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Why can't the current transformer in operation be open circuit? What is the principle? What are the adverse effects of an open circuit?
Most welders often hear such a sentence: "the secondary side of current transformer shall not be open circuit, and the secondary side of voltage transformer shall not be short circuit fault". At work, everyone will treat this as the Koran. When disassembling the secondary line of the current transformer, it is very appropriate to add a short rotating piece or short wiring to connect the secondary side, and then carry out surgical wire removal or wiring, so as to ensure the safety of the person. When I removed the current line of the electricity meter before, I encountered the charging and discharging conditions of loose wiring and crackling at the wiring terminal. Then why can't the secondary current transformer be open circuit? What is the principle? What adverse effects will it cause? Below is a careful analysis of everyone. I hope everyone can understand.
When the current transformer is working normally, the load connected to the secondary coil is ammeter or ammeter current electromagnetic coil and its intelligent transmitter. The characteristic impedance of this electromagnetic coil is small, and most of it operates in the case of short-circuit fault. Under such conditions, the magnetic flux caused by the primary current of the current transformer and the secondary coil current offset each other, so that the relative density of the magnetic flux in the iron core is maintained at a moderate level, which is generally a few tenths of Tesla (the unit of magnetic flux density: T). Because the secondary coil resistor is small, the working voltage of the secondary coil is also very low.
Under all normal operating conditions, the magnetic flux of current transformer is offset, and the relative density of magnetic flux is small. When the secondary winding of the current transformer is open circuit, if the primary current does not change, the secondary circuit is broken, or the resistor is very large, then the current on the secondary side is 0, or very small, and the magnetic flux of the secondary electromagnetic coil or iron core is not large, which cannot offset the primary magnetic flux. At this moment, all the primary currents become self induced electromotive force, making the iron core saturated. This change is sudden, called sudden change. Its relative magnetic flux density reaches many Tesla cars.
Adverse effect of secondary open circuit of current transformer
When this kind of situation occurs, it will cause the following adverse effects:
1. It will cause thousands of volts of working voltage for the second time (this is a basic theory that has not been certified, which is copied). The high voltage will penetrate the insulation layer of the current transformer and make all the enclosures of the substation equipment inductively energized. It will also cause an electric shock accident for the maintenance staff, which is life-threatening.
2. The sudden change of the saturation state of the iron core will increase the loss of the iron core of the voltage transformer, and the iron core will be hot and destroy the voltage transformer.
3. When the voltage transformer is saturated, the measurement verification is inaccurate, and the CT ratio difference and angle difference increase.
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