Electrical Current Transformer for protection is divided into: 1. Overload protection current transformer, 2. Differential protection current transformer, 3. Ground protection current transformer (zero sequence current transformer); protection current transformer is mainly related to relay The device cooperates to provide a signal to the relay device to cut off the fault circuit in the event of a short circuit overload and other faults to protect the safety of the power supply system. The working conditions of the current transformer for protection are completely different from the current transformer for measurement, and the protection transformer only starts to work effectively when the current is several tens of times larger than the normal current. The main requirements for the protection transformer are: 1. Reliable insulation, 2. Large enough accurate limit coefficient, 3. Sufficient thermal stability and dynamic stability.