Abstract

Overcurrent relays are the most economical protective relays that are used as main/backup protection in distribution networks and as backup protection in transmission and sub-transmission networks. The most effective overcurrent relays have inverse time-current characteristics such as Standard (Normal) Inverse (SI) characteristic, Very Inverse (VI) characteristic and Extremely Inverse (EI) characteristic. Lately, the use of numerical protective relays is considered due to their programmable functionality, greater flexibility, higher speed and accuracy. Numerical overcurrent relays usually allow the users to define and setup arbitrary time-current characteristics. This paper aims to develop a new time-current characteristic to be used in numerical overcurrent relays to overcome some drawbacks of the conventional characteristics. This characteristic curve is constructed by connecting successive straight lines and can easily be tabulated to write to the numerical relays.