Step Up Transformer
To step up the generation voltage level of 11 KV to that of transmission level of 132KV generator is connected to transformer.
Off load tap changer is proposed on high voltage side. An allowance of 10% continuous over load is also proposed.
SideThe high voltage side is 132KV. An off load tap changer with a range of ± 2*2.5 is included.
This would facilitate to feed WAPDA system at 132kv. The BIL for 132KV side may be rated at 650KV. The high voltage winding will be connected in three phase star connection with neutral taken out.
Low Voltage Side
The nominal rating of low voltage side shall be 11kv and the three phase winding connection will be made in delta in order to minimize the possible third harmonic current originating from generator. The connection from generator terminal to the transformer terminal will be made through cables as per IEC specification.
Cooling System of transformer
It is proposed to use the oil natural & air natural cooling system (ONAN) for the transformer. The air fans will be controlled from thermal detectors within the transformer tank. During a short time, an over loading capacity of about 10% is required.
Reserve fans in the cooling system shall be provided so that failure of two fans will not shut down the transformer. The number of fans should not be less than four in total.
Structural details of the civil engineering design shall take into consideration a good airflow to the transformer location, so that no accumulation of high temperature air volume takes place. In order to keep the consumption of the cooling system as low as possible, a temperature controlled sequence of turn-on and shut-off of the fans shall be included.
Fire Protection of Transformer
Transformers shall be installed outside of the power house building, so that the most favorable solution for this type of protection will be the water sprinkle system. An adequate tank reserve of pressurized water is needed for this purpose. AC/DC driven pumps should be included to keep the dimensions of the storage tank economically small. An adequate volume of the oil reception basin underneath transformer is required. Gravel shall cover the surface of oil receipt.
Two auxiliary transformers of capacity 300 KVA (one standby) shall supply the electrical power to the auxiliary equipments of one generating unit and for the station services. Each transformer can be connected to 132 KV bus through 132KV breaker.
Oil filled distribution type transformers with a capacity of 300KVA, 132KV/0.4kV, have been selected based on the required capacity of load in the plant. Natural air circulation cooling has been chosen due to the operational safety. The insulation material shall be self-extinguishing and varnish shall prevent any spreading of fire. The units shall be installed close to the space where the power is needed
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