The current transformer connection for relay protection is usually a star connection when it is used as a protective device in a power system directly grounded to neutral. In non-directly grounded neutral power systems, incomplete star wiring is widely used due to the short-time single-phase-to-ground operation and, in most cases, single phase grounded signaling devices. Protection of the current transformer delta wiring used in Y / â–³ wiring transformer differential protection. The main current transformer for protection cooperates with the relay device. When there is a fault such as a short circuit overload in the circuit, a signal is provided to the relay device to cut off the fault circuit so as to protect the power supply system. Protection of micro-current transformer working conditions and measuring transformer is completely different, the protection of the transformer is only a few times larger than the normal current several times the current began to work effectively. Protection of the main requirements of the transformer: 1, reliable insulation, 2, large enough accurate limit factor, 3, sufficient thermal stability and dynamic stability. Protection of the transformer at rated load to meet the exact level of the maximum current called the rated accuracy of a current limit. The exact limit factor is the rated current limit and rated primary current ratio. When a current large enough core will not be able to saturate the role of reflecting a current, the exact limit of the coefficient is that this feature. Accuracy level of protection transformer 5P, 10P, which means that the allowable error 5%, 10% at the rated accuracy limit primary current. The impact current in case of line failure generates heat and electromagnetic force, and the current transformer for protection must withstand. Secondary winding short circuit case, the current transformer can withstand without damage in one second rms current, said the rated short-term thermal current. Secondary winding short circuit case, the current transformer can withstand without damage to a peak current, said the rated dynamic current stability. Protection current transformer is divided into: 1, overload protection current transformer, 2, differential protection current transformer, 3, ground protection current transformer (zero sequence current transformer).