This paper expounds the excited inrush current problem and the influences of TA secondary circuit abnormality on differential protection in transformer with actual examples,and advances some suggestions on solving these problems.

Pointing to the defect of TA abnormal criterion,the author gives a new criterion of abnormal TA and also analysizes the protective actions after TA abnormal and during self-recovery of abnormal TA.

An accident analysis caused by a secondary circuit connection wrong;

Faults frequently found in secondary circuit of circuit breaker and improvements;

The methods of looking up the fault of secondary circuit;

Analysis of voltage transformer secondary loop grounding point;

Secondary loop includes current transformer,voltage transformer,secondary wiring,electronic energy meter and burden monitoring units.

In view of occasional erroneous wiring of current often met with in the power industry, the paper discusses the mutual influence among different parameters of current transformers, as well as the effect of erroneous wiring of transformer terminals on the secondary loop current, on secondary load impedance and an the reliability of protection by relays.