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Current transformer parameter description
Apr 07, 2018

A current transformer model [3] :

The first letter: L - current transformer

Second letter: A—through wall type; Z—post type; M—bus type; D—single pass through type; V—inverted type; J—zero sequence

Grounding detection; W—anti-fouling; R—winding exposed

The third letter: Z-epoxy resin casting; C-ceramic insulation; Q-gas insulation medium; W- and microcomputer protection

The fourth letter: B - with protection level; C - differential protection; D - D level; Q - enhanced type; J - enhanced ZG

Fifth Number: Voltage Level Product Number

Second, the main technical requirements

2.1 Rated capacity: The apparent power consumed when rated secondary current passes the secondary rated load. The rated capacity can be represented by the apparent power V.A, or it can be expressed by the secondary rated load impedance Ω.

2.2 Primary rated current: The load current that is allowed to pass through the primary winding of the current transformer. The current transformer used in the power system has a rated current of 5 to 25,000 amps, and the precision current transformer for test equipment is 0.1 to 50,000 amps. The current transformer can run for a long time under a rated current. When the load current exceeds the rated current value, it is called overload. If the current transformer runs for a long time, it will burn out the winding or reduce the service life.

2.3 Secondary rated current: One-time induced current through the secondary winding of the current transformer is allowed.

2.4 Rated current ratio (ratios): ratio of primary rated current to secondary rated current.

2.5 Rated voltage: The maximum voltage that the primary winding can withstand for a long period of time (effective value in kV) and shall not be lower than the rated phase voltage of the connected line. Current transformer rated voltage is divided into several voltage levels of 0.5, 3, 6, 10, 35, 110, 220, 330, 500kV.

2.6 10% multiple: The current transformer's current error is -10% at the specified secondary load and any power factor, and the primary current is a multiple of its rated value. The 10% multiple is a technical indicator related to relay protection.

2.7 Accuracy level: It represents the level of error (ratio difference and angle difference) of the transformer itself. The accuracy level of the current transformer is divided into 0.001 to 1 levels, and the accuracy is greatly improved compared to the original. The power meter used in power plants, substations, and power distribution units generally uses 0.5-level or 0.2-level electrical equipment; the relay protection for equipment and lines is generally not less than 1; when used for electrical energy measurement, it is considered as How much load capacity or power consumption is measured depends on the requirements of the regulations (see first lecture).

2.8 Difference: The error of the transformer includes two parts: difference ratio and angle difference. The ratio error, referred to as the ratio difference, is 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.

2.9 Angle difference: The phase angle error is referred to as the angle difference. It is generally expressed by the symbol δ, which is the phase difference between the secondary current vector and the primary current vector after rotating 180°. It is specified that the secondary current vector leads the primary current vector δ to a positive value, and vice versa to a negative value, using the fraction (') as the calculation unit.

2.10 Thermal stability and dynamic stability factor: When the power system fails, the current transformer is affected by the thermal effect and electrodynamic force of the huge current caused by the short-circuit current. The current transformer should have the ability to withstand without being damaged. Represented by thermal stability and kinetic stability factor. The thermal stability factor refers to the ratio of the current of the current transformer that exceeds the allowable limit of current to the current rating of the current transformer within 1 s of the thermal stabilization current. The dynamic stability factor is the ratio of the maximum current instantaneous value that the current transformer can withstand to its rated current.