Principles For The Use Of Current Transformers

1) The wiring of current transformers should follow the principle of series connection: the primary winding should be connected in series with the tested circuit, while the secondary winding should be connected in series with all instrument loads.
2) Choose the appropriate transformation ratio according to the magnitude of the measured current, 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 entering the low voltage side of the secondary side in case of insulation damage, causing personal and equipment accidents.
3) Open circuit is absolutely not allowed on the secondary side, as once it is opened, all the current I1 on the primary side becomes magnetizing current, causing a sudden increase in φ m and E2, resulting in excessive saturation magnetization of the iron core, severe heating, and even burning of the coil; At the same time, excessive saturation magnetization of the magnetic circuit increases the error. During normal operation, the secondary side of a current transformer is connected 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 magnitude of the CT secondary current 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 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 a high peak wave when the magnetic passes through zero, which can reach thousands or even tens of thousands of volts, endangering the safety of workers and the insulation performance of instruments.
In addition, the open circuit on the secondary side causes the voltage on the secondary side to reach several hundred volts, which can lead to electric shock accidents if touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent an open circuit on the secondary side. During use, if there is an open circuit on the secondary side, the circuit load should be immediately removed, and then the power should be cut off. Everything must be handled before reuse.
4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment, and fault filtering devices, 2-8 secondary winding current transformers are installed in circuits such as generators, transformers, outgoing lines, bus section circuit breakers, bus circuit breakers, and bypass circuit breakers.
5) The installation location of protective current transformers should be set to eliminate the unprotected area of the main protection device as much as possible. For example, if there are two sets of current transformers and their positions allow, they should be placed on both sides of the circuit breaker to ensure that the circuit breaker is within the cross protection range.
6) In order to prevent busbar faults caused by flashover of the bushing of the pillar type current transformer, the current transformer is usually arranged on the outgoing line or transformer side of the circuit breaker.
7) In order to reduce the damage caused by internal faults in the generator, the current transformer used for automatic adjustment of the excitation device should be arranged on the outgoing side of the generator stator winding. In order to facilitate analysis and detect 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.