Survey of Rate of Change of Frequency Relays and Voltage Phase Shift Relays for Loss of Mains Protection

In the UK, private generators that operate in parallel with the electricity supply from a Regional Electricity Company (REC) generally referred to as embedded generators, must fit loss of mains (LOM) protection if their plant has a rating of more than 150 kw, as required by engineering recommendation G59/1.

The application of LOM protection has raised a lot of debate within the industry, mainly centred around "best" settings for LOM relays, but also on nuisance trips.

This report considers why these relays are required.

It discusses in particular how a loss of mains condition which leads to "islanding" has adverse safety and operational implications for both the generator user and the REC supply system within which the generator is embedded.

It surveys some of the more popular relays currently on the market and their performance under loss of mains condition and network faults, and provides a technical understanding of their method of operation and an appreciaton of the factors that influence their performance.

All the LOM relays surveyed are either of the rate of change of frequency (ROCOF) type or voltage vector shift type. A number of different techniques are used in the implementation of each relay type, and the most common are highlighted and discussed on a comparative basis.