Application of ct transformer

From the wire with large current, a small current can be induced in a certain proportion for measurement, and it can also provide power for relay protection and automatic devices.   Example: For example, there is a very thick cable with a very large current. If you want to measure its current, you need to disconnect the cable and connect the ammeter in series in this circuit. Because it is very thick and the current is very large, a large ammeter is needed. But in fact, there is no such large ammeter, because the specifications of the current meter are all below 5A. What should I do? At this time, you need to use a current transformer.   First select a suitable current transformer, and then pass the cable through the current transformer. At this time, the current transformer will induce current from the cable, and the magnitude of the induced current is just reduced by a certain multiple. Send the induced current to the instrument for measurement, and then multiply the measured result by a certain multiple to get the real result.   For example, now you need to measure the current magnitude of a cable. First, pass a cable through a 500/5 current transformer (500/5 is actually 100 times), then connect the current transformer to the ammeter, and the ammeter measures 4A. From this, we can calculate that the real current of the cable is 4*100=400A.   Wiring diagram: Some friends may say that a clamp meter can also achieve this goal. In fact, there is a current transformer inside the clamp meter, and the principle is similar.   Use: 1) The wiring of the current transformer should comply with the series principle: that is, the primary winding should be connected in series with the circuit to be measured, and the secondary winding should be connected in series with all instrument loads 2) According to the size of the measured current, select the appropriate change, 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 personal and equipment accidents 3) The secondary side is absolutely not allowed to be open-circuited, because once the circuit is open, the primary side current I1 will all become magnetizing current, causing φm and E2 to increase sharply, causing the core to be over-saturated and magnetized, causing serious heat and even burning the coil; at the same time, after the magnetic circuit is over-saturated and magnetized, the error increases. When the current transformer is working normally, the secondary side is similar to a short circuit. If it is suddenly opened, the excitation electromotive force will suddenly change from a very small value to a very large value, and the magnetic flux in the core will present a severely saturated flat-top wave. Therefore, the secondary winding will induce a very high peak wave when the magnetic field passes through zero, and its value can reach thousands or even tens of thousands of volts, which will endanger the safety of the staff and the insulation performance of the instrument.In addition, the secondary side open circuit makes E2 reach hundreds of volts, which will cause electric shock accidents once touched. Therefore, the secondary side of the current transformer is equipped with a short-circuit switch to prevent the primary side from opening. As shown in Figure 1, K0, during use, once the secondary side is open, the circuit load should be removed immediately, and then the vehicle should be stopped for processing. It can be used again after everything is processed. 4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault recording devices, current transformers with 2 to 8 secondary windings are installed in the generator, transformer, outgoing line, busbar section circuit breaker, bus tie circuit breaker, bypass circuit breaker and other circuits. For large current grounding systems, it is generally configured in three phases; for small current grounding systems, it is configured in two or three phases according to specific requirements 5) The installation location of the protective current transformer should be set to eliminate the non-protected area of ​​the main protection device as much as possible. For example: if there are two sets of current transformers, and the position allows, they should be installed on both sides of the circuit breaker so that the circuit breaker is within the cross protection range 6) In order to prevent the flashover of the pillar current transformer bushing from causing busbar failure, 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 of the generator, the current transformer used for automatic adjustment of the excitation device should be arranged on the outgoing line side of the generator stator winding. In order to facilitate analysis and find internal faults before the generator is connected to the system, the current transformer used for measuring instruments should be installed on the neutral point side of the generator.   Principle: In the power supply and electricity lines, the current and voltage vary greatly from a few amperes to tens of thousands of amperes. In order to facilitate the measurement of secondary instruments, it needs to be converted into a relatively uniform current. In addition, the voltage on the line is relatively high. If it is measured directly, it is very dangerous. The current transformer plays the role of current conversion and electrical isolation. Earlier, most of the display instruments were pointer-type current and voltage meters, so most of the secondary currents of the current transformers were ampere-level (such as 5A, etc.).   The current ratio of the primary winding current I1 to the secondary winding current I2 of the current transformer is called the actual current ratio K. The current ratio of the current transformer when working at the rated working current is called the rated current ratio of the current transformer, expressed as Kn.   Kn=I1n/I2n I. What are the common faults of the current transformer? How to judge and deal with it? What items should be checked during normal inspections. A. Common faults of current transformers are: 1. The secondary side of the current transformer is open 2. The current transformer is overheated during operation 3. Smoke or odor inside the current transformer 4. The screws of the current transformer coil are loose, and there is a short circuit between turns or layers 5. Discharge inside the current transformer, abnormal sound or discharge sparks between the lead and the shell 6. The oil-filled current transformer has serious oil leakage or the oil level is too low   B. Usually, judgment and treatment should be carried out according to the abnormal phenomenon that occurs. For example, use a temperature test wax sheet to check the heating condition, and judge whether it is open circuit according to the sound and meter indication value. Once a fault is found, it should be repaired or replaced immediately. The items of normal patrol inspection generally include:   1. Check for overheating and abnormal odor 2. Regularly check the insulation condition 3. Check whether the three-phase indication value of the ammeter is within the allowable range and whether it is overloaded 4. Whether the porcelain part is clean and complete, whether there is damage and discharge 5. Whether the oil level of the oil-filled current transformer is normal, and whether there is oil leakage 2. Why is the secondary side of the current transformer not allowed to be open circuit? What are the dangers after opening the circuit?   1. Usually, the primary current of the current transformer has nothing to do with the current of the secondary load. When the current transformer is operating normally, since the secondary side impedance is very small (close to the short circuit state), most of the magnetic lines of force generated by the primary current are compensated by the secondary current, the total magnetic flux density is not large, and the secondary side potential is not high. But when the secondary circuit is open, the secondary current is equal to zero, and the primary current completely becomes the excitation current, generating a very high potential in the secondary coil (the peak value can reach several thousand volts or even higher), which may not only damage the secondary insulation, but also threaten personal safety. In addition, excessive increase in the core magnetic flux density may also cause the core to overheat and be damaged.   III. Hazards of long-term overload of current transformer Once the current transformer is overloaded for a long time, the core magnetic flux density will reach saturation, increase the error of the current transformer, and the meter indication will be incorrect, so it is not easy to grasp the actual load or operation. In addition, due to the increase in magnetic flux density, the core and secondary coil will overheat and the insulation will be damaged.   IV. Signs and treatment of the secondary side of the current transformer open circuit: When the secondary side of the current transformer is open, it is often accompanied by the following phenomena:   1. The ammeter and power meter indicate zero, the electric meter does not rotate, and a buzzing sound is emitted. 2. The current transformer itself has a squeaking discharge sound or other abnormal sounds, and the terminal block may be burnt.   When the current transformer is open-circuited, the potential generated is related to the primary current. Therefore, when dealing with the open-circuit fault of the current transformer, it is necessary to reduce the load or make the load zero, and then use insulating tools to handle it. The corresponding protection device should be disabled during the handling.