Diesel Generator Review

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Diesel Generator Review and Operational Characteristics

A diesel generator is the combination of a diesel engine with an electrical generator (often called an alternator) to generate electric energy. Diesel generating sets 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 1 kVA to 10 kVA may be used as power supplies on construction sites, or as auxiliary power for vehicles such as mobile homes.

The packaged combination of a diesel engine, a generator and various ancillary devices (such as base, canopy, sound attenuation, control systems, circuit breakers, jacket water heaters and starting system) is referred to as a generating set or a genset for short.

Set sizes range from 8 to 30 kW (also 8 to 30 kVA single phase) for homes, small shops & offices with the larger industrial generators from 8kW (11 kVA) up to 2,000 kW (2500 kVA three phase) used for large office complexes, factories. A 2,000 kW set can be housed in a 40 ft ISO container with fuel tank, controls, power distribution equipment and all other equipment needed to operate as a standalong power station or as a standby backup to grid power. These units, referred to as power modules are gensets on large triple axle trailers weighing 85,000 lbs or more. Combination of these modules are used for small power stations and these may use from one to 20 units per power section and these sections can be combined to involve 100's of power modules. In these larger sizes the power module (engine and generator) are brought to site on trailers separately and are connected together with large cables and a control cable to form a complete sychronized power plant.

Diesel generators, sometimes as small as 200 kW (250 kVA) are widely used not only for emergency power, but also many have a secondary function of feeding power to utility grids either during peak periods, or periods when there is a shortage of large power generators.

Ships often also employ diesel generators, sometimes not only to provide auxiliary power for lights, fans, and winches, etc. but also indirectly for main propulsion. With electric propulsion the generators can be placed in a convenient position, to allow more cargo to be carried. Electric drives for ships were developed prior to WW I. Electric drives were specified in many warships built during WW II because manufacturing capacity for large reduction gears was in short supply, compared to capacity for manufacture of electrical equipment.[1] Such a diesel-electric arrangement is also used in some very large land vehicles.

Generating sets are selected based on the load they are intended to supply power for, taking into account the type of load, i.e. emergency or for continuous power and the size of the load, and the size of any motors to be started which is normally the critical parameter. Factors such as maximum power, prime power rating and load combinations must be considered when selecting generator sizes. Environmental conditions such as altitude, temperature and emissions regulations must be taken into account as well. Often there is a least one complete spare generator in a set of generators to allow for maintenance and to have a spare unit in the event that a breakdown occurs.

One or more diesel generators operating without a connection to an electrical grid are referred to as operating in "Island Mode". In island mode, several parallel generators provide the advantages of redundancy and better efficiency at partial loads. The plant brings generator sets online and takes them off line depending on the demands of the system at a given time. An islanded power plant intended for primary power source of an isolated community ("Prime Power") will often have at least three diesel generators, any two of which are rated to carry the required load. Groups of up to 20 are not uncommon.

Generators can be electrically connected together through the process of synchronization. Synchronization involves matching voltage, frequency and phase before connecting the generator to a live bus-bar. Failure to synchronize before connection could cause a high current short-circuit or wear and tear on the generator and/or its switchgear. 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 bus-bar voltages, while regulating the speed through the engine governor or ECM (Engine Control Module). Typical manufacturers are DSE, ComAp, GAC, DEIF, Woodward and Heinzman, who dominate this market

Load can be shared among parallel running generators through load sharing. Load sharing can be achieved by using [droop speed control] controlled by the frequency at the generator, while it constantly adjusts the engine fuel control to shift load to and from the remaining power sources. A diesel generator will take more load when the fuel supply to its combustion system is increased, while load is released if fuel supply is decreased. 

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