in wave pattern of the secondary currents of the two CTs which will be 180º or more than that during internal faults and less than 180º during external faults or no fault. Keywords— CT, Differential protection, Fault, Microprocessor, Phase comparison. I. INTRODUCTION Different schemes used for differential type of protection of feeder are - pilot wire protection of line, use of two relays, one at each end, pilot –wire protection of a two- ended line (circulating current scheme), basic scheme of Pilot Wire Feeder protection. Other methods are also suggested to achieve the purpose of differential relaying. A unique new approach [1] to numerical current differential line protection is there that offers improved performance over the conventional percentage restraint approach. This current differential algorithm employs a new method of Fourier calculation that allows the protective system to use ―partial‖ Fourier signals called ―phaselets‖ rather than the normal fixed window Fourier approach. The phaselet approach also facilitates in the calculation of a confidence level for the measured currents. This permits the current differential system to adapt its operating characteristics based on the quality of the measured current values. Another approach to current differential protection of transmission lines is to transform the instantaneous line current(s) by using a moving window averaging technique [2]. If the time span of moving window is equal to one- cycle time, then the steady-state value of the transformed current is zero for a periodic signal which is composed of fundamental and harmonic frequencies. Signal distortions (e.g., a fault) cause the transformed currents to deviate from the nominal zero value. This permits the development of a sensitive, secure, fast, and yet simple current differential protection scheme. The scheme can be applied into series-compensated transmission lines. Zhenyu Chen presented a paper [3] on the Intelligence Structure of Current Differential Protection in Transmission Line of UHV Based on Agent. Here the action criterions of Phase-segregated current differential protection in Transmission line of UHV include the criterion based on whole current, instantaneous value of fault and other changing from it. In different areas of fault, reach criterion has different reliability and sensitivity. On the basis of deeply analyses and study of the principle for Phase-segregated current differential protection, this paper combines intelligence Agent technology, and creates an intelligent protection structure which can choose reasonable action criterion according to the place of fault, and it can make the reliability and sensitivity of Phase- segregated current differential protection in Transmission line of UHV improved. Another approach presents Transmission lines differential protection based on the energy conservation law [4]. This paper proposes a differential protection scheme for transmission lines using the energy conservation law. The real power signals at both ends (sending and receiving) of the line are compared with a pre-specified value, if the resultant is more than this value, an internal fault is indicated in the transmission line zone. In another approach [5] for a power line current differential protection system, where all three phase current values (IA, IB and IC) are obtained from both the local end and the remote end of a power transmission line. The magnitude of the ratio of the remote current values to the local current values are calculated. Also, the angle difference between the local and the remote current values for each phase are calculated. Comparison elements then compare the ratio and angle values against preselected values which establish a restrain region in the current ratio plane. Current values which result in the ratio being within the region do not result in a tripping signal for the circuit breaker on the power transmission line, while current values which result in a ratio outside of the region result in a tripping of the circuit breaker. Similar circuitry is used for negative sequence current quantities, with the negative sequence preselected values being set substantially lower to produce a more sensitive response to possible faults in the line.