Diesel generator

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

Contents

[edit] Power plants

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.

[edit] Supporting main utility grids

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.



[edit] Engine Damage

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).

Glazing occurs due to low combustion temperatures and pressures in the engine cylinder. When an engine is loaded correctly, the load resists the movements of the crankshaft and piston during combustion. This causes the combustion pressure to rise as the volume of the cylinder cannot increase directly in line with the increase in pressure during combustion.

Running an engine under low loads low cylinder pressures and consequent poor piston ring sealing – these rely on the gas pressure to force them against the oil film on the bores to form the seal. Low initial pressure causes poor combustion and resultant low combustion pressures and temperatures.

This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings. This causes a further drop in sealing efficiency and exacerbates the initial low pressure. Glazing occurs when hot combustion gases blow past the poorly-sealing piston rings, causing the lubricating oil on the cylinder walls to 'flash burn', creating an enamel-like glaze which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface.

Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks maintains the piston seal and pressures.

Un-burnt fuel leaks past the piston rings and contaminates the lubricating oil. At the same time the injectors are being clogged with soot, causing further deterioration in combustion and black smoking.

This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required.

Under loaded running inevitably causes not only white smoke from unburnt fuel due to the engines failure to heat up rapidly, but over time as the engine is destroyed it is joined by the blue smoke of burnt lubricating oil leaking past the damaged piston rings, and the black smoke caused by the damaged injectors. This pollution is unacceptable to the authorities and any neighbours.

In fact at the diesels at Weymouth’s Radipole pumping station, before Wessex Water took them over and before they were converted to load management, on more than one occasion the fire brigade were erroneously called out to a supposed fire as they suffered just such an eventuality. The thick white smoke was routinely reported as a traffic hazard. Now, however, whilst housing estates have subsequently been built close up to the station there are no complaints. With a fully loaded diesel there is only a very short puff of white smoke which rapidly disappears once the diesels warm up in a matter of seconds.

Once glazing or carbon build up has occurred, it can generally only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If detected in the early stages, running an engine at maximum load and a high throttle setting for a long period, to raise the internal pressures and temperatures, might allow the piston rings to expand and scrape glaze off the bores and allow carbon buildup to be burnt off. However, if glazing has progressed to the stage where the piston rings have seized into their grooves due to glaze, this will not have any effect.

In the 1987 storms, 50% of Thames Water generators failed to start or stay running due to the failure to undertake these regular full load runs.[1]

[edit] Ratings

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.

[edit] Typical operating costs

  • 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.

[edit] Popularity

Wessex Water has 550 generators of capacity 110MW in the range 50 kWe to 1.2 MWe. Presently, it only uses 32 generators of the larger sets (greater than 250 kW) with 18MW total capacity for Load Management / Triads / Reserve Service. Many of the other smaller sets have started to be converted also.

According to EA Technology nationally there are up to 20 GW of emergency diesels. With the right financial incentives and explanations of the benefits large numbers of these could be brought into the Reserve Service type of scheme. Over 20 years this practice and associated technology will probably become standard.

[edit] See also

[edit] External links

[edit] References

  1. ^ Harry Maurer – personal communication – ex Thames Energy Manager