Problems and Common Fault Analysis of Current Transformers During Operation

1. What problems should be paid attention to during the operation of the current transformers?

1) The secondary side of the current transformer must not be opened during operation. Once the secondary side is open, it will burn out due to excessive iron loss and high temperature, or the secondary winding voltage will rise and the insulation will be broken down, resulting in the risk of high-voltage electric shock. Therefore, when exchanging meters, such as exchanging ammeters, active meters, reactive meters, etc., the current loop should be short-circuited first, and then the measuring instruments should be exchanged. When the meter is adjusted, first connect it to the secondary circuit, then remove the short-circuit and check whether the meter is normal.

2) If sparks are found when the short circuit is removed, and the current transformer E is open at this time, it should be re-shorted immediately, and the short circuit can be removed only when it is confirmed that there is no open circuit in the circuit of the metering instrument. When removing the short circuit of the current transformer, stand on the insulating pad, and consider the maintenance and installation of the current transformer circuit. After the work is completed, the maintenance and installation can be put into operation.

3) If the current transformers have a humming sound, check whether the internal dead center is loose, and the dead center bolt can be tightened. At one end of the secondary side of the current transformer, the shell should be firmly grounded.

4) When the insulation resistance of the secondary side coil of the current transformer is lower than 10~20 megohms, the drying treatment must be stopped to restore the insulation before use.

2. Common faults of current transformers are often related to manufacturing shortcomings, as detailed below:

1) The insulations of current transformers are very thick, some of the insulation is loosely wrapped, and there are wrinkles between the insulation layers. In addition, the vacuum treatment is poor, and the impregnation is incomplete, resulting in an air-containing cavity, which is easy to cause partial discharge failure.

2) The size and arrangement of the capacitive screen do not meet the design requirements, and even less capacitive screens are discharged, and the capacitive plates are not smooth and flat, or even dislocated or broken, which destroys the voltage equalization characteristics. Therefore, when the electric field strength along the surface of the local solid insulation reaches a certain value, partial discharge will be caused.

The direct consequence of the above partial discharge is to crack the insulating oil, generate a large amount of x-wax between the insulating layers, and increase the dielectric loss. This discharge has a cumulative effect, and if it continues to develop, the gas in oil analysis may have the characteristics of arc discharge.

3) Creeping discharge may occur on the surface of the insulating material due to uncleanness or high moisture content. This situation is more common when the terminal lead discharges along the surface of the pad.

4) Some loose connections or potential suspension of metal parts will lead to spark discharge. For example, the looseness of the primary winding support nut will cause the potential suspension of the shielding aluminum foil of the primary winding.

5) The loose connection of the splint, bolts and nuts at one time, the loose grounding nut of the last screen, the looseness of the tap fastening nut all may increase the contact resistance, resulting in local overheating failure.

In addition, improper on-site maintenance management should also be paid attention to. For example, the water inflows of current transformers are damp, although it may be related to the sealing structure and sealing material of the manufacturer, but there are also maintenance management problems. Generally speaking, the on-site vacuum degassing is not sufficient or the vacuum drying is not carried out during the maintenance, resulting in easy saturation of the dissolved gas in the oil or high residual air bubbles and moisture content in the oil-paper insulation. All of these will leave a security risk to the device.

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