Measuring current transformers (or windings of current transformers): providing grid current information to measuring equipment within the normal operating current range. Measure the CT current flowing through the primary equipment under normal operating conditions. The measured current is used in the metering circuits of relay protection devices, integrated automation devices, and metering devices. High measurement accuracy is required under normal load current.

Current transformers (or protective windings of current transformers) used for protection: In the event of a power grid fault, the fault current information of the power grid can be provided to relay protection and other equipment. Protecting CT means that in the event of faults such as short circuits in primary equipment or circuits, and when a large fault current flows through the primary equipment, the output current of the CT secondary side can still reflect the magnitude of the primary side current. When the fault current is large, that is, when N times the rated current flows through the primary side, the output current error on the secondary side is still very small, which requires CT to remain unsaturated.

Usually, the currents used in protection circuits and measurement circuits are provided by the protection CT and measurement CT respectively, which cannot be used interchangeably, and most measurement circuits have precision components. In order to protect the safety of these components in the event of a line fault, it is usually necessary to measure the saturation of CT when a large short-circuit current flows through the primary side.