Principles of use of current transformers
1) The wiring of the current transformer should follow the series connection 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.
2) Select the appropriate transformation ratio according to 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 on the primary side from entering 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 open, the primary side current I1 will all become magnetizing current, causing φm and E2 to increase sharply, resulting in excessive saturation magnetization of the iron core, serious heat generation and even burning of the coil; , which increases the error. 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 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 the circuit is suddenly opened, the excitation electromotive force will suddenly change from a small value to a large value, and the magnetic flux in the iron core will present a severely saturated flat-top wave. Very high peak waves, whose value can reach thousands or even tens of thousands of volts, endanger the safety of workers and the insulation performance of the instrument.
In addition, the open circuit of the secondary side makes the voltage of the secondary side reach several hundreds of volts, which will cause an electric shock accident if touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the secondary side from being open. In the process of use, once the secondary side is open, the circuit load should be removed immediately, and then the power outage should be processed. It can be reused after everything is disposed of.
4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering, etc., all circuits are installed in generators, transformers, outgoing lines, bus sectional circuit breakers, bus circuit breakers, bypass circuit breakers and other circuits. 2 to 8 current transformers with secondary windings.
5) The installation site of the protective current transformer should be set as far as possible to eliminate the non-protection zone of the main protection device. For example, if there are two sets of current transformers, and the location allows, they should be installed on both sides of the circuit breaker, so that the circuit breaker is within the scope of cross protection.
6) In order to prevent the busbar fault caused by the bushing flashover of the pillar-type current transformer, the current transformer is usually arranged on the outgoing line or the transformer side of the circuit breaker.
7) In order to reduce the damage caused by the internal fault of 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 find internal faults before the generator is integrated into the system, the current transformer used for measuring instruments should be installed on the neutral side of the generator.
