Current Transformer Configuration Guide: Select the Appropriate “Protection Shield” According to the Scenario

2025-09-01

Content: How to select current transformers (CTs) for different power circuits? The following three configuration principles will help you “precisely match”!

Principle 1: Select windings based on functional requirements (measurement vs. protection)
The number of secondary windings of a CT depends on its purpose:

Measurement CT: Usually equipped with 1-2 secondary windings to meet the accuracy requirements of ammeters and energy meters, usually with an accuracy level of 0.5 or 1.
Protection CT: Usually equipped with 2-8 secondary windings to meet the needs of multiple sets of protection devices, such as overcurrent protection and differential protection, with an accuracy level of 5P or 10P.
Combined CT: It can measure and protect windings at the same time, avoiding repeated installation and saving costs.

Principle 2: Select the connection method according to the system type (high current vs. low current grounding)
The configuration of CTs will vary depending on the grounding method of the power system:

High current grounding system (such as 110kV and above substations): adopt a three-phase configuration (CT is installed on each phase to fully monitor the current of each phase and provide more sensitive protection).
Low current grounding system (such as 10kV distribution network): A two-phase configuration can be used (saving costs, the third phase is calculated based on the two-phase current, meeting basic protection requirements).

Principle 3: Determine location based on protection objectives (preventing dead zones + reducing damage)
The installation position of CT directly affects the protection effect:

No dead zone for main protection: Try to install two sets of CTs on both sides of the circuit breaker (for example, the line CT is installed before the circuit breaker and the transformer CT is installed after the circuit breaker). In this way, when the circuit breaker trips, the CTs on both sides can detect the fault current and avoid the “protection dead zone”.
Prevent busbar failure: Pillar-type CTs should be installed on the outgoing side of the circuit breaker to reduce busbar short circuits caused by bushing flashover.
Generator protection: The measuring CT should be installed on the neutral point side (to facilitate the detection of minor internal faults before grid connection), and the excitation regulating CT should be installed on the outgoing line side (to reduce the high current impact in the event of a fault).

For example:
In a 10kV distribution station, the main incoming line needs to monitor the current and protect the transformer at the same time. The configuration scheme can be:

Mains Incoming CT: Three-phase configuration (high current earthing system) with 2 secondary windings (1 for measurement + 1 for protection).
Transformer CT: Installed on the low-voltage side (close to the transformer), equipped with two secondary windings (for protecting the main transformer and measuring power).

Reasonable configuration of CT is like installing a “multi-faceted protection shield” for the power system, which can not only accurately monitor but also quickly respond to faults, making power operation safer and more reliable!