Error limit of protective current transformer
accuracy level | Under rated primary current Current error （±%） | Under rated primary current phase difference | Composite error under rated accuracy limit of one current % | |
±（´） | ±crad | |||
5P 10P | 1 3 | 60 - | 1.8 - | 5 10 |
Example: A current transformer with the symbol 5P20, 30VA, 1000/5 is represented as follows: transformation ratio 1000/5, rated secondary current 5A. If the secondary load impedance is less than 1.25 Ω (Z=S/I2=30/25=1.25 Ω), when the primary fault current is 20000A (20 × 1000A), the composite error of the secondary current is less than 5%.
Based on the rated secondary current, accurate limit coefficient, and rated load, the voltage achieved during the excitation characteristic test can be roughly estimated
U=KALFIsnZbn
In this example, U=20 × 5 × 1.25=125V
