Main Technical Requirements for Current Transformers

Rated capacity of current transformers

The apparent power consumed when the rated secondary current passes through the rated secondary load. The rated capacity can be expressed in terms of apparent power V.A, or in terms of secondary rated load impedance Ω.

Primary rated current of current transformers

The allowable electrical load current passing through the primary winding of the CT current transformer. The primary rated current of current transformers used in power systems is 5-25000A, whereas precision current transformers used in testing equipment are 0.1-50000A. Current transformers can operate for a long period of time under the primary rated current, and when the load current exceeds the rated current value, it is called overloading. If a CT current transformer operates overloads for a long period, the winding will burn or its lifespan will be reduced.

Secondary rated current of current transformers

The primary induced current allowed to pass through the secondary winding of the CT current transformer.

Rated current ratio (turns ratio) of current transformers

The ratio of primary rated current to secondary rated current.

Rated voltage of current transformers

The voltage (expressed in kV) that the primary winding can withstand for a long period of time against earth should not be less than the rated phase voltage of the connected circuit. The rated voltage of current transformers is divided into several voltage levels, such as 0.5, 3, 6, 10, 35, 110, 220, 330, 500kV, etc.

10% multiple of current transformers

The multiple of the primary current to its rated value when the current error of the CT current transformer is -10% under the specified secondary load and any power factor. The 10% multiple is a technical index related to relay protection.

Accuracy class of current transformers

It indicates the level of errors (ratio error and phase displacement) of the transformer itself. The accuracy class of current transformers varies from 0.001 to 1, and accuracy has improved significantly compared to before. Electric instruments used in power plants, substations and power distribution control panels usually have an accuracy class of 0.5 or 0.2; those used in equipment and line relay protections are generally not less than 1; and those used in electricity metering should be selected according to the regulations of the tested load capacity or electricity consumption.

Ratio error of current transformers

The error of current transformers includes both ratio error and phase displacement. Ratio error, also known as voltage ratio error, is denoted by the symbol f, and it is equal to the percentage difference between the actual secondary current and the one calculated from the primary current reduced to the secondary side value.

Phase displacement of current transformers

Phase displacement is denoted by the symbol δ, and it refers to the phase angle difference between the secondary current vector, which rotates 180°, and the primary current vector. The vector of the secondary current is ahead of that of the primary current, and δ is a positive value; otherwise, it is a negative value. The unit of calculation is the minute (').

Thermal and dynamic stability multiples of current transformers

When power system faults occur, the current transformers are subject to the effects of heat and electrodynamic forces caused by the large current induced by short-circuit currents. Therefore, current transformers should be capable of withstanding such forces without being destroyed. Such resistance is expressed in thermal and dynamic stability multiples. Thermal stability multiples refer to the ratio between the current that does not make the CT current transformer generate excessive heat within 1s and the rated current of the CT current transformer.

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