The residual current transformer can convert the primary current with a large value to a secondary current with a small value through a certain transformation ratio, which is used for protection and measurement purposes. Installed in the switchgear in order to connect instruments such as ammeter and relay protection. Each meter cannot be connected to a wire or bus with a large actual value, so it must be converted into a secondary value with a smaller value by a transformer, and the actual value of the primary is reflected by the transformation ratio.
Residual current transformer working principle: the primary winding is connected in series in the circuit under test, and the number of turns is very small; the secondary winding is connected to a low impedance load such as ammeter and relay current coil, which is approximately short circuited. The primary current and secondary current depend on the load of the line under test and have nothing to do with the load on the secondary side of the residual current transformer. When the residual current transformer is running, the secondary side cannot be opened. Because the primary current becomes the excitation current, the magnetic flux and the secondary voltage will greatly exceed the normal value and endanger personal and equipment safety. Therefore, it is not allowed to connect the fuse in the secondary circuit of the residual current transformer, and it is not allowed to disassemble the ammeter and relay and other equipment without bypass during operation.
The characteristics of are: 1 The primary coil is connected in series in the circuit with few turns, the current is completely dependent on the load current of the circuit under test, and has nothing to do with secondary current. 2 The current coil impedance of the instrument and relay connected to the secondary coil is very small, and the residual current transformer operates in a near short-circuit state.
Residual current transformers are inevitably burned out during use, and the main causes of burnout are also caused by the following points:
1. The residual current transformer is opened for a second time, which generates a high voltage and burns the residual current transformer.
2. The service life of the residual current transformer is too long. The insulation is aging, and breakdown or discharge occurs locally, resulting in overvoltage.
3. The contact surface of the residual current transformer once connected to the aluminum row is too oxidized, the contact resistance is too large, and the residual current transformer is burned due to heat.
4. The user is overloaded at certain times.