Why is impedance important in Transformer?

It is used for determining the interrupting capacity of a circuit breaker or fuse employed to protect the primary of a transformer.

Example: Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 KVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source. Calculate as follows:

Normal Full Load Current =

Nameplate Volt Amps 10,000 VA

__________________ = __________ =

                            Line Volts 480 V                             

                             20.8 Amperes

Maximum Short Circuit Amps =

Full Load Amps 20.8 Amps

_____________ = _________ =

     4%                           4%

520 Amps

The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.

Example:


Determine the interrupting capacity, in amperes, of a circuit breaker or fuse required for a 75 KVA, three phase transformer, with a primary of 480 volts delta and secondary of 208Y/120 volts. The transformer impedance (Z) = 5%. If the secondary is short circuited (faulted), the following capacities are required:

Normal Full Load Current = 

          

 Volt Amps 75,000 VA

______________ __________________________

Square Root 3 x Line Volts Square Root 3 x Line Volts 

90 Amps

Maximum Short Circuit Line Current = 

Full Load Amps 90 Amps

______________ = _______

           5%                        5%

1,800 Amps

The breaker or fuse would have a minimum interrupting rating of 1,800 amps at 480 volts.

 The secondary voltage is not used in the calculation. The reason is the primary circuit of the transformer is the only winding being interrupted.