The power distribution system mainly consists of lines and transformers. Line is mostly identified by voltage, how many volts and how many kV (kilovolt) of line and transformer is also identified by voltage but primary (primary) and secondary (secondary) voltage. That is to say, the line is identified by one voltage and the transformer is identified by two voltages, how much voltage the transformer takes and how much voltage it draws out. For example - 33KV, 11KV, 0.4KV Line etc and Transformer 33/11KV, 33/0.4KV 11/0.4KV etc. Transformer is the device which changes the voltage while the line voltage does not change. Transformer capacity is in KVA (kilo volt ampere) or MVA (mega volt ampere). Therefore, speaking voltage and capacity are both necessary for any transformer. e.g. 63 KVA, 11/0.4 kV transformers. Another main reason is that there is no connection between the two bindings (primary and secondary) of the transformer, yet both the bindings are on the same core. The working principle of transformer is electromagnetic induction.