Transformer Differential Protection


Which Data of the Transformer must be known ?

1. Amplitude matching

  • Nominal Power (from there primary and secondary nominal current)
  • Current transformer ration primary and secondary

2. Phase position matching

  • connection group e.g. Yd 5

3. At automatically regulated Transformers

  • Control range e.g. +/- 10%

4. Two- or Three windings transformer

  • One critical event for the differential protection of transformer is the starting.
  • Current flows in the transformer and it is used for magnetizing the core,but no current flows out on secondary side.
  • For prevention of tripping of the protection device special actions are to be taken.
  • The magnetizing current is identificated by a high component 2.Harmonics (100Hz).
  • It will be filtered-out and used for stabilisation. With that the transformer can be started safely.

Transformer Differential Protection



but also in normal operation:
I1 is not I2 !

because of actual transfrormation ratio of the transformer 

Amplitude matching 

because of connection group of the transformers 

Phase position matching

  • at conventional and analog devices: matching transformers
  • at digital devices computationaly in protection device