Fault Risks & Protection Measures for Dry-Type Transformers in High-Humidity & Condensation Environments

A dry-type transformer's end of life often does not begin at the moment of short-circuit — it begins the moment the first droplet of condensation settles on the winding surface.

Common Problems

• Insulation Performance Degradation (Core Issue):Condensation forms a conductive water film on winding surfaces, causing insulation resistance to plummet. This is especially severe during the southern plum-rain season, where insulation resistance can drop from normal levels to far below qualification standards.
• Winding Surface Tracking & Partial Discharge:Once insulation degrades, the water film increases surface leakage current, producing abnormal discharge sounds. Sustained discharge carbonises the insulation layer, ultimately leading to inter-turn or phase-to-phase short-circuits; in severe cases, the transformer burns out completely.
• Metal Component Corrosion:Condensation accelerates oxidation and rusting of busbars, terminal connections, and fasteners. Increased contact resistance triggers local overheating, creating a vicious cycle of "corrosion → overheating → accelerated corrosion"; in severe cases, bolts rust solid and become impossible to service.
• Synergistic Deterioration by Dust & Moisture:In underground environments, dust (especially metallic dust) adheres to windings and combines with moisture to form conductive paths, drastically increasing tracking risk.
• Core Rusting:Dry-type transformer cores are exposed to air and rust easily in high humidity, increasing no-load losses, reducing efficiency, and shortening service life.

Systematic Protection Measures

Dry-Type distribution transformer

1. Environmental Humidity Control (Fundamental Measure)

• Continuous Dehumidification: During the plum-rain season and high-humidity periods, industrial dehumidifiers should operate continuously to maintain relative humidity within a safe, condensation-free range. Dehumidifier sizing must be calculated based on substation volume, sealing condition, and local climate.
• Cabinet-Mounted Constant-Temperature Dehumidification Heaters: Install heaters inside the transformer cabinet to automatically maintain internal temperature above the dew point, suppressing condensation at the source.
• Online Temperature-Humidity Monitoring: Deploy sensors interlocked with dehumidification equipment; set humidity thresholds for automatic start/stop, eliminating human response lag.

2. Equipment-Body Protection

• Pre-Commissioning Drying & Insulation Testing: Newly installed or long-idle dry-type transformers must undergo drying treatment before energisation, with megohmmeter insulation resistance testing; only pass-fail units may be energised.
• Periodic Moisture-Drive for Standby Units: Standby transformers should be periodically energised no-load, using self-generated heat to drive off moisture.
• High-IP-Rated Enclosures : For tunnels, underground utility corridors, and similar environments, select enclosures with high protection ratings; ensure both enclosure sealing and terminal protection meet requirements.
• Stainless-Steel Fasteners: In humid environments, replace terminal bolts, base fasteners, and similar hardware with stainless steel to eliminate rust at the source.
• Moisture-Resistant Transformer Selection: Some manufacturers offer specialised models with moisture-resistant