Principles for using current transformer
1) The wiring of the current transformer should comply with the series connection principle [8]: 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
2) Select the appropriate transformation ratio according to the current being measured, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the high voltage from the primary side from flowing into the secondary low voltage side once the insulation is damaged, causing personal and equipment accidents.
3) The secondary side is absolutely not allowed to open circuit, because once the circuit is opened, the primary side current I1 will all become the magnetizing current, causing a sudden increase in φm and E2, causing the core to be oversaturated and magnetized, causing serious heating and even burning the coil; at the same time, after the magnetic circuit is oversaturated and magnetized , causing the error to increase. When a current transformer is working normally, the secondary side is used in series with current coils such as measuring instruments and relays. The impedance of current coils such as measuring instruments and relays is very small, and the secondary side is approximately short-circuited. The size of the CT secondary current is determined by the primary current. The magnetic potential generated by the secondary current balances the magnetic potential of the primary current. If it is suddenly open-circuited, the excitation electromotive force will suddenly change from a very small value to a very large value, and the magnetic flux in the iron core will show a severely saturated flat-top wave. Therefore, the secondary winding will induce an electromagnetic wave when the magnetic field passes through zero. The value of very high peak waves can reach thousands or even tens of thousands of volts, endangering the safety of workers and the insulation performance of instruments.
In addition, an open circuit on the secondary side will cause the voltage on the secondary side to reach several hundred volts. Once touched, it will cause an electric shock accident. 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 should be cut off. It can be used again after everything has been taken care of.
4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering devices, circuit breakers are installed in generators, transformers, outlet lines, bus section circuit breakers, bus circuit breakers, bypass circuit breakers and other circuits. Current transformer with 2 to 8 secondary windings.
5) The installation location of the protective current transformer should be set up to eliminate the non-protection zone of the main protection device as much as possible. For example: if there are two sets of current transformers, and if the location permits, they should be placed on both sides of the circuit breaker so that the circuit breaker is within the cross protection range.
6) In order to prevent the busbar failure caused by the flashover of the pillar-type current transformer bushing, the current transformer is usually arranged at the outlet of the circuit breaker or the transformer side
7) In order to reduce damage when the generator internal fault occurs, the current transformer used for automatically adjusting 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 integrated into the system, the current transformer used for measuring instruments should be installed on the neutral point side of the generator.
