Study of Combined Time & Current Grading Protection Scheme for Distribution System

The paper paraphrases the power system protection principle using overcurrent relaying, distribution network configurations and relay coordination to enhance reliability of power supply. The distribution network topology and its protection schemes are described in detail. The most simple, cheapest & high speed scheme of protections i.e. overcurrent and their limitations are reported. Rules for setting of relays operating time and pickup current setting under time graded, current graded and in combined time & current graded protection schemes for various network configurations are examined. A PSCAD/EMTDC software environment is used to simulate the radial distribution network and MATLAB software for analysis. The variation in fault current and fault MVA with change in fault position are simulated. Result indicates the time and current setting for relays under combined time and current protection scheme. Instant of fault clearance and relay trip signal reflects coordination of primary and back protection. by proper selecting the faulty zone in case of combine setting of both time and current of relays. In addition the classification of overcurrent relay as per ANSI/IEEE, IEC standards and the protection zones for radial, ring main and parallel network are inferred & the study engrossed on a small portion of distribution network consisting four subsection with load on each.