Causes and precautions of secondary electrification of current transformerWhen the current transformer (hereinafter referred to as CT) is working, the secondary circuit must not be open circuit, otherwise it will cause high voltage and electrification in the secondary circuit, which may damage the electrical equipment or endanger personal safety. Therefore, in case of electrification once but twice, the power shall be cut off immediately for inspection. The following is a preliminary analysis of the causes of CT secondary electrification, and some problems that should be paid attention to.1. Cause of failure
(1) Due to negligence of work or unclear use principle of CT, the secondary circuit is disconnected for operation or CT secondary line is not installed, so that the secondary circuit is in open circuit state and charged.(2) Since the secondary pile heads (i.e. K1 and K2 pile heads) of CT are not connected properly or in a loose state, the secondary circuit is charged.(3) The secondary circuit is electrified because the pile head of the electrical equipment connected in the secondary circuit of CT is not connected properly or in a loose state (such as electrical instruments, protective relays, electric energy meters, etc.).
(4) The CT secondary is charged due to the disconnection of the secondary conductor. The reasons for disconnection are generally bitten by small animals, aging fracture, mechanical strength tension fracture, etc.2. Cause of wrong wiringThe secondary wrong wiring of CT will cause secondary electrification. The author has encountered two cases:
(1) Two ammeters are connected in series from KL of two-phase CT, as shown in Figure 1. Although there is no secondary open circuit in this connection, when the CT secondary connection flows from phase a of CT to phase C of CT and passes through two CT secondary coils with large impedance, the CT secondary tends to open circuit, so the secondary is charged. And with the increase of load, the secondary voltage appears higher.(2) An ammeter is connected in series from KL of two-phase CT, as shown in Figure 2. There is no open circuit in t secondary in this wrong wiring; However, the principle of electrification is the same as above, and one ammeter has no indication.3. Causes of irregular wiring
The informal wiring in Figure 3 is to introduce the voltage from the CT secondary KL pile head into the voltage and current coil of the electric energy meter to make the current secondary part of the electric energy meter charged. The essence of this informal wiring is to connect the I of CT "L" is connected with KL, and the voltage pile heads L, 4 and 7 of the electric energy meter are respectively connected with the current pile heads 2, 5 and 8. This connection method omits the three voltage secondary lines a, B and C, and uses the CT secondary connection line to introduce the electric energy meter, so the CT secondary is charged with 380 / 220V voltage. Although this connection method has little impact on the measurement, it is extremely nonstandard and unsafe, so it should be eliminated.
4. Cause of insulation damage(1) The insulation is damaged due to overload heating, and the primary voltage jumps into the secondary voltage to make it live.(2) The secondary may also be charged due to poor product quality, insulation aging and other reasons.
(3) Some CT's are located in wet places for a long time, and their insulation is damaged by moisture all the year round, which makes the primary voltage jump into the secondary voltage and make it charged.5 safety problems needing attention in the use of current transformer
1 During the operation of CT, the secondary measurement shall not be open circuit. During the normal operation of CT, because the secondary impedance is very small and close to the short-circuit state, the excitation current of its iron core tends to zero, so the secondary circuit does not carry voltage. However, when the secondary open circuit or the secondary impedance is very large, the excitation current increases, the CT iron core heats due to the significant increase of magnetic flux, and the induction of CT secondary coil is high Voltage endangers personal safety and equipment safety. Therefore, open circuit is never allowed during CT secondary operation. During installation, attention must be paid to reliable wiring, and each connecting pile head shall be tight and firm.
(2) One end of CT secondary measurement must be grounded (generally KZ grounding) to prevent the voltage of primary measurement from channeling to secondary measurement in case of CT primary and secondary coil breakdown.(3) In case of abnormal vibration, noise and heating of CT during operation, it shall be stopped for inspection and treatment.(4) For CT in operation, the secondary line shall not be disconnected at will and work between the secondary lines, otherwise it will cause open circuit danger.
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