Grid support

Emergency standby diesel generators such as those used in hospitals, water plant etc, are, as a secondary function, widely used in the US and the UK to support the respective national grids at times for a variety of reasons. In the UK for example, some 0.5 GWe of diesels are routinely used to support the National Grid, whose peak load is about 60 GW. These are sets in the size range 200 kW to 2 MW. This usually occurs during say the sudden loss of a large conventional plant of say 660 MW, or a sudden unexpected rise in power demand eroding the normal spinning reserve available.

This is extremely beneficial for both parties - the diesels have already been purchased for other reasons; but to be reliable need to be fully load tested. Grid paralleling is a convenient way of doing this.

In this way the UK National Grid can call on about 2 GW of plant which is up and running in parallel as quickly as two minutes in some cases. This is far quicker than a base load power station which can take 12 hours from cold, and faster than a gas turbine, which can take several minutes. Whilst diesels are very expensive in fuel terms, they are only used a few hundred hours per year in this duty, and their availability can prevent the need for base load station running inefficiently at part load continuously. The diesel fuel used is fuel that would have been used in testing anyway. See Control of the National Grid, National Grid Reserve Service[2][3]

A similar system operates in France known as EJP, where at times of grid extremis special tariffs can mobilize at least 5 GW of diesel generating sets to become available.In this case, the diesels prime function is to feed power into the grid