1. 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, while the secondary winding should be connected 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 low voltage on the secondary 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 magnetizing current, causing ∅m and E2 to increase sharply, resulting in excessive saturation magnetization of the iron core, serious heat generation and even burning of the coil; at the same time, the magnetic circuit is excessive After saturation magnetization, the error increases. 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 secondary current of the current transformer 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 suddenly change from a small value to a large value, and the magnetic flux in the iron core will show a flat top wave with severe saturation. The high peak wave, whose value can reach several or even tens of thousands of volts, endanger the safety of the staff 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 hundred volts, which will cause an electric shock accident if touched. Therefore, the secondary side of the current ear sensor 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.
(4) In order to meet the needs of measuring instruments, relay protection, circuit breaker failure judgment and fault filtering and other devices, all circuits such as generators, transformers, outgoing lines, bus sectional circuit breakers, bus circuit breakers, and bypass circuit breakers are used. Set up current transformers with 2 to 8 secondary windings.
(5) The installation site of the protective current transformer should be set according to the non-protection zone of the main protection device as far as possible. 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 the 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.
2. Wiring mode of current transformer
The wiring mode of the current transformer is determined according to the operation requirements of the load it is connected to. The most common wiring methods are single-phase, three-phase star and incomplete star.
3. Calibration methods of current transformers
Tests such as checking polarity and demagnetization are usually required before conducting a current transformer error test.
(1) Polarity check.
The primary windings of the current transformer are marked as P1 and P2, and the secondary windings are marked as S1 and S2. If P1 and S1 are the same name terminals, this kind of sign is called minus polarity. The primary current goes in from P1, and the secondary current goes out from S1. Polarity check is simple, in addition to checking on a transformer calibrator, a DC check method can also be used.
(2) Demagnetization check.
In the case of a sudden drop in current of the current transformer, the core of the transformer may produce residual magnetism, such as the sudden cut off of the power supply of the current transformer under the condition of high current, the sudden open circuit of the secondary winding, etc. The core of the transformer has residual magnetism, which reduces the magnetic permeability of the core and affects the performance of the transformer. Transformers should be demagnetized after long-term use. The transformer should also be demagnetized before inspection.
Demagnetization is to give the iron core an alternating magnetic field with an alternating excitation current through the primary or secondary winding. Gradually increase the alternating magnetic field (excitation current) from 0 to make the iron core reach saturation, and then slowly reduce the excitation current to zero to eliminate residual magnetism.
For the demagnetization of the current transformer, the primary winding is open, and the secondary winding is supplied with power frequency current, which gradually increases from zero to a certain current value (this current value is related to the design measurement upper limit of the transformer, generally 20% of the rated current~ About 50%. If the current suddenly increases sharply, it means that the iron core has entered the magnetic saturation stage). Then slowly reduce the current to zero, repeat 2~3 times. Before disconnecting the power supply, the primary winding should be short-circuited before disconnecting the power supply, and the demagnetization of the iron core is completed. This method is called the open-circuit demagnetization method. For some current transformers, the number of gates of the secondary winding is relatively large. If the open-circuit demagnetization method is used, the open-circuit winding may generate high voltage. Therefore, a larger resistance (10 to 20 times the rated impedance) can be connected to the secondary winding. The primary winding is energized with current, and it gradually changes from zero to the maximum current allowed by the primary winding of the transformer, and then gradually changes to zero, and so on 2~3 times. Since the iron core connected to the load may not be completely demagnetized, the maximum current of the primary winding is limited. If it is too large, the primary winding may be burned out. If the voltage generated by the secondary winding connected to the load is not too high, the load resistance of the secondary winding can be increased, which can improve the demagnetization effect.
(3) Accuracy check.
The transformer error test generally uses the tested transformer to compare with the standard transformer. The difference between the secondary currents of the two transformers is the tested transformer error. This test method is called the comparison method. The standard transformer is required to be two grades higher than the tested transformer, and the error of the standard transformer can be ignored at this time. If the standard transformer is only one level higher than the tested transformer, the error of the test result should be considered plus the standard transformer error.
The secondary current difference between the tested transformer and the standard transformer is generally measured by a transformer calibrator, and the ratio difference fx (%) and phase difference δx (‘) are directly read from the transformer calibrator. The calibrator measures the ratio of the current difference between the tested transformer and the standard transformer to the secondary current, so the requirements for the transformer calibrator are not high. What level of transformer is to be verified is basically determined by the standard transformer.
4. Precautions for daily inspection of current transformers
(1) Whether the foundation of the current transformer has cracks, subsidence or collapse.
(2) Whether the supporting steel frame is deformed, corroded, cracked, etc.
(3) Whether the current transformer number plate and safety sign plate are missing, falling off, or the font is seriously blurred and unrecognizable.
(4) Whether the current transformer is installed and fixed loose or skewed, and whether the composite material or porcelain surface is dirty, damaged, corona and discharge traces.
(5) Whether the paint layer on the outside of the device is dirty, peeling and peeling.
(6) Whether there is abnormal vibration and abnormal noise in the current transformer.
(7) Whether the primary lead has loose strands, excessive wind swing, and whether the joint has poor contact, heat, discharge, and hanging debris.
(8) Whether there is virtual connection, falling off, fracture, corrosion, etc.
(9) Whether the oil level of the oil level indicator is abnormal, whether the seal is bad, whether the oil color is turbid, and whether there is oil leakage.
(10) Whether there is any abnormality in the position of the metal expander and whether there is oil leakage.
(11) Whether the explosion-proof membrane is broken.
(12) Whether the secondary circuit of the terminal box is poorly grounded at one point, whether the connection terminals are not fastened, whether there is overheating, ignition, and whether the secondary wires and cables are corroded and damaged.
