The cause of burning of residual current transformer

- Mar 06, 2020-

Current transformer is composed of closed core and winding according to electromagnetic induction principle. The number of winding turns is small. In the circuit that needs to measure the current, there is often all the current flowing through the circuit. The number of secondary winding turns is relatively large. In the measuring instrument and protection circuit, the secondary circuit is closed when the current transformer is working, so the impedance of the series coil of the measuring instrument and protection circuit is very small, and the working state of the current transformer is close to short circuit.
The current transformer can convert the primary current with larger value into the secondary current with smaller value through a certain transformation ratio, which can be used for protection, measurement and other purposes. It is installed in the switch cabinet for connecting ammeter and other instruments and relay protection. It is impossible for each instrument to be connected to the conductor or bus with a large actual value, so it is necessary to convert it into a secondary value with a small value through the transformer, and reflect the primary actual value through the ratio of change.
Working principle of current transformer: the number of turns of the primary winding in series in the circuit under test is very small; the secondary winding is connected with low impedance load such as ammeter and relay current coil, which is nearly short circuited. The primary side current and the secondary side current are determined by the load of the line under test, independent of the secondary side load of the current transformer. When the current transformer is running, the secondary side shall not be open circuited. Because the primary side current becomes the excitation current, it will cause the magnetic flux and the secondary side voltage to exceed the normal value greatly and endanger the safety of people and equipment. Therefore, it is not allowed to connect fuse in the secondary circuit of current transformer, or dismantle ammeter, relay and other equipment without bypass during operation.
The characteristics are as follows: 1. The number of turns of the primary coil in series in the circuit is very small, and the current completely depends on the load current of the circuit under test, independent of the secondary current. 2. The impedance of the current coil of the instrument and relay connected to the secondary coil is very small, and the current transformer operates under the condition of near short circuit.
It is inevitable that residual current transformer will be burnt out in the process of using, and the main reasons for burning out are as follows:
1. The secondary open circuit of the residual current transformer produces high voltage, which makes the residual current transformer burn out.
2. The residual current transformer's service life is too long, the insulation is aging, partial breakdown or discharge occurs, and overvoltage occurs.
3. The contact surface of the primary connection aluminum bar of the residual current transformer is too oxidized, the contact resistance is too large, and the heating causes the current transformer to burn out.
4. The user is overloaded at some time.