Differenet Transformers in Power Station
Generation Transformer is employed in power plant for stepping up the voltage for transmitting the power to the grid. Electrical power is generated in the power plant at lower voltages (typically generation voltage will be between 11kV to 33kV). In order to transmit the power to long distances voltage has to step up to reduce the losses. Hence in power plants, generation transformer is employed.
Rating of the generation transformers will be almost equal to the rating of the generator (500MW generating unit will have generating transformer rating about 588MVA). Connection between the generator transformer and power plant generator will be through isolated Phase Bus Duct (IPBD).
Air Force Oil Force (AFOF) cooling is to be provided for these transformers. Generator transformers are provided with on-load tap changing mechanism to regulate the terminal voltage. Generator transformer will be Delta (LV side) and Star (HV side) connected with star connection is connected to earth through resistor to reduce the fault currents and protecting the transformer.
If Generator Circuit Breaker (GCB) is provided at the generator terminals, power from the grid can be supplied to plant auxiliary loads through generator transformer by opening the GCB when generating unit is not in operation.
Some of the points about generator transformer is given below:
- This is the main transformer of generating unit used for stepping up the voltage from generating station for the transmission
- In a generating plant for every generating unit one generating transformer is required
- Rated voltage on LV side corresponds to the rated generating voltage
- Rated voltage on the HV side corresponds to rated voltage of the HV bus
- Usually these transformers are outdoor type
- LV terminals are connected to the generating terminals via isolated phase bus systems
- HV terminals are connected to the outdoor busbars by flexible ACSR conductors via overhead flexible bus
Unit auxiliary Transformer (UAT):
Power plant is provided with Unit Auxiliary Transformers (UAT) connected to the generator terminals through Isolated Phase Bus Duct (IPBD). Unit Auxiliary Transformers provides electrical power to power plant distribution buses by stepping down the voltage from 11kV to 6.6kV (example).
Unit Auxiliary Transformers are rated based on the rating of the loads it has to supply. On-load tap changing mechanism is provided to UATs. They are usually Delta (HV side) and Star (LV side) connected transformers. The neutral of the star point is connected to earth through resistor to limit the fault current during ground faults and to protect the transformer. These transformers are usually provided with Air Force Oil Force (AFOF) cooling
Some of the points associated with Unit Auxiliary Transformers are listed below:
- The Purpose of Unit auxiliary Transformer is to feed power to generator auxiliaries of that unit
- These transformers are connected to generators and are used as stepping down transformers. The HV side transformer voltage corresponds to the voltage of the generating unit and the LV side voltage is stepped down to 6.6KV
- Rated KVA of Unit Auxiliary Transformers is approximately 15% of the generating rating
- Usually these transformers are outdoor transformers
- One Unit auxiliary transformer is present for every generating unit.
Station Service Transformer
Station Transformers are employed for supplying power to plant auxiliary loads during the event of starting of the plant or when generating unit is not generating power. Station Transformers are connected to the swichyard bus. LV side of the station transformer is connected to the auxiliary load buses.
Station transformer is normally rated for supplying power to the auxiliary loads. On-load tap changing mechanism is provided to regulate the terminal voltage of the transformer. These transformers are provided with Air Force Oil Force (AFOF) type of cooling. The star point of the station transformer is grounded through resistor.
Some the points related to station transformer are given below:
- In general station service transformer is used for supplying power to auxiliary equipment in the power plant when the plant is not generating any power.
- Rated HV voltage corresponds to the rated voltage of the outer busbars
- Rated LV voltage corresponds to the auxiliary bus voltage
- Rated KVA corresponds to the load of common auxiliaries of the station. This corresponds to the 10% to 15% of the rating of the generating power.
- These transformers are Outdoor type.
Electrical Interview Questions With Answers
These transformers are employed in the power plants for delivering power to low voltage loads (voltage below 1kV). These transformers connects between HV distribution buses and LV distribution buses of the plant. Their rating will be around 1 to 5MVA. Natural oil cooling or air cooled transformers are used.
Some of the points related to station auxiliary transformers are listed below:
- These transformers are located in power plant to step down voltage from 6.6KV to 415V.
- The rating for this transformer corresponds to the rating of the auxiliary load it should be bearing.
- These transformers are indoor type and usually dry type transformers are used.
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