In the power generation, substation, transmission, distribution and power lines, the currents vary widely in size from several to several tens of thousands of amps. In order to facilitate measurement, protection, and control, it is necessary to convert to a more uniform current. In addition, the voltage on the line is generally higher. If direct measurement is very dangerous. Current transformers perform current conversion and electrical isolation.
Current transformer use principle:
1) The wiring of the current transformer should comply with the principle of series connection: that is, one winding resistance should be connected in series with the circuit under test, and the second winding resistance should be connected in series with all load of the meter.
2) According to the measured current size, select the appropriate ratio, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the insulation of the primary side from breaking into the secondary low-pressure side when the insulation is damaged, causing personal and equipment accidents.
3) The open circuit is absolutely not allowed on the secondary side. Once once opened, the primary current I1 becomes a magnetizing current, causing a sudden increase in φm and E2, causing the core to oversaturate and magnetize, causing serious heating and even burning the coil; at the same time, the magnetic circuit is over-saturated and magnetized. , increase the error. When the current transformer is in normal operation, the secondary side is used in series with current coils such as measuring instruments and relays. The current coils of the measuring instruments and relays have very low impedance, and the secondary side is similar to a short circuit. The secondary current of the CT 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 opened, the excitation electromotive force is suddenly changed from a very small value to a very large value. The magnetic flux in the iron core shows a severely saturated flat top wave, so the secondary winding will induce the magnetic flux to pass through zero. The high spire wave, whose value can reach thousands or even tens of thousands of volts, endangers the safety of the staff and the insulation performance of the instrument.
In addition, the secondary side of the open circuit makes the secondary voltage reach several hundred volts, once touched will cause electric shock accidents. Therefore, the short circuit switch is provided on the secondary side of the current transformer to prevent the secondary side from opening. In use, once the secondary circuit is opened, the circuit load should be immediately removed, and then the power should be cut off. It can be reused after everything is handled.
4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering devices, they are set in generators, transformers, outlets, bus sectional breakers, bus breakers, and bypass breakers. 2 to 8 secondary winding current transformers.
5) The installation location of the protective current transformer should be set as far as possible to eliminate the unprotected area of ​​the main protection device. For example, if there are two current transformers and the position is allowed, they should be set on both sides of the circuit breaker to make the circuit breaker in the cross protection range.
6) In order to prevent busbar faults caused by the flashover of the pillar-type current transformer bushings, the current transformers are usually arranged on the outgoing or transformer side of the circuit breaker.
7) In order to reduce the damage in the internal fault of the generator, the current transformer for the automatic adjustment of the excitation device shall be arranged on the outlet side of the stator winding of the generator. In order to facilitate analysis and find internal faults before the generator is incorporated into the system, the current transformers used for the measuring instruments should be mounted on the neutral side of the generator.
Current transformer use principle:
1) The wiring of the current transformer should comply with the principle of series connection: that is, one winding resistance should be connected in series with the circuit under test, and the second winding resistance should be connected in series with all load of the meter.
2) According to the measured current size, select the appropriate ratio, otherwise the error will increase. At the same time, one end of the secondary side must be grounded to prevent the insulation of the primary side from breaking into the secondary low-pressure side when the insulation is damaged, causing personal and equipment accidents.
3) The open circuit is absolutely not allowed on the secondary side. Once once opened, the primary current I1 becomes a magnetizing current, causing a sudden increase in φm and E2, causing the core to oversaturate and magnetize, causing serious heating and even burning the coil; at the same time, the magnetic circuit is over-saturated and magnetized. , increase the error. When the current transformer is in normal operation, the secondary side is used in series with current coils such as measuring instruments and relays. The current coils of the measuring instruments and relays have very low impedance, and the secondary side is similar to a short circuit. The secondary current of the CT 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 opened, the excitation electromotive force is suddenly changed from a very small value to a very large value. The magnetic flux in the iron core shows a severely saturated flat top wave, so the secondary winding will induce the magnetic flux to pass through zero. The high spire wave, whose value can reach thousands or even tens of thousands of volts, endangers the safety of the staff and the insulation performance of the instrument.
In addition, the secondary side of the open circuit makes the secondary voltage reach several hundred volts, once touched will cause electric shock accidents. Therefore, the short circuit switch is provided on the secondary side of the current transformer to prevent the secondary side from opening. In use, once the secondary circuit is opened, the circuit load should be immediately removed, and then the power should be cut off. It can be reused after everything is handled.
4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering devices, they are set in generators, transformers, outlets, bus sectional breakers, bus breakers, and bypass breakers. 2 to 8 secondary winding current transformers.
5) The installation location of the protective current transformer should be set as far as possible to eliminate the unprotected area of ​​the main protection device. For example, if there are two current transformers and the position is allowed, they should be set on both sides of the circuit breaker to make the circuit breaker in the cross protection range.
6) In order to prevent busbar faults caused by the flashover of the pillar-type current transformer bushings, the current transformers are usually arranged on the outgoing or transformer side of the circuit breaker.
7) In order to reduce the damage in the internal fault of the generator, the current transformer for the automatic adjustment of the excitation device shall be arranged on the outlet side of the stator winding of the generator. In order to facilitate analysis and find internal faults before the generator is incorporated into the system, the current transformers used for the measuring instruments should be mounted on the neutral side of the generator.
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