Diesel generator
A diesel generator is the combination of a diesel engine with an electric generator to generate electrical energy. This is a specific case of an engine generator. A diesel compression-ignition engine is usually designed to run on diesel fuel, but some types are adapted for other liquid fuels or natural gas.
Diesel generating sets are used in places without connection to a power grid or as an emergency power supply if the grid fails, as well as for more complex applications such as peak-lopping, grid support, and export to the power grid.
Diesel generator size is crucial to minimize low load or power shortages. Sizing is complicated by the characteristics of modern electronics, specifically non-linear loads. Its size ranges around 50 MW and above, an open cycle gas turbine is more efficient at full load than an array of diesel engines, and far more compact, with comparable capital costs; but for regular part-loading, even at these power levels, diesel arrays are sometimes preferred to open cycle gas turbines, due to their superior efficiencies.
Diesel generator set
The packaged combination of a diesel engine, a generator, and various auxiliary devices is referred to as a "generating set" or a "genset" for short.Set sizes range from 8 to 30-kW for homes, small shops, and offices, with the larger industrial generators from 8-kW up to 2,000-kW used for office complexes, factories, and other industrial facilities. A 2,000-kW set can be housed in a ISO container with a fuel tank, controls, power distribution equipment, and all other equipment needed to operate as a standalone 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 or more.
A combination of these modules is used for small power stations, and these may use from one to 20 units per power section, these sections can be combined to involve hundreds of power modules. In these larger sizes, the power modules are brought to the site on trailers separately and are connected with large cables and a control cable to form a complete synchronized power plant.
Several options also exist to tailor specific needs, including control panels for AutoStart and mains paralleling, acoustic canopies for fixed or mobile applications, ventilation equipment, fuel supply systems, exhaust systems, etc.
Diesel generators are not only for emergency power but may also 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. In the UK, this program is run by the national grid and is called STOR.
Ships often also employ diesel generators, sometimes not only to provide auxiliary power for lights, fans, 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 before World War I. Electric drives were specified in many warships built during World War II because manufacturing capacity for large reduction gears was in short supply, compared to the capacity for the manufacture of electrical equipment. Such a diesel-electric arrangement is also used in some very large land vehicles, such as railroad locomotives.
Generator size
Generating sets are selected based on the electrical load they are intended to supply, the electrical load's characteristics, such as kW, kVA, var, harmonic content, surge currents, and non-linear loads. The expected duty, as well as environmental conditions, must also be considered.Most of the larger generator set manufacturers offer software that will perform the complicated sizing calculations by simply inputting site conditions and connected electrical load characteristics.
Power plants – electrical "island" mode
One or more diesel generators operating without a connection to an electrical grid are referred to as operating in island mode. Operating generators in parallel provides the advantage of redundancy and can provide better efficiency at partial loads. The plant brings generator sets online and takes them offline depending on the demands of the system at a given time. An islanded power plant intended for a primary power source of an isolated community 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 through the process of synchronization. Synchronization involves matching voltage, frequency, and phase before connecting the generator to the system. Failure to synchronize before a connection could cause a high short circuit current or wear and tear on the generator or its switchgear. The synchronization process can be done automatically by an auto-synchronizer module, or manually by the instructed operator. 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 ECM.
The 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 the load is released if the fuel supply is decreased.
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 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-GW of diesel have at times been used to support 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. 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 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 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 is being made in Britain to balance the fluctuating output from renewable energy sources, such as 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. 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. 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.