Main technical requirements for current transformers
1. Rated capacity: the apparent power consumed when the rated secondary current passes through the secondary rated load. The rated capacity can be expressed by the apparent power V.A or by the secondary rated load impedance Ω.
2. Primary rated current: the power load current allowed to pass through the primary winding of the current transformer. The primary rated current of the current transformer used in the power system is 5-25000A, and the precision current transformer used in the test equipment is 0.1-50000A. The current transformer can operate under a rated current for a long time. When the load current exceeds the rated current value, it is called overload. The long-term overload operation of the current transformer will burn out the windings or reduce the service life.
3. Secondary rated current: the primary induced current allowed to pass through the secondary winding of the current transformer.
4. Rated current ratio (transformation ratio): the ratio of the primary rated current to the secondary rated current.
5. Rated voltage: the maximum voltage that the primary winding can withstand to the ground for a long time (effective value is in kV), which should not be lower than the rated phase voltage of the connected line. The rated voltage of the current transformer is divided into 0.5, 3, 6, 10, 35, 110, 220, 330, 500kV and other voltage levels.
6. 10% multiple: under the specified secondary load and any power factor, when the current error of the current transformer is -10%, the multiple of the primary current to its rated value. The 10% multiple is a technical indicator related to relay protection.
7. Accuracy level: Indicates the level of the error (ratio difference and angle difference) of the transformer itself. The accuracy level of the current transformer is divided into 0.001 to 1 level, and the accuracy is greatly improved compared with the original. The electrical instruments used in power plants, substations, and power distribution control panels of power-consuming units generally adopt class 0.5 or 0.2; the relay protection used for equipment and lines is generally not lower than class 1; when used for electric energy measurement, depending on the The measured load capacity or the amount of electricity consumption is selected according to the requirements of the regulations (see the first lecture).
8. Ratio difference: The error of the transformer includes two parts: the ratio difference and the angle difference. The ratio error is abbreviated as the ratio difference, generally represented by the symbol f, which 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: The phase angle error is referred to as the angle difference, generally represented by the symbol δ, which is the phase difference between the secondary current vector and the primary current vector after rotating 180°. It is stipulated that the secondary current vector is a positive value ahead of the primary current vector δ, otherwise it is a negative value, and the unit of calculation is the minute (').
10. Thermal stability and dynamic stability multiples: When the power system fails, the current transformer is subject to the thermal effect and electrodynamic action of the huge current caused by the short-circuit current. The current transformer should have the ability to withstand without being damaged. Capability is expressed in terms of thermal and dynamic stability multiples. 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 1s of the thermally stable current to the rated current of the current transformer. The dynamic stability multiple is the ratio of the instantaneous value of the maximum current that the current transformer can withstand to its rated current.
