Overcurrent protection co-ordination: a modern approach for modern devices

One of the most demanding tasks in applying power system protection is to choose suitable settings for primary or backup overcurrent relays. These must offer adequate sensitivity, acceptable fault clearance times and discrimination of protective relays at various locations and under variable system operating conditions. The relay co-ordination task is to make relays remote from the location of a fault operate more slowly than those which are required to trip. The difference in relay operating times is the co-ordination or grading margin. This paper briefly reviews established approaches to relay co-ordination and it offers a revised approach to address problems faced by many power system protection engineers today, especially when dealing with different relay designs. The approach is well suited to the use of spreadsheet methods, which enables it to be used very easily