Module 10 :
Differential Protection of Bus, Transformer and Generator
Lecture 39 :
Transformer Protection
Introduction
Differential protection of transformer was introduced in lecture 2. Traditionally, it involves establishing circulating current through a pair of matched CTs installed on the primary and secondary winding of the transformer. If there is no internal fault in the transformer, zero current flows through the differential overcurrent element. However, in case of an internal fault, the CT secondary currents are not matched and hence the differential current is not zero. This causes the overcurrent element to pick up and operate the circuit breakers to isolate the transformer.
Selection of CT Turns Ratio
Let the transformer turns ratio given by and the corresponding CT ratio be given by and as given in fig 39.1. Then,
Current in CT - 1 primary =
Current in CT - 1 secondary =
Current in CT - 2 primary =
Current in CT - 2 secondary =
If there is no fault, then with proper connections account for the CT polarity, we should obtain circulatory current through CT secondary.
Hence,
is not zero. This causes the overcurrent element to pick up and operate the circuit breakers to isolate the transformer. 
and the corresponding CT ratio be given by 
and 
as given in fig 39.1. Then,
