Diesel generator
From Wikipedia, the free encyclopedia
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.
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 due to the fact that 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)
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[edit] Using generators for supporting main 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 Natnal 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 for other reasons, but to be reliable need to be full load tested. Grid paralleling is a convenient way of doing this.
[edit] Testing Diesels off Load Damages them
Diesels must be run regularly at least once a month and preferably once a week to ensure they will work when called on unexpectedly in an emergency power failure, which is their prime function. Failure to regularly run diesel generators means they are very unlikely to start in an emergency usually due to a variety of simple failures – most commonly flat batteries, contaminated fuel, or corroded contacts. Even if they do start they are likely to stop after a short while – usually overheating due to failures of the cooling systems.
(A recent survey by another water company indicated that whereas its Load Management generators had near to 100% reliability for emergency use, out of 30 non LM sets, 11 would not be able to operate in an emergency)
There is a general tendency to assume that it is preferable to run diesels off load, it somehow being assumed that this is less harmful than wearing them out by running them at full load. However this ignores the fact that diesel generators are designed to run at their stated rating (see later).
In fact testing diesels off load is extremely harmful and can very quickly ruin an engine – in as little as only 50 hours of accumulated running. This is because under-loading causes a series of interlocking damaging sequence of events.
[edit] Engine Damage Due To Under Load Running
Initially it means 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 yet further causing a further drop in sealing efficiency and exacerbating the initial low pressure.
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.
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. (Harry Maurer – personal communication – ex Thames Energy Manager)
[edit] Diesel generator Ratings
There are internationally agreed definitions of the rating levels for diesel engines.
- Standby - short term use only for 10s of hours per year ie an emergency generator at a maximum but not continuous 100% of the standby rating.
- Prime Power - where the generator is meeting the sole power for an off grid site such as a mining camp or construction site and continuously varying.
- Continuous – output which can be maintained 8760 hours per year.
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] Testing diesels – good practice - and how paralleling with the utility can help
A diesel engine can be tested on full load by connecting it to a load bank but this usually means hiring in a load bank and the specialist to physically connect it which is an expensive operation.
Alternatively a dedicated load bank is sometimes provided but this itself has a cost and obviously is merely a fuel waster.
The generator could of course be used to run the emergency load to which it is connected, but this usually means an undesirable break in supply unless short term paralleling devices are fitted. Generally the load connected to a generator is found to be only about 1/3 of its maximum standby rating so this can lead to long term problems as well, though not nearly as bad as no load running.
It is often found that major defects are pre-emptively identified by Load Management runs – for example, in a recent case at the Weymouth head works site, the generator caught fire due to a failed turbo oil seal – this would have occurred sooner or later but it was greatly to Wessex Water’s advantage that the failure occurred during a Load Management run and not during an emergency run, and was therefore able to be repaired before the next real power failure.
So Load Management by paralleling the the utility is the ideal way to prove diesels without destroying them because it gives a readily available full load test against and which earns income rather than merely wasting fuel.
[edit] Typical conversion and operating Costs for a diesel generator
- 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] How Many Diesels Are There in the UK?
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
Forest City have been in production of such machines for over 100 years dating back to 1902 [[1]]
