I.Characteristics of transformer excitation inrush current

(1) The waveform presents a pointed shape, indicating the presence of significant non periodic components and second and third harmonic components. Over time, the second harmonic content of a certain phase may exceed half of the fundamental component.

(2) The amplitude of excitation inrush current is directly related to the initial phase angle of the voltage when the transformer is put into no-load operation. For single-phase transformers, the amplitude of the excitation inrush current is maximum when the voltage zero crossing point is applied. However, due to the 120 ° phase difference between each phase of the three-phase transformer, the inrush current is also different.

(3) The time constant of surge attenuation is related to the impedance, capacity, and core material of the transformer.

II.The hazards of transformer excitation inrush current

(1) The sudden increase in voltage caused by the short-circuit fault of the transformer output line can trigger the misoperation of the transformer protection, resulting in a complete power outage on all sides of the transformer load.

(2) The misoperation of relay protection devices such as transformer differential protection and quick break overcurrent often leads to the failure of transformer operation.

(3) The running transformer caused a "synchronous excitation inrush current" and mistakenly tripped, resulting in a large-scale power outage.

(4) High amplitude excitation inrush current can cause damage to transformers.

(5) Inducing overvoltage during operation and damaging electrical equipment.

(6) The DC component in the excitation inrush current leads to a decrease in measurement accuracy and the correct operation rate of the relay protection device.

(7) The large amount of harmonics in the excitation inrush current causes serious pollution to the power quality of the power grid.

(8) Causing a sudden rise or drop in grid voltage, affecting the normal operation of other electrical equipment.

(9) Causing voltage waveform distortion and affecting power supply quality.

(10) Causing increased transformer noise, high losses, and other issues.

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