Power Plant Capacity

In general selection of a power plant type and its capacity is made after forming the list of electrical devices. A power plant mode should be clarified at the project design stage.

Analysis of every electrical consumer, its capacity characteristics, i.e. start current, start capacity and cos(phi) should be fulfiled for all newly developed power plants.
This job must to be done for all industrial and remote oil/gas deposit sites.

Combined cycle power plant

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Operational mode can be either isolated or parallel with a grid.

EPC contractor of power generating equipment or project design company should coordinate operational mode with the owner of a power plant in advance. A category of reliability for all electrical devices must be also confirmed.

A category of electrical reliability for power provision gives information about acceptable temporary breaks in power supply.

These data will be used to determine a type of reserve generating equipment. For instance, they can help to install a reserve diesel generator set with an automatic power breaker of needed current.

Such diesel generator should supply all devices of the first category of reliability with electrical power. According to the Russian Electrical Code (PUE) all electrical devices of the second category will be switched off in case of emergency shutdown.

Many factors should be taken into account while solving the problem of power supply. The most important one is the total electrical consumption of a site.

The next factor is possibility of connection to the local electrical grid. Usually it is not possible to do it at remote sites such as oil/gas deposits. In most cases electrical lines are not available in such distant places of Russia.

In this case selection of a power generator set will depend on maximum total electrical consumption and maximum power of the biggest electrical device.

Start current and capacity of a big electrical motor without variable frequency drive is very important while designing a project of a power plant in isolated electrical systems. This starting power should be compensated by a gas engine or gas turbine generator set. Allowable immediate power increase in such isolated systems should not exceed 20 per cent.

When a single electrical consumer has a starting power more than 20 per cent of the rated generator capacity variable frequency drive is recommended for installation. VFD can help to avoid engine or turbine overload in isolated systems. Usage of VFD for big electrical motors increases reliability of the power plant operation in isolated mode.

Generally installation of variable frequency drive is a very expensive unit for big electrical motors. VFD can be applied at sites which are already in use and where connection to electrical grid is not possible.

When there is a capacity increase/decrease of more than 20 per cent in isolated mode, considerable frequency fluctuations appear. These fluctuations can be up to ±5 Hz. As a result there electrical devices can be switched off by their frequency control protection. Mechanical overload of a gas engine or gas turbine can also happen.

Manufacturers of high quality engine or gas turbine generator sets provide diagrams of allowable capacity increase for operation in isolated mode.

Diesel generators can accept immediate capacity increment which is much bigger than that for gas engines. Usually diesel generator sets are designed for power supply in isolated electrical systems. Immediate load increase of 50 per cent or more will not result in switching off generator breaker of a diesel genset.

Electrical systems of bigger scale can be equipped with reserve gas turbine generators and gas booster compressors. Such gas turbine sets are used when reservation with diesel generators is not possible.

Gas compressor boosters should have continuously operating oil lubrication pump for fast automatic start and gas supply to the reserve gas turbine. In this case start period of reserve gas booster can last about one minute.

Usually the problem of gas pipeline volume at the discharge line is not properly taken into account. When a gas turbine operates in isolated mode there is no option to reduce electrical load. If the volume of gas pipeline after gas booster is not sufficient it can be followed by emergency shutdown of a gas turbine. It may happen because of low inlet gas pressure which can result in turbine emergency shutdown.

Electrical capacity of a single device and gas discharge volume of gas pipeline are not so important when industrial or oil/gas deposit site is connected with electrical grid. There will be no big fluctuations of gas pressure before turbine or gas engine because they can work with unchangeable power value.

In parallel with a grid mode capacity of a power plant should be approximately equal to the basic capacity of the site.

Peak power will be compensated by electrical grid. Thus the power plant will always generate maximum possible capacity from its generator sets.

It will help to provide minimal possible payback period for installed equipment. Maximum saving will be reached due to difference in prices between the produced electricity and electricity bought from the grid. Surely lower price is supposed for the electricity produced at a site.

