The working principle of the current transformer is similar to that of a transformer, and its main structure consists of an iron core, a primary winding, a secondary winding, a lead wire and an insulating structure. The difference is that the primary winding turns of the current transformer are few, only a few or even one.
When in use, the primary winding is connected in series in the current circuit under test, and the current passing through the primary winding is the current of the circuit under test, and its magnitude does not depend on current transformers and its secondary load, but on the load state of the circuit under test. The number of turns of the secondary winding is more than that of the primary winding, ranging from a dozen to hundreds of turns, depending on the size of the converter ratio. The secondary side of the current transformer is used in series with the current coil of the measuring instrument and the relay. The impedance of the secondary load is very small, which is close to the short-circuit state and is equivalent to a short-circuit operation transformer.
The main function of the current transformers is to convert large currents into small currents suitable for electrical measuring instruments and relay maintenance. In high-voltage systems, the current transformers isolate the current circuits of electrical instruments and relays from high-voltage conductive circuits to ensure the safety of equipment such as measuring instruments and relays, and to ensure personal safety in the maintenance work of measuring instruments and relays.
The secondary side of the current transformer is not allowed to open circuit. If the secondary side of the current transformer is open, the magnetic flux in the iron core will increase sharply with the increase of the primary current, which will not only cause serious saturation of the iron core, but also induce high voltage on the secondary side, seriously endanger the insulation of the secondary circuit, and even destroy and burn out. At this time, if someone touches the secondary circuit, it is easy to cause electric shock damage.
When the current transformer is open on the secondary side, not only the iron core increases, the secondary side induces a high voltage, but also the iron core is saturated, the change range of the magnetic induction intensity curve increases, and the induced electromotive force on the secondary side produces a high sharp wave, and its voltage amplitude reaches the risk value of 2-3KV. Therefore, the current transformer is not allowed to open circuit twice during operation.
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