Common faults inside voltage transformers - Solutions - Huaqiang

(l) For voltage transformers, one common issue is related to the design structure, which can lead to the core-through bolt becoming suspended and causing discharge during operation. This type of failure was more prevalent in the 1980s. However, with advancements in domestic design improvements, modern voltage transformers have significantly reduced such issues. Another frequent problem involves poor insulation brackets, followed by issues with end seals and moisture ingress. Table 6 presents a summary of faults detected in 25 voltage transformers through gas analysis in oil. Table 6-10 provides detailed statistics on the causes of these faults. Fault natureFailure causeNumber Floating potential dischargeCore bolt and iron core connection loose, causing the bolt to be in suspension potential7 Metals foreign matter in suspension potential discharge1 Insulation bracket nut potential suspension1 Arc discharge cascade 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 current transformers can be too thick, with some layers loosely wrapped, leading to wrinkles between them. Poor vacuum treatment and incomplete impregnation may result in gas-filled cavities, increasing the risk of partial discharges. 2) Improper size or arrangement of the capacitor screen can affect voltage equalization. If the capacitive screen is missing, uneven, or broken, it can lead to local electric field concentration, causing partial discharges. These discharges can degrade the insulating oil, increase dielectric loss, and eventually lead to arcing as seen in gas analysis. 3) Contamination or high humidity in green material can cause surface creeping discharge, especially along the pad’s surface. 4) Loose metal parts or suspended connections may cause spark discharges. For example, a loosened primary winding support nut could lead to the shield aluminum foil being suspended, while poor soldering or broken wires in the last screen lead could also cause similar issues. 5) Loose secondary connection clamps, bolts, or nuts, as well as a loose grounding nut on the final screen, can increase contact resistance and lead to overheating failures. In addition, improper on-site maintenance practices should not be overlooked. For instance, if the transformer gets wet, it may be due to both sealing issues from the manufacturer and inadequate maintenance. Insufficient vacuum degassing or drying during maintenance can leave residual bubbles and moisture in the oil-paper insulation, posing long-term safety risks. Table 6-11 shows the fault statistics for 45 current transformers, serving as a reference only. Table 6-11 provides detailed data on the causes of these faults.

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 the main screen, the paper near the end screen and the surface of the aluminum platinum has a large amount of X wax, tana increases the apparent3 U-shaped capacitor core bottom to the ground dischargeDiaphragm rupture, seal damage, water damp3 Last screen capacitive screen breakdown or poor grounding to the ground discharge, the last screen is unsoldered, the insulation is damp.4 Other discharge faults are unknown (exit operation, no internal inspection)12 Local overheatingOnce the lead fastening nut is loose, the tapping nut is loose, and the end screen grounding nut is loose14