Electric Motors and Generators Menu
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
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. 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.