Main Technical Requirements For Current Transformers

1. Rated capacity: The apparent power consumed when the rated secondary current passes through the rated secondary load. The rated capacity can be used as apparent power V A can also be represented by the secondary rated load impedance Ω.
2. Primary rated current: The electrical load current allowed to pass through the primary winding of the current transformer. The primary rated current of current transformers used in power systems is 5-25000A, and the precision current transformers used in testing equipment are 0.1-50000A. Current transformers can operate for a long time at a rated current. When the load current exceeds the rated current value, it is called overload. Long term overload operation of current transformers can burn out the windings or reduce their service life.
3. Secondary rated current: Allow the primary induced current through the secondary winding of the current transformer.
4. Rated current ratio (transformation ratio): the ratio of primary rated current to secondary rated current.
5. Rated voltage: The maximum voltage (effective value in kV) that a primary winding can withstand to ground over a long period of time should not be lower than the rated phase voltage of the connected line. The rated voltage of current transformers is divided into several voltage levels such as 0.5, 3, 6, 10, 35, 110, 220, 330, and 500 kV.
6. 10% multiplier: The multiple of the rated current of a current transformer when the current error is -10% under a specified secondary load and any power factor. The 10% multiple is a technical indicator related to relay protection.
7. Accuracy level: Refers to the level of error (ratio difference and angle difference) of the transformer itself. The accuracy level of current transformers is divided into multiple levels ranging from 0.001 to 1, which greatly improves the accuracy compared to the original. The electrical instruments used on the distribution control panel of power plants, substations, and electricity consuming units generally adopt level 0.5 or 0.2; The relay protection used for equipment and lines is generally not lower than level 1; When used for energy metering, the selection should be based on the measured load capacity or electricity consumption according to regulatory requirements (see the first lecture).
8. Ratio difference: The error of the transformer includes two parts: ratio difference and angle difference. Ratio error, abbreviated as ratio error, is generally represented by the symbol f. It is equal to the difference between the actual secondary current and the primary current converted to the secondary side, and the ratio of the primary current converted to the secondary side, expressed as a percentage.
9. Angle difference: Phase angle error, abbreviated as angle difference, is generally represented by the symbol δ. It is the phase difference between the secondary current vector rotated 180 ° and the primary current vector. The second current vector leading the first current vector δ is a positive value, and vice versa is a negative value, calculated in minutes (').
10. Thermal stability and dynamic stability multiple: In the event of a power system failure, current transformers are subjected to the thermal and electrodynamic effects of the huge current caused by short-circuit currents. Current transformers should have the ability to withstand without being damaged, and this ability to withstand is represented by the thermal stability and dynamic stability multiple. The thermal stability multiple refers to the ratio of the current that does not cause the heating of the current transformer to exceed the allowable limit within 1 second of the thermal stability current to the rated current of the current transformer. The dynamic stability multiple is the ratio of the maximum instantaneous current that a current transformer can withstand to its rated current.