Selection of power generating equipment depends mostly on total electrical load of a site. For electrical load from 50 kW up to 10 MW it is reasonable to choose gas engine generator sets.

Usage of gas turbine sets with capacity less than 10 MW is not beneficial because of their low efficiency in comparison with gas engines. Gas engines with generated power of more than 1 MW have efficiency of around 38…40 per cent. 10 MW engines sometimes have efficiency around 48…50 per cent.

Usually capacity of the most gas engines does not exceed 10 MW. The more the power of a gas engine the bigger are the dimensions and weight. 1 MW gas engine genset can be installed into a standard prefabricated 40 feet container package (2.4 m width, 3.0 m height) with auxiliary equipment. This a typical power generation solution for the Russian market.

Gas engine genset with capacity 2 MW should be installed into a container of double width. It can not be transported to a customer site without disassembling. For instance the weight of 2 MW gas generator set Caterpillar CG170-20 itself is around 18 tons. The weight of the total generator set packaged into the container with auxiliary equipment and heat recovery system will be around 30 tons.

Such heavy gensets sometimes can not be transported to a site without disassembling because of road restrictions for transportation of heavy cargo. It is especially actual for remote oil/gas deposits.

It is necessary to take into consideration that repair and overhaul procedures are usually very expensive for gas engines.

When designing a project for power supply at remote sites such as oil/gas sites installation of gas turbine generator set can be more preferable. This is due to compact dimensions and low weight of gas turbines below 10 MW.
Sometimes they are the only possible technical solution for power supply of remote sites thanks to their compact size and low weight.

While operating with associated gas fuel derating for gas engines is applied. It helps to reduce the risk of detonation in engine cylinders. Derating can be up to 35 per cent for heavy hydrocarbon gases with low methane number.

Usage of associated oil gas continuously increases in the Russian oil/gas industry because of steady penalties rise for flare gas burning. The main requirement for operation of gensets with associated oil gas is the following: gas temperature should be about 10…15 °C higher than water and hydrocarbon dew point temperatures.

Generally gas turbines operate with associated oil gas more stably in comparison with gas engines. However they have derating as well.

While designing a project of a power plant it should be taken into consideration that gas engine gensets don’t need high inlet gas pressure. In most cases gas turbine gensets need inlet gas pressure at least 1,2 MPa. Exact value of pressure for gas turbine depends on its model and capacity.

That’s why gas turbines need gas booster compressors. For example screw compressor are used as the most inexpensive solution for fuel gas boosting. If an oil lubrication system is used for compressor, fuel gas in discharge line should have a rated value of residual oil content before gas turbine. Usually they can accept 0,5…1,0 ppm of residual oil content in fuel gas.

Gas turbines have a very wide power range from 4 MW up to 560 MW. Gas turbines have high efficiency around 38…40 per cent with capacity above 40 MW.

However, Russian turbine manufacturers didn’t have their own gas turbines for capacity of more than 25 MW by 2019. For this reason power plants can not be equipped with Russian gas turbines with capacity of more than 150 MW.

Power stations with both gas engines and gas turbines should be configured with cogeneration / heat recovery system. Heat recovery includes heat exchange equipment for combined production of thermal and electrical energy.

Cogeneration includes cooling circuits equipment of gas engines and turbines and recovery of heat from exhaust gases. Heat recovery helps to reduce considerably fuel expenses for generation of thermal energy.

If low pressure steam is needed at an industrial site waste heat boiler can be installed at an exhaust gas part of a gas turbine. In this case waste heat boiler can be used to heat up water for consequent evaporation. Gas engines are usually supplied not with steam but water waste heat boilers.

Steam turbines with high steam flow are used for big power plants. Usually they are equipped with steam turbine generators and waste heat utility sets. Such big power plants require only upgrade of the existing steam turbines. Efficiency of a large steam turbine set can be around 40 per cent. Its capacity can be up to 1800 MW.

When a power plant has capacity of more than 150 MW it is reasonable to use combined gas-steam turbine cycle. Maximum capacity of the steam-gas cycle can be up to 700 MW for a single block. Its efficiency may reach 55 per cent.