When the transformer external fault occurs, it will produce over-current; in the transformer internal failure, it will produce differential protection and gas protection of the backup, in the transformer, should be installed current protection device. According to the transformer capacity and the system short circuit current, different protection methods to choose.
The current transformer for relay protection is: reliable insulation; sufficient exact limit factor; sufficient thermal stability and dynamic stability. Protection transformer in the rated load to meet the requirements of the exact level of the largest current called rated accurate limit of a current. The exact limit factor is the rated accuracy limit of the primary current and the rated primary current ratio. When the current is large enough when the iron core will not be able to reflect the role of a current, accurate limit coefficient is that this feature. Protection Transformer Accuracy Level 5P, 10P, indicates the allowable error of 5%, 10% of the rated current at the rated accuracy limit.