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Protective current transformer
Apr 07, 2018

Protection current transformer is divided into: 1. Overload protection current transformer, 2. Differential protection current transformer, 3. Ground protection current transformer (zero sequence current transformer)

The protective current transformer is mainly matched with the relay device. When a fault such as short-circuit overload occurs on the line, it provides a signal to the relay device to cut off the fault.

Road to protect the safety of the power supply system. Current transformer condition and measurement current for protection

The transformer is completely different, and the protective transformer only starts to work effectively when the current is several times higher than the normal current. The main requirements for protection transformers are: 1. Insulation reliability, 2. Accurate enough limit coefficient, 3. Sufficient thermal stability and dynamic stability.

The protective transformer can meet the requirement of accurate level under the rated load. The maximum primary current is called the rated accurate limit primary current. The accurate limit factor is the ratio of the rated current to the rated primary current. When the primary current is large enough, the iron core will not saturate and reflect the effect of the primary current. Accurate limit factor indicates this characteristic. Protection transformer accuracy level 5P, 10P, indicating that the allowable current error is 1%, 3% when the rated accuracy is limited to a primary current, and the compound error is 5% and 10% respectively.

The inrush current at the time of the line failure generates thermal and electromagnetic forces, and the protective current transformer must withstand. Under the short-circuit condition of the secondary winding, the primary current value of the current transformer that can withstand without damage in one second is called rated short-term thermal current. Under the short-circuit condition of the secondary winding, the primary current peak that the current transformer can withstand without damage is called rated stable current.

The accuracy level of protection current transformers is 5P/10P, and 10P indicates that the composite error does not exceed 10%.