A diesel generator of 500kVA in a tourist resort in Egypt.

A diesel generator is the combination of a diesel engine with an electrical generator (often called an alternator) to generate electric energy.

Diesel generators are used in places without connection to the power grid or as emergency power-supply if the grid fails. Small portable diesel generators range from about 1kVA to 10kVA, while the larger industrial generators can range from 8kVA - 30kVA for homes, small shops & offices up to 2000kVA used for large office complexes, factories and power stations. These generators are widely used not only for emergency power, but also many have a secondary function for providing back up power to utility grids.

Ships often also employ diesel generators, sometimes not only to provide energy for electric systems, but also for propulsion. The use of diesel generators for propulsion is actually becoming more common because in this arrangement the generators do not need to be close to the propeller and instead they can be placed in better positions, usually allowing more cargo to be carried. Such a diesel-electric arrangement is also used in some very large land vehicles.

Power generators are selected based on the load they are intended to supply power for, and that load's "mission critical" needs (e.g. a hospital needs to have 100% redundancy and up-time, a backyard standby unit to keep a hot tub warm isn't nearly as critical)

Power plants Edit

Welder - hollowell 2011 - Picture 1024

A Typical older portable generator / welder unit as used on construction sites and in shipyards

Diesel generators can be operated together (in parallel). The use of parallel running generators provides the advantages of more capacity, efficiency and redundancy. A power plant driven by diesel generators will typically include between three and six machines.

Generators can be connected together through the process of synchronization. Synchronization involves matching voltage, frequency and phase before connecting the generator to a live busbar. Failure to synchronize before connection could cause a high current short-circuit or wear and tear on the generator and/or its switch gear. The synchronization process can be done automatically by an auto-synchronizer module. The auto-synchronizer will read the voltage, frequency and phase parameters from the generator and busbar voltages, while regulating the speed through the engine governor or ECU (Engine Control Module).

Load can be shared among parallel running generators through load sharing. Like auto-synchronization, load sharing can be automated by using a load sharing module. The load sharing module will measure the load and frequency at the generator, while it constantly adjusts the engine speed to shift load to and from the remaining power sources. As the prime mover of a diesel generator runs at constant speed, it will take more active load when the fuel supply to its combustion system is increased, while load is released if fuel supply is decreased.

Supporting main utility grids Edit

Emergency standby diesel generators in for example hospitals, water plant etc, are widely used in the US and the UK to support the respective national grids at peak times. In the UK for example, some 2 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 200kW to 2 MW. Control of the National Grid (UK)

This is extremely beneficial for both parties - the diesels have already been paid for other reasons, but to be reliable need to be full 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 in as short a time as 2 minutes in some cases. This is far quicker than a base load power station which can take 12 hours from cold. Whilst diesels are very expensive in fuel terms, they are only used a few hundreds of hours per year, 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.

Engine Damage Edit

Diesel engines can suffer damage under certain conditions that are sometimes encountered when used in a generating set- namely internal glazing and carbon buildup due to prolonged periods of running at low speeds and/or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, if the engine powering the set is over-powered for the load applied to it by the alternator, or if the generator set's maximum output is far in excess of the normal loads placed on it, causing the diesel unit to be under-loaded. This is a common problem in generator sets. For example, a diesel generator set powering the lighting circuit of a building would be designed to be able to cope with the load of every light in the building being on. However, this situation rarely occurs, so for the vast majority of its operating life the diesel engine in the set will not be heavily loaded (maybe as little as 10% of the maximum load). Ideally diesel engines should run at least around 60-75% of their maximum rated load, and at around 75% of their maximum speed (although the phasing requirements of engines in generator sets can make these speeds hard to achieve).

Ratings Edit

There are internationally agreed definitions of the rating levels for diesel engines.

  • Standby - Output available with varying load for the duration of the normal source of electrical supply. In essence it is the "prime overload" condition with no time limit for an engine which is normally not operated.
  • Prime - Output available with varying load between 25% and 100% of the rating for an unlimited time. The unit can be overloaded to 110% of the rating for one hour in twelve.
  • Continuous - Output available without varying the load for an unlimited time.

If the standby rating were 1000 kW, then a Prime Power rating might be 850 kW, and the continuous rating 800kW.

Wessex Water sets are sized initially on the standby rating for emergency use, but are run on Load Management at the Continuous rating level which is about 80% of the standby rating.

Typical operating costs Edit

  • Approx. £3k to fit the PLC to the set
  • Paralleling and synchronising gear and G59 equipment (this allows grid connection) Approx £5k
  • Tidying up set (noise, larger fuel tank) Approx another £5k
  • So for a 1MW set…£13/kW
  • 50 kW…maybe £260/kW
  • Running costs - fuel 10p/kWh
  • maintenance about 0.5p/kWh

This is very cheap capacity considering power stations are about £350/KW) for a CCGT. A diesel set itself is about 150/kW fully installed and connected.

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