Why can't the voltage transformer be short-circuited? Why can't the current transformer open?

- Jul 19, 2019-

When the current transformer is in normal operation, the magnetomotive force generated by the secondary current demagnetizes the magnetic flux potential generated by the primary current, the excitation current is very small, the total magnetic flux in the iron core is small, and the induced electromotive force of the secondary winding Not more than tens of volts. If the secondary side is open, the demagnetization of the secondary current disappears, and the primary current completely changes to the excitation current, causing the magnetic flux in the core to increase sharply, the iron core is in a highly saturated state, and the number of turns of the secondary winding is large, according to The law of electromagnetic induction is = 4.44/fNB, which will generate a very high voltage (even thousands of volts) across the secondary winding, which may not only damage the insulation of the secondary winding, but also seriously endanger personal safety. Moreover, due to the sharp increase in the magnetic induction strength, the core loss is increased, the heat is severely generated, and even the insulation is burned out. Therefore, the secondary side of the current transformer is absolutely not allowed.

The secondary side of the current transformer is absolutely not allowed to open, because once the circuit is open, the primary current I1 becomes a magnetizing current, causing a sudden increase in φm and E2, causing excessive saturation magnetization of the core, causing severe heat or even burning the coil; at the same time, excessive saturation magnetization of the magnetic circuit After that, the error is increased. When the current transformer is in normal operation, the secondary side approximates the short circuit. If it is suddenly opened, the excitation electromotive force suddenly changes to a large value from a small value, and the magnetic flux in the iron core presents a severely saturated flat top. Wave, so the secondary winding will induce a very high apex wave when the magnetic flux passes through zero, the value can reach thousands or even tens of volts, the safety of the crisis workers and the insulation performance of the instrument.

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