Introduction: Studies have been carried out to assess the capabilities of loss of mains relays to withstand system disturbances. Whilst this is an important characteristic to maintain generation as systems move increasingly towards active networks the prime consideration in determining a suitable setting must be safety and compliance with regulations.

The stability setting requirements to ride through anticipated system disturbances may form the minimum desired setting. Previous work carried out on testing the stability of relays to genuine network disturbances, show that there is a wide variation in the response of relays from different manufacturers to the disturbances. The results also show that relays from the same manufacturers have different responses at different settings.

Objectives: It is equally important to understand how sensitive a loss of mains relay is to a genuine loss of mains.

• How many cycles are required to detect the condition i.e. how many cycles does the relay need to sample before it can detect a loss of mains?

• What percentage change or mismatch of load compared to generator rating is required for the relay to detect a loss of mains? This can vary with construction and size of generator?