Common faults in current transformers are often related to manufacturing defects, specifically:
The insulation of current transformers is often thick, but sometimes the insulation wrapping is loose, with wrinkles between the insulation layers. Combined with poor vacuum treatment and incomplete impregnation, this creates air-containing cavities, making them prone to partial discharge faults.
The size and arrangement of the capacitor bank do not meet design requirements, or even the number of capacitor banks is insufficient. The capacitor plates are not smooth or flat, or even misaligned or broken, thus compromising their voltage equalization characteristics. Therefore, when the electric field strength along the surface of the local solid insulation reaches a certain value, partial discharge will occur.
The direct consequence of the above partial discharge is the cracking of the insulating oil, generating a large amount of wax between the insulation layers, increasing dielectric loss. This discharge has a cumulative effect; if left unchecked, gas analysis in the oil may show characteristics of arc discharge.
Due to unclean or highly moist insulating materials, surface discharge may occur on their surface. This is commonly seen in primary terminal leads discharging along the surface of the pad.
Loose connections or floating potential in metal parts can lead to spark discharge. For example, a loose primary winding support nut can cause floating potential in the primary winding shielding aluminum foil. Poor contact or welding of the end screen leads, or even breakage, can also cause this type of fault.
Loose secondary connection plates, bolts, and nuts, loose end screen grounding nuts, and loose tap fastening nuts can all increase contact resistance, leading to localized overheating faults. Furthermore, improper on-site maintenance and management should also be taken seriously. For example, water ingress into the transformer, while possibly related to the manufacturer's sealing structure and materials, also indicates maintenance issues. Generally, insufficient vacuum degassing on-site or failure to perform vacuum drying during maintenance can lead to easy saturation of dissolved gases in the oil or residual air bubbles and high moisture content in the oil-paper insulation. All of these will create safety hazards for the equipment.