Updated: Mar 11
What is Power Transformer?
The voltage at the generating station is stepped up by a transformer for long distance transmission to reduce the line losses, and at the receiving end the voltage is stepped down to utilization voltage level. The transformer used for this application is known as power transformer. Power transformers of rating more than 200 MVA is used at 400 KV,220 KV,132 KV,66 KV,33 KV is classified in the category of power transformer. The power transformer acts as a step-up and step- down transformer at sending and receiving end respectively.
What is Distribution Transformer?
Distribution transformers are used at lower voltage rating to utilize the voltage at end use. The distribution transformers are of less than 200 MVA capacity in the voltage grade of 11 KV,6.6 KV,3.3 KV,0.440 V,230 V. The secondary voltage tends to be lower and secondary voltage is used by end user.
Further, we can differentiate the power transformer and distribution transformer on the following basis.
Size of Transformer
Power transformers are large rating transformer of higher voltage grade (>200MVA, 400KV), therefore the size of the transformer is bigger than the distribution transformer.
Power transformers have higher voltage grade 400 KV, 220 KV etc. therefore more insulation is required. The transformer is generally of outdoor duty and insulation level is kept high to withstand lightning impulse. Distribution transformers have less insulation requirement because the voltage grades of the transformer is below 33 KV.
Power transformers are supposed to be operated at full load and designed for 100 % efficiency. Efficiency of the transformer is calculated by measuring ratio of the power output to power input. The loading on the distribution transformer varies all day and its All day efficiency is about 50 -70 %.The efficiency of the distribution transformer is calculated on all day basis.
Iron Losses & Copper Losses
Power transformers operate at 100 % loading for 24 hours. Therefore, the copper loss of the transformer should be as minimum as possible to have maximum efficiency. Power transformers are designed to operate at the maximum flux density. The flux density of the power transformer is kept in between 1.7- 1.8 Tesla. The higher flux density cause higher core losses and subsequently on account of higher flux density fewer turns are required and hence copper loss of the transformer gets reduced at 100 % loading.
The distribution transformer operates all day and loading on the transformer is not constant. Therefore, the distribution transformer is designed to operate at lower flux density to have low iron losses. The distribution transformer is designed for 60-70 % average loading. Trade off between iron loss and copper loss is made to have best efficiency of the distribution transformer.
Thus, the power transformer has higher iron losses as compared to distribution transformer and, it has lower copper loss as compared to distribution transformer.