When a power or a distribution transformer is first put into operation, a phenomenon called excitation inrush current occurs. Although this inrush current is generally not as destructive as fault currents, the duration of excitation inrush currents is approximately seconds, compared to cycles with fault currents.
The frequency of excitation inrush current is much higher than that of short-circuit, so this phenomenon is worth discussing.
Consider what happens when the single-phase transformer is initially energized. The magnetic flux in the magnetic core is equal to the integral of the excitation voltage.
If the circuit is closed when the voltage passes through zero and the initial magnetic flux is zero, the sinusoidal magnetic flux will be completely offset from zero. The peak of full offset flux is twice that of symmetric sine wave flux. In other words, the peak flux of fully offset waves can be close to twice the normal peak flux, which is usually sufficient to drive the magnetic core into a saturated state.
At this point, the only limiting excitation inrush current is the empty core impedance of the winding, which is several orders of magnitude smaller than the normal magnetization impedance. Therefore, during the half cycle of magnetic core saturation, the excitation inrush current is much greater than the normal excitation inrush current. During the opposite half cycle, the magnetic core is no longer saturated and the excitation inrush current is approximately equal to the normal excitation inrush current. When there is residual magnetic flux in the magnetic core and the direction of the residual magnetic flux is the same as the offset direction of the sinusoidal magnetic flux wave, the situation becomes even more extreme.
If there is no resistance in the circuit, each continuous peak will have the same value, and the current influx will continue indefinitely. However, in the presence of resistance in the circuit, the voltage drop at both ends of the resistance will be large, and the increase in flux does not need to be as high as the previous cycle.
The integral of voltage drop represents the net decrease in magnetic flux required to support the application of voltage. Due to i × The R voltage drop always follows the same direction, so each cycle will reduce the required magnetic flux. When the peak value of magnetic flux drops below the saturation value of the magnetic core, the inrush current disappears. The decay rate is not exponential, although it is similar to exponential decay current. For large power transformers, inrush currents can last for several seconds until they eventually disappear.
By simply adding inductance to the air-core inductance of the winding, the line reactance has the effect of reducing peak surge current. There is a definite relationship between inrush current and short-circuit current, as both are related to the empty core inductance of the winding.
Please remember that short circuits often eliminate magnetic flux from the core.
Usually, our experience shows that the peak excitation inrush current is slightly higher than 90% of the peak short-circuit current. However, the magnetic force caused by excitation inrush current is usually much smaller than the short-circuit force. Since each phase only contains one winding, there is no magnetic repulsion between the windings.
When it comes to three-phase power transformers, analyzing the entire problem of excitation inrush current becomes more difficult. This is because the phase angles of the excitation voltage are separated by 120 °, there is an interaction between current and voltage between the phases, and the three poles of the switching device are not fully closed at the same time.
However, it can be said with certainty that the peak surge current of the three-phase transformer is close to the short-circuit current level. One of the interesting features of excitation inrush current is that due to the complete cancellation of the current, there is a large proportion of even harmonics. Even harmonics are rarely encountered in power circuits.
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