In selecting the power transformer for large capacities for power station, the factors which are to be considered in deciding the suitabiity of particular type of the transformer for particular application are initial cost, losses in transformers, methods of cooling, insulation employed, percentage of impedance voltage, regulation of the voltage, floor area, weight of the transformer, continuity of the service and load factors
The system engineers has to decide the emphasis to be given to each of the above points depending on particular application
In power plants major transformers employed are:
Generator transformer: Which is used for step up the generation voltage for transmission.
Unit Auxiliary Transformers: Which is connected to plant generator is employed for supplying power to plant auxiliary loads
Station Transformers: Which is connected to switchyard bus is employed to supply power to the plant auxiliary loads during plant start up or when the generator is not in operation.
Auxiliary Transformers: Connected between the plant High voltage buses and Low voltage buses is employed to supply power to low voltage auxiliary loads of the power plant
Factors consider for Selecting Transformers:
The operating characteristics and the capital cost should be considered while selecting the transformer in power plant.
Generator Transformer Selection:
Rating of Generator transformer should be almost equal to the rating of the generating unit (generator rating). If the capacity of the generating unit increases, the rating of the generator transformer should also be increased. Therefore for higher capacity generating units (above 500MW), a choice of employing a bank of three single phase transformers connected in delta connection (each of rating about 150MVA for 500MW generator) or 3phase transformer is possible. Both the options have their advantages and disadvantages. The following points should be considered while deciding the choice between three single phase transformers or 3-phase transformer
Advantages of three single phase generator transformer bank:
- Transportation of 3-phase single transformer is difficult because of the larger in size and requires more clearances.
- In the case of single phase transformer banks, one spare transformer can be made possible with lesser cost. Therefore if any single transformer in the transformer bank fails, spare transformer can be used. This improves the availability and reliability of the power transmission capability. In the case of 3 winding single transformer failure of transformer can leads to power delivery halt.
- It is possible to connect three single phase transformers in delta -delta connection with full capacity and in open delta on both the sides for a smaller loads
- In case of the fault in one transformer, two single phase transformers can be used to supply the load
- Single phase transformer occupies less space and weights less than the corresponding 3 phase transformer. This a factor to be considered particularly for underground power station as the space carries lot of importance and handling of heavy weights would be difficult.
The losses in the transformer are also important criteria for their selection. A 3-phase transformer is preferable compared to a bank of three single phase transformers as the core loss is 20% less in 3-phase transformer
The impedance of the transformer is another consideration for the selection as it plays important role in the operation. The transformer must have some percentage impedance voltage for their efficient working when operation in parallel. The impedance of the transformer increases with the insulation level and method of cooling used. Forced cooling transformers are better for the system requiring high impedance. The cost of the transformers increases rapidly if they are required with lower impedance for their normal standard values