Depending upon the type of service, in the field of power system, transformers are classified in to two
- Power Transformers
- Distribution Transformers
- They may be single phase or 3-phase; 3-wire Δ/Δ or Δ/Υ connected; and voltage rating 220/11 kV or in high voltage range.
- They are put in operation during load hours and disconnected during light load hours i.e power transformers are usually operated on approximately full load.
- This is possible because they are arranged in banks and can be thrown in parallel with other units or disconnected.
- So power transformers are designed to have maximum efficiency at or near full load (iron loss to full load copper loss ratio of 1:1)
- Power transformers are designed to have considerable greater leakage reactance than is permissible in distribution transformers.
- It is because in power transformers inherent voltage regulation is not as much important as current limiting effect of the higher leakage reactance.
- Usual flux density of power transformers are of 1.5 T to 1.7 T.
- Percentage Impedance ranging from 6-18%.
- Regulation ranging from 6-10%.
- They may be self oil cooled, forced air cooled or forced water cooled.
- They are kept in operation all the 24 hours a day whether they are carrying any load or not.
- In such transformers iron loss occurs all the time where copper loss occurs only when they are loaded.
- Therefore distribution transformers should be designed with iron loss smaller in comparison with full load copper loss (say of the ratio 1:3).
- In other words we can say, distribution transformers are designed for good all day efficiency and not for highest efficiency on full load.
- They are of self cooling type.
- The distribution transformers are usually 3-phase 4 wire 11 kV/400 V Δ/Υ connected.
- They are designed to have an inherently good voltage regulation. It is possible by arranging the coils in minimum leakage reactance.
- Percentage impedance of distribution transformers ranging from 4 – 5%.
- Voltage regulation is of 4-8%.
- They are made with cold rolled steel with flux density of 1.7 T.