Selecting Transformers For Hydro Power Plants
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27-01-2009, 09:36 AM
Electrical energy is a major requirement for the economic and industrial growth of any country. Most of the electrical energy is generated from the combustion of fossil fuels (coal, oil, gas). The fossil fuels are not only limited in nature, but generating electricity with these fuels have serious environmental, economical and safety consequences.
Energy from the sun, water, wind biomass and the ocean is renewable, biomass clean and environment friends. India has sufficient potential to develop hydro-electric power, but the development rate of large, hydro power plants is still low because of high capital cost, long gestation period, increasing difficulties in implementation detoriating socioeconomic aspects and adverse environmental impacts.
On the other hand, small, hydro power plants not only overcome these draw-backs but also provide advantages like continuous power availability on demand, concentrated and predictable source, limited maintenance low running costs, long lasting and robust in construction. The small hydro power plant may be subdivided as micro (upto 100 KW), mini (101 KW to I MW) and small (1001 KW to 5 MW).
Transformer plays a very important role to transfer electric power from the generating station to the consuming centre. The generating site of consume centre. Generating site of hydro power plants are situated very far away from the consuming centre, necessitating very long transmission systems for which much higher transmission voltage are required to reduce the transmission cost as well as transmission loss. In India, the generating voltage is generally up to 11 KV.
The unit system for voltage step up is adopted universally m which each generator is directly connected to the unit step up transformer (called main transformer without any circuit breaker in between. A generator and its unit step up transformer form the unit. Each unit has its own turbine, control panel and auxiliaries. The unit auxiliaries are supplied through unit auxiliary transformer.
The station auxiliaries are supplied through another transformer called station service transformer. A general connection diagram of a unit step up transformer is shown in figure. 1. The unit system for voltage set up is available for large hydro power plant.
However, other ways for stepping up the voltage are by generator step up transformer, interbus transformer and outgoing feeder transformer. These methods are very much suitable for the small hydro power plants where several different rating generators are operating simultaneously. In generator step up transformer, each generator is connected to the HV bus through a step up transformer. Here the number of transformers used are equal to the number of generators.
In interbus transformer, the transformer is connected between two busbars-one at generating voltage and other at high level transmission voltage. In this case, one or more transformers may be connected in parallel between the LV and HV buses. Hence the number of transformers required are less than the total number of generators.
In outgoing feeder transformer, the transformer is connected between the generating voltage bus bar and directly to the transmission lines on the high voltage side. In this method, one HV bus and its switchgear has been saved, and the number of transformers required is less than the total number of generators connected. This method is economical and simpler over the above two types. The single line connection diagrams of these types are shown in figure.2(a), 2(b) and 2© respectively.
The sum of MVA rating of transformers must be equal to the sum of MVA rating of the generator /generators connected. The auxiliary transformers required for hydro power stations are about 15 per cent of the generator rating. They are therefore, universally of ON type with cooling tubes welded to the main tank.
However, such transformers should be located outdoors in open cells flanked by fire-barrier walls. It is sometimes convenient to minimize cable connections to generator unit transformers by locating them indoors.
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