Dry-Type Transformer Insulation Degradation: Root-Cause Analysis & Solutions

Principal Causes of Insulation Degradation

• Moisture Ingress & Condensation：The most common cause. The winding body is exposed to air; when humidity is high, water vapour condenses on winding surfaces, significantly reducing insulation performance. This is particularly prominent in southern coastal areas and during rainy seasons.
• Dust & Contamination Accumulation：Dry-type transformers rely on air cooling with multiple ventilation ducts. After long-term operation, dust and metal particles adhere to winding surfaces. When combined with moisture, they form conductive paths, drastically reducing insulation resistance.
• Thermal Ageing：Prolonged high temperature causes epoxy resin and other insulation materials to crack, embrittle, and carbonise. According to the thermal ageing law of insulation: for Class F insulation, every 6 °C rise in temperature halves the insulation life; for Class A, every 8 °C rise halves the life.
• Electrical Ageing & Partial Discharge：Micro-voids in resin castings are prone to partial discharge, which continuously erodes the insulation structure, eventually leading to breakdown.
• Terminal Loosening or Oxidation：Loose bolts cause local overheating, charring surrounding insulation; copper busbar oxidation increases contact resistance, generating high temperatures that damage insulation.
• Manufacturing Quality Defects：Insufficient purity of insulation materials, voids or bubbles in resin casting — these create hidden risks for future insulation degradation.
• Mechanical Damage：Collisions and scratches during transport and installation, or long-term electromagnetic forces causing insulation abrasion.
• Core Insulation Issues：Core multi-point grounding, degraded core coating, damp core insulation boards — all reduce overall insulation resistance.

Solutions & Treatment Measures

1. Cleaning & Drying Treatment

• Surface Cleaning — Remove dust and contamination with dry cloth or vacuum cleaner; washing with water is strictly prohibited.
• Light Moisture — Bake windings with incandescent or infrared lamps (approximately 0.5 m from windings), with fans running for ventilation, for several consecutive days.
• Moderate Moisture (insulation resistance low but not completely failed) — Use the no-load drying method: high-voltage side open-circuited, low-voltage side fed with 380 V mains power; bake until insulation resistance recovers to qualified value before returning to service.
• Severe Moisture — Use the short-circuit method: low-voltage side shorted, high-voltage side fed with three-phase power not exceeding impedance voltage; must be performed by qualified personnel.

2. Improve Operating Environment

• Ensure good substation ventilation; ambient temperature not exceeding 40 °C, relative humidity not exceeding 85%.
• In humid environments, install dehumidifiers or space heaters to prevent condensation.
• In dusty environments, install filtration devices and perform regular cleaning.

3. Strengthen Temperature Control & Load Management

• Ensure temperature controllers and fans operate normally: fans start at 100 °C, alarm at 130 °C, trip at 150 °C.
• Avoid long-term overload; recommended load factor ≤ 85%.

4. Tightening & Inspection

• Regularly use a torque wrench to tighten terminal bolts; check for oxidation, discoloration, or burn marks.
• Check core grounding reliability; eliminate multi-point grounding faults.

4. Periodic Testing & Prevention

• Use a 2500 V megohmmeter to measure winding-to-ground insulation resistance; calculate absorption ratio (R60/R15 ≥ 1.3).
• Disconnect three-phase connecting copper bars to test each phase separately and identify the phase with degraded insulation.
• Where conditions permit, conduct partial discharge testing.
• Perform regular comprehensive maintenance (dust removal, tightening, temperature controller verification, fan maintenance); regular DC resistance and turns-ratio tests.