What is transformer inrush current

FAQ | Feb 07,2023

The transformer inrush current is the maximum instantaneous current drawn by the primary of the transformer when their secondary is open circuit.The characteristics of inrush current are as follows,
1, the inrush current contains a large number of high harmonic components (mainly second and third harmonic), mainly even harmonic, therefore, the change curve of excitation inrush current for the peak wave
2. The attenuation constant of excitation inrush current is related to the saturation degree of the iron core. The deeper the saturation, the smaller the reactance and the faster the attenuation. Therefore, at the beginning of the instant attenuation is very fast, and then gradually slow down, after 0.5 ~ 1s its value does not exceed (0.25 ~ 0.5)
3. In general, the larger the transformer capacity, the longer the duration of attenuation, but the general trend is that the attenuation speed of inrush current is often slower than that of short circuit current.
4. The value of excitation inrush current is very large, up to 6 ~ 8 times of rated current. When setting a circuit breaker to control a transformer, its speed break can be set according to the transformer excitation current.

The inrush current is related to the saturation degree of the core, and the remanence of the core and the phase Angle of the closing voltage can affect its magnitude.

The inrush current is not dangerous to the transformer because the inrush current exists for a short time. Of course, it is not good to charge the transformer repeatedly, because the multiple impact of large current will cause the mechanical force between the windings, which may gradually loosen its fixed objects. In addition, the excitation inrush current may cause the differential protection action of the transformer, so the transformer operation should be paid attention to.


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