The precision transformer for protection mainly cooperates with the relay device to provide a signal to the relay device to cut off the fault circuit when the line is short-circuit overloaded, etc., to protect the safety of the power supply system. The working conditions of the miniature current transformer for protection are completely different from those of the measuring transformer. The protective transformer only starts to work effectively when it is several times larger than the normal current. The main requirements of the protection transformer: 1. reliable insulation, 2. large enough accuracy limit coefficient, 3. sufficient thermal stability and dynamic stability.
The precision transformer for protection can meet the requirements of the accuracy class under the rated load. The current is called the rated accuracy limit primary current. The accuracy limit factor is the rated accuracy limit of the primary current to the rated primary current ratio. When the primary current is large enough, the iron core will not saturate to reflect the primary current. The accuracy limit coefficient indicates this characteristic. The accuracy of the protection transformer is 5P, 10P, which indicates the allowable error of 5% and 10% at the rated accuracy limit current.
The inrush current in the event of a line fault generates thermal and electromagnetic forces that must be withstood by the protective current transformer. In the case of a secondary winding short circuit, the current transformer can withstand the damage of the primary current in one second without damage, called the rated short-time thermal current. In the case of a secondary winding short circuit, the current transformer can withstand a primary current peak without damage, called the rated dynamic steady current.
Protection current transformers are divided into: 1. Overload protection current transformer, 2. Differential protection current transformer, 3. Ground protection current transformer (zero sequence current transformer).