What is the excitation inrush current of the power transformer?
The excitation inrush current of the electrical transformer, which is also called "closing inrush current", is a large current phenomenon that suddenly occurs in the winding of the transformer when the transformer is closed at no-load, that is, when it starts working or reconnects to the power supply. Simply put , just like when our high-power appliances (such as air conditioners) are started, because the coils and magnets inside the equipment need to quickly reach the working state, a large amount of current will be temporarily consumed. The excitation inrush current of the transformer is also based on a similar principle, but it occurs in the iron core and windings of the transformer. This current is a special current phenomenon in the early stage of the transformer's operation.
What causes the excitation inrush current of the power transformer?
Residual magnetic flux and working magnetic flux are superimposed
We know that the iron core of the oil immersed transformer itself is magnetically conductive, and there is a hysteresis property inside the iron core material, that is, under the action of the alternating magnetic field, the iron core will produce a process of magnetization and demagnetization. Before the transformer is put into operation, there may be residual magnetic flux in its iron core. What is residual magnetic flux?
Residual magnetic flux refers to the magnetic flux remaining in the iron core and coil of the transformer after the AC power supply is interrupted. This is because when the transformer is working normally, the iron core will be magnetized, and when the power is cut off, the magnetization will not disappear immediately, but a part of the magnetic flux will be retained.
When the distribution transformer is put into operation, when the magnetic flux generated by the working voltage is in the same direction as the residual magnetic flux in the iron core, the two will be superimposed, resulting in a total magnetic flux far exceeding the saturation magnetic flux of the iron core.
Core saturation
If the total magnetic flux after superposition exceeds the maximum value that the core can withstand (saturation magnetic flux), the core will be like "full" and can no longer absorb more magnetic flux. At this time, a very large current, namely the excitation inrush current, will be generated.
The magnitude of the excitation inrush current is also related to the power supply voltage value and the initial phase angle of closing, the core magnetic flux value and residual magnetic direction before closing, the system equivalent impedance value and phase angle, the wiring method of the transformer winding and the neutral point grounding method, the magnetization characteristics of the core material, the hysteresis characteristics, etc., the core structure type, and the process assembly level.
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