Evaluation of Rocof Relay Performances on Networks with Distributed Generation

Due to the increasing levels of distributed generation in the distribution network, the correct operation of loss-of mains protection is of utmost importance. Utilities have voiced their concern relating to the false operation of ROCOF relays, which are one of the most commonly employed loss-of-mains detection method. Whilst ´standard settings´ are used, the performances of commercially available ROCOF relays are reported to vary considerably. This paper presents an investigation on the characteristic of ROCOF relays that contribute to their diverse operating responses. The factors investigated are frequency measuring techniques, measuring windows, time delays and the under-voltage interlock function. With the increasing penetration of DG into the network, it is becoming common to have multiple DG units connected to the same network location. Two generators connected at similar location and employing ROCOF relays with the same setting but different characteristics were simulated. When subjected to the same network disturbances, the possible interference between the two relays was analyzed.