Diesel generator
Supporting main utility grids
Supporting main utility grids
-----------------------------

In addition to their well\-known role as power supplies during power failures, diesel generator sets also routinely support main power grids worldwide in two distinct ways:

### Grid support

Emergency standby diesel generators, such as those used in hospitals and water plants, are, as a secondary function, widely used in the US and, in the recent past, in Great Britain to support the respective national grids at times for a variety of reasons. In the UK, the tenders known as the [Short Term Operating Reserve](/wiki/National_Grid_Reserve_Service "National Grid Reserve Service") have exhibited quite variable prices, and from 2012 on, the volume of demand\-side participation, which mainly entails the use of on\-site diesel, has dropped as the tendered prices fell. Some 0\.5\-GWe of diesel have at times been used to support the [National Grid](/wiki/Control_of_the_National_Grid "Control of the National Grid"), whose peak load is about 60 GW. These are sets in the size range of 200\-kW to 2 MW. This usually occurs during, for example, the sudden loss of a large conventional 660\-MW plant, or a sudden unexpected rise in power demand eroding the normal spinning reserve available.

This is 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. This method of operation is normally undertaken by a third\-party aggregator who manages the operation of the generators and the interaction with the system operator.

These diesels can in some cases be up and running in parallel as quickly as two minutes, with no impact on the site (the office or factory need not shut down). 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 diesel is very expensive in fuel terms, they are only used a few hundred hours per year in this duty, and its availability can prevent the need for a base load station running inefficiently at partial load continuously. The diesel fuel used is the fuel that would have been used in testing anyway.

In Great Britain, National Grid can generally rely upon about 2 GW of customer demand reduction via backup diesel being self\-dispatched for about 10 to 40 hours a year at times of expected peak national demand. National Grid does not control these diesels \- they are run by the customer to avoid "triad" transmission network use of system (TNUoS) charges, which are levied only on consumption of each site, at the three half\-hours of peak national demand. It is not known in advance when the three half\-hours of peak national demand (the "triad" periods) will be, so the customer must run his diesel for a good deal more half\-hours a year than just three.

The total capacity of reliably operable standby generation in Britain is estimated to be around 20 GW, nearly all of which is driven by diesel engines. This is equivalent to nearly 29% of the British system peak, although only a very small fraction will ever be generated at the same time. Most plants are for large office blocks, hospitals, supermarkets, and various installations where continuous power is important such as airports. Therefore, most are in urban areas, particularly city and commercial centers. It is estimated that around 10% of the plant exceeds 1\-MW, about 50% is in the 200\-kW\-1\-MW range, and the remaining 40% is sub\-200\-kW. Although it is growing, only a very small proportion is believed to be used regularly for peak lopping, the vast majority just being only for standby generation. The information in this paragraph is sourced from section 6\.9 of the government report: "Overcoming Barriers To Scheduling Embedded Generation to Support Distribution Networks"

Increasing use of banks of diesel generators (known as "diesel farms") is being made in Britain to balance the fluctuating output from renewable energy sources, such as [wind farms](/wiki/Wind_farms "Wind farms").

A similar system to Great Britain's Short\-Term Operating Reserve operates in France. It is known as EJP; at times of grid stress, special tariffs can mobilize at least 5 GW of diesel\-generating sets to become available. In this case, the diesel's prime function is to feed power into the grid.

During normal operation in synchronization with the electricity net, powerplants are governed with a five percent [droop speed control](/wiki/Droop_speed_control "Droop speed control"). This means the full load speed is 100% and the no\-load speed is 105%. This is required for the stable operation of the net without hunting and dropouts of power plants. Normally the speed changes are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a [centrifugal governor](/wiki/Centrifugal_governor "Centrifugal governor"). Generally, this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication.Speed Droop and Power Generation. Application Note 01302\. 2\. Woodward. Speed


What are the benefits for both parties when diesels are used to support national grids?