What is the principle of use of open transformer

The wiring of the open transformer should follow the series principle: that is, the primary winding should be connected in series with the circuit under test, and the secondary winding should be connected in series with all instrument loads. According to the measured current, select the appropriate transformation ratio, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the primary side high voltage from entering the secondary low voltage side once the insulation is damaged, causing an accident.

The secondary side of the open transformer is excellent and does not allow an open circuit. Once open, the primary current I1 will all become the magnetizing current, causing φm and E2 to increase sharply, causing the core to be over-saturated and magnetized, causing serious heating and even burning the coil; at the same time, the magnetic circuit is over-saturated and magnetized. After that, the error increases. When the current transformer is working normally, the secondary side is used in series with current coils such as measuring instruments and relays. The impedance of the current coils such as measuring instruments and relays is very small, and the secondary side is similar to a short circuit.

The magnitude of the secondary current of the open transformer is determined by the primary current, and the magnetic potential generated by the secondary current balances the magnetic potential of the primary current. If it is suddenly opened, the excitation electromotive force will suddenly change from a very small value to a very large value. The magnetic flux in the iron core presents a severely saturated flat-top wave, so the secondary winding will induce when the magnetic passes through zero. The very high peak wave, its value can reach thousands or even tens of thousands of volts, endangering the safety of workers and the insulation performance of the instrument.

In addition, an open circuit on the secondary side causes the secondary side voltage to reach hundreds of volts, which will cause an electric shock if touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side from opening. During use, once the secondary side is open, the circuit load should be removed immediately, and then the power failure should be dealt with. It can be reused after everything is handled.

In order to prevent the bushing flashover of the pillar current transformer from causing a bus failure, the current transformer is usually arranged on the outgoing line or transformer side of the circuit breaker. In order to reduce the damage caused by the internal fault of the generator, the current transformer used for the automatic adjustment of the excitation device should be arranged on the outlet side of the generator stator winding. In order to facilitate analysis and find internal faults before the generator is incorporated into the system, the current transformer used for the measuring instrument should be installed on the neutral side of the generator.