The testing standards and specifications for current transformers (CTs) are the fundamental basis for ensuring the accuracy of power system metering, the reliability of protection, and the safety of equipment. They cover key aspects such as accuracy grades, error limits, testing items, and test methods.
Standard for accuracy
Test accuracy standard for measuring CT:
Normal measurement level (0.1, 0.2, 0.5, 1 level);
Special measurement grade (0.2S, 0.5S grade):
Low precision level (3, 5 levels)
Test accuracy standard for Protective CT;
With the composite error as the core indicator, the standard grades are 5P and 10P. For example, at the rated accuracy limit current, the composite error of grade 5P is ≤ 5%, and at the rated primary current, the current error is ≤ ±1%.
Current Transformer Inspection Items and Test Standards
Error test: Apply different proportional currents (such as 5%, 20%, 100%) using a standard current source. Measure the current error and phase difference using precise instruments, and ensure they comply with the corresponding accuracy level limits.
Insulation resistance test: Use an ohmmeter to measure the insulation resistance between the windings and the ground, as well as between the windings themselves. Generally, it is required to be ≥ 1000 MΩ (specifically according to the equipment's voltage level)
AC withstand voltage test: Apply industrial frequency high voltage (e.g., 3kV for 1 minute) to the windings, with no breakdown or flashover phenomena.
DC resistance test of windings: Measure the resistance value of the secondary winding, with the deviation being within ±10% of the designed value, to ensure the conductivity of the windings.