During current transformer manufacture, we reach for high accuracy and low error, while, do you know the Factors Affecting the Current Transformer Error?

Here we clarify it in below six points:

1. Current Input

The higher current input is, the higher magnetic density of the core proportionally, at same time,  the magnetic permeability ( μ) and the loss angle (ψ) increase,  which reduce the transformer ratio difference , the phase difference decrease, final transformer error decreases.

2. The square of the number of turns of the secondary winding

The error is inversely proportional to the square of the number of turns of the secondary winding.

In theory, increasing the number of turns of the secondary could reduce the transformer error. However, the bigger of secondary turns, the higher of internal resistance of the secondary winding, which limited the error decline. The single-turn or bus-type current transformer has simplest structure and most material-saving, but once with small primary current, the error increase rapidly, which can't meet accuracy level request. Therefor, we suggest multi-turn when primary current below 300A, and single-turn or bus-type for primary current is greater than 300A~600A.

3.The average magnetic path length

The error is proportional to the average magnetic path length.

The core's window area influence the core’s magnetic circuit length, window size must ensure that the primary & secondary winding can be installed and insulated. After meeting this requirement, we try to reduce window area as small as we can. Because, the smaller the core's window area, the smaller magnetic path length of the core, the smaller the error, and material saving.

4.The core's cross section area & shape

The error is inversely proportional to the  core cross section area.

In theory, increasing core section area is a way to reduce error. However, the core magnetic permeability will decrease as the area increases. As the average magnetic circuit length increases, the internal impedance of the secondary winding increases. All those limited the error reduction, even increase the error, wastes material.

The core section shape affect transformer error, too. Because for same core's cross section area, the higher of core, the shorter of the average magnetic circuit length. For the core with same height & width, request shortest copper wire of winding, and minimal resistance. That's why we need consider about proportional relationship between the core height & width during design.

5. The core magnetic permeability

Transformer error is inversely proportional to the core magnetic permeability. Thus, improving core raw material's magnetic properties is a main way to reduce ct's finished size, special for the high accuracy ct.

6. The secondary load

The transformer's error is proportional to the magnitude of the secondary load. During the secondary load increasing, the core magnetic density & the magnetic permeability increases slightly. So the transformer error increase with the secondary load increase, but less than a proportional increase.