Common faults inside voltage transformers - Solutions - Huaqiang

(l) For voltage transformers, one common issue arises from an unreasonable design structure, leading to the core-through bolt becoming suspended and causing discharge during operation. This type of failure was more prevalent in the 1980s. However, with improved domestic design standards, modern voltage transformers have significantly reduced such problems. Another frequent fault is poor insulation brackets, followed by issues with end seals and moisture ingress. Table 6 presents the statistics of faults found in 25 voltage transformers through gas analysis in oil. Table 6-10 shows the breakdown of fault causes for 25 voltage transformers.

Fault natureFailure causeNumber Floating potential dischargeCore bolt and iron core connection loose, causing the bolt to be in suspension potential7 Metal foreign matter in suspension potential discharge1 Insulation bracket nut potential suspension1 Arc dischargeCascade winding on iron core discharge, poor insulation support7 Insulated and water damp2 The end of the primary winding is not grounded1 Other unexplained reasons (exit operation is not checked)3 Overheating fault cause is unknown, but large and insulation performance degraded (no internal inspection, shutdown)3

(2) Common faults in current transformers are often linked to manufacturing defects. Here’s a brief overview:

1) The insulation of the current transformer is often too thick, with some layers loosely wrapped, creating wrinkles and air pockets due to poor vacuum treatment or incomplete impregnation. These cavities can lead to partial discharges.

2) The size and placement of the capacitor screen may not meet design specifications. If the capacitive screen is missing, uneven, or misplaced, it can disrupt voltage equalization. When the electric field strength reaches a critical level along the solid insulation surface, partial discharges occur. These discharges can break down insulating oil, generate x-wax between layers, and increase dielectric loss. Over time, this leads to arcing, which becomes detectable through gas analysis in oil.

3) Contamination or high humidity on the green material can cause creeping discharges on its surface. This is especially common in discharges along the pad surface of terminal leads.

4) Loose metal parts or suspended connections can cause spark discharges. For example, a loosened primary winding support nut can cause the shield aluminum foil to become suspended, while poor soldering or broken wires in the last screen lead can also trigger such faults.

5) Loose secondary connection clamps, bolts, nuts, or grounding nuts on the final screen can increase contact resistance, leading to local overheating. In addition, improper on-site maintenance, such as insufficient vacuum degassing or lack of drying, can result in high residual bubbles and moisture in the oil-paper insulation, posing a safety risk.

Table 6-11 provides statistics on the faults of 45 current transformers, serving as a reference only. Table 6-11 lists the causes of faults in 45 current transformers.

Fault natureFailure causeNumber Floating potential dischargePrimary winding support nut loose, causing primary winding shield aluminum platinum potential suspension4 Secondary winding to ground breakdownSecondary open circuit, insulation moisture1 2 Shielding layer breakdownMain screen has a disconnect, less end screen, resulting in uneven potential distribution2 Partial discharge between main screen, paper near end screen, and surface of aluminum platinum has a large amount of X wax, tanδ increases3 U-shaped capacitor core bottom to ground dischargeDiaphragm rupture, seal damage, water damp3 Last screen capacitive screen breakdown or poor grounding to ground discharge, last screen unsoldered, insulation damp4 Other discharge faults (unknown, exit operation, no internal inspection)12 Local overheatingLoose lead fastening nut, tapping nut loose, end screen grounding nut loose14