Diesel_generator

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Diesel generator

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. Sizes up to about 5 MW are used for small power stations and these may use from one to 20 units. These generators 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 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. 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 suppply, compared to capacity for manufacture of electrical equipment. 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)

Power plants - electrical "Island" mode

Typically diesel generators operate as and electrical island, ie not connected to any public grid referred to as "island mode" or in "island". If more than one, then several diesel generators can be thus operated together (in parallel). The use of parallel running generators provides the advantages of more capacity, efficiency and redundancy. An island 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 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 ECU (Engine Control Module). Typical manufacturers are Woodward and Heinzman who dominate this market

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 fuel control 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 load when the fuel supply to its combustion system is increased, while load is released if fuel supply is decreased.

Supporting main utility grids

Emergency standby diesel generators such as those used in hospitals, water plant etc, are, as a secondary function, widely used in the US and the UK to support the respective national grids at times for a variety of reasons. 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. This usually occurs during say the sudden loss of a large conventional plant of say 660 MW, or a sudden unexpected rise in power demand eroding the normal spinning reserve available.

This is extremely 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.

In this way the UK National Grid can call on about 2 GW of plant which is up and running in parallel as quickly as two minutes in some cases. 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 diesels are very expensive in fuel terms, they are only used a few hundred hours per year in this duty, 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. See Control of the National Grid (UK), National Grid (UK) reserve service ,

A similar system operates in France known as EJP, where at times of grid extremis special tariffs can mobilize at leas 5 Gw of diesels to become available.In this case, the diesels prime function is to feed power into the grid.

Typical operating costs

Fuel consumption is the major portion of diesel plant owning and operating cost for power applications, whereas capital cost is the primary concern for backup generators. Specific consumption varies, but a modern diesel plant will consume between 0.28 and 0.4 litres of fuel per kilowatt hour at the generator terminals.

Fuel cost 0.18-0.26p/kWh (using farm diesel at 0.65p/litre)
lifetime engine maintenance about is 0.5p/kWh - 1.0/kWh

Typical costs of conversion to paralleling for grid operation

To be able to operate in parallel with the mains certain modifications are necessary which include the following:

Approx. £3k to fit a 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

This capital cost of £13/kW - £260/kW is low compared to combined cycle gas turbines that cost £350/kW.

Peak Shaving and Triads

Maximum demand tariffs in many areas encourage the use of diesels to come on either at times of peak tariff charges, or maximum demand. This is also known as peak shaving. So called Triad demand National Grid (UK)charges in the UK also encourage private generators to start, but here the payment effectively comes from National Grid seeking to limit its maximum demand.

Generator Sizing and Ratings

Rating

Generators must be capable of delivering the power required for the hours per year anticipated by the designer to allow reliable operation and prevent damage. Typically a given set can deliver more power for fewer hours per year, or less power continuously. That is a standby set is only expected to give its peak output for a few hours per hear, whereas a continuously running set, would be expected to give a somewhat lower output, but literally continuously, and both to have reasonable maintenance and reliability.

To meet the above criteria manufactures give each set a rating based on internationally agreed definitions.

These standard rating definitions are designed to allow correct machine selection and valid comparisons between manufacturers to prevent them misstating the performance of their machines. and to guide designers:

Generator Rating Definitions

Standby Rating based on Applicable for supplying emergency power for the duration of normal power interruption. No sustained overload capability is available for this rating. (Equivalent to Fuel Stop Power in accordance with ISO3046, AS2789, DIN6271 and BS5514). Nominally rated.

Typical application - emergency power plant in hospitals, offices, factories etc. Not connected to grid.

Prime (Unlimited Running Time) Rating based on: Applicable for supplying power in lieu of commercially purchased power. Prime power is the maximum power available at a variable load for an unlimited number of hours. A 10% overload capability is available for limited time. (Equivalent to Prime Power in accordance with ISO8528 and Overload Power in accordance with ISO3046, AS2789, DIN6271, and BS5514). This rating is not applicable to all generator set models.

Typical application - where the generator is the sole source of power for say a remote mining or construction site, fairground, festival etc.

Base Load (Continuous) Rating based on: Applicable for supplying power continuously to a constant load up to the full output rating for unlimited hours. No sustained overload capability is available for this rating. Consult authorized distributor for rating. (Equivalent to Continuous Power in accordance with ISO8528, ISO3046, AS2789, DIN6271, and BS5514). This rating is not applicable to all generator set models

Typical application - a generator running a continuous unvarying load, or parallelled with the mains and continuously feeding power at the maximum permissible level 8760 hours per year. This also applies to sets used for peak shaving /grid support even though this may only occur for say 200 hour per year.

As an example if in a particular set the Standby Rating were 1000 kW, then a Prime Power rating might be 850 kW, and the Continuous Rating 800kW. However these ratings vary according to manufacturer and should be taken from the manufacturer's data sheet.

Often a set might be given all three ratings stamped on the data plate, but sometimes it may have only a standby rating, or only a prime rating.

Sizing

Typically however it is the size of the maximum load that has to be connected and the acceptable maximum voltage drop which determines the set size, not the ratings themselves. If the set is required to start motors, then the set will have to be at least 3 times the largest motor, which is normally started first. This means it will be unlikely to operate at anywhere near the ratings of the chosen set.

Manufactures have sophisticated software that enables the correct choice of set for any given load combination.

Correct Generator Installation

To ensure correct functioning, reliability and low maintenance costs generators must be installed correctly. To this end manufacturers provide detailed installation guidelines covering such things as:

*Sizing and selection

*Electrical factors

*Cooling

*Ventilation

*Fuel storage

*Noise

*Exhaust

*Starting systems

These are frequently ignored causing problems for users

Engine damage due to mis-application or mis use

Diesel engines can suffer damage under certain conditions as a result of mis-application or mis use - namely internal glazing and carbon buildup. This is a common problem in generator sets but is caused by failure to follow application and operating guidelines. Ideally diesel engines should run at least around 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Internal glazing and carbon buildup is 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, (mis use)if the engine powering the set is over-powered (mis application) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (mis use).

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. which are there partly to hold oil and return it to the crankcase via the scraper ring. 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. The problem is increases further by the fact that a lightly-loaded diesel engine may never reach its designed operating temperature. This can cause carbon build up from poor combustion, oil dilution and the formation of acids in the engine oil caused by condensed water and combustion by-products which would 'boil off' at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces. 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.

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.

For example, a diesel generator set, normally running in island, and powering the lighting circuit of a building may be designed to be able to cope with the load of every light in the building being on. However, this situation may rarely occur, so for the vast majority of its operating life the diesel engine in the set may not be heavily loaded (maybe as little as 10% of the maximum load).

The situation can be prevented by carefully selecting the generator set in accordance with manufacturers printed guidelines, and possibly in this case having 2 (perhaps one being markedly smaller than the other) or more to ensure that any running set is never run for long periods under loaded; by routine testing at full load with a hired in load bank; or permanently installing a load bank.

For emergency only sets, which are islanded, the problem is that the emergency load is often only about 1/4 of the sets standby rating, for reasons due to starting loads and minimising starting voltage drop. Hence the available load is not usually enough for load testing and again engine damage will result if this us used as the weekly or monthly load test, albeit not as severe or rapid as entirely off load running.

Often the best solution in these cases will be to convert the set to parallel running and feed power into the grid, if available, once a month on load test, thereby gaining revenue from the fuel burnt.