Principle of Differential protection for transformer
Differential protection for detecting faults is an attractive option when both ends of the apparatus are physically located near each other. e.g. on a transformer, a generator or a bus bar.
Consider an ideal transformer with the CT connections, as shown in Figure.
To illustrate the principle let us consider that current rating of primary winding is 100A and secondary winding is 1000A. Then if we use 100:5 and 1000:5 CT on the primary and secondary winding, then under normal (no fault) operating conditions the scaled CT currents will match in magnitudes. By connections the primary and secondary CTs with due care to the dots (polarity markings), a circulating current can be set up as shown by dotted line.
No current will flow through the branch having overcurrent current relay because it will result in violation of KCL.
Now if an internal fault occurs within the device like interturn short etc., then the normal mmf balance is upset i.e. N1I1 ≠ N2I2. Under this condition, the CT secondary currents of primary and secondary side CTs will not match. The resulting differential current will flow through overcurrent relay. If the pick up setting of overcurrent relay is close to zero, it will immediately pick up and initiate the trip decision.
In practice, the transformer is not ideal. Consequently, even if I2=0, I1≠0, it is the magnetization current or (no load) current. Thus, a differential current always flows through the overcurrent relay.
