Why can't the current transformer in operation be open? What is the principle?

- Dec 27, 2019-

When the current transformer works normally, the load connected to the secondary is an ammeter or a current meter current coil and a transmitter. The impedance of these coils is very small and basically runs in a short circuit state. In this case, the magnetic flux generated by the primary current and the secondary current of the current transformer cancel each other out, so that the magnetic flux density in the iron core is maintained at a low level, usually at a fraction of a few Tesla (the unit of the magnetic flux density). :T), because the secondary resistance is small, the secondary voltage is also low.

The magnetic flux of the current transformer is canceled under normal operation, and the magnetic flux density is very small.

Current transformer secondary side open circuit

When the secondary winding of the current transformer is open, if the primary current does not change, the secondary circuit is disconnected, or the resistance is large, then the current on the secondary side is 0, or very small, the magnetic flux of the secondary coil or the core It is very small and cannot offset the magnetic flux once. At this time, all the currents become the excitation current, which saturates the core. This change is sudden, called abrupt change, and its magnetic flux density is as high as several Tesla.

Abrupt magnetic flux density, high secondary voltage

The consequences of the secondary open circuit of the current transformer

After this happens, it will have the following consequences:

1. The second generation of several thousand volts (this is not verified, is the theory of copying), high voltage may penetrate the insulation of the current transformer, so that the entire distribution equipment housing is charged, it may also cause the maintenance personnel to get an electric shock, life-threatening .

2. The saturation of the iron core will increase the core loss of the transformer, and the iron core will heat up and damage the transformer.

3. The transformer is saturated with iron core, the measurement is out of alignment, and the CT ratio difference and angular difference are increased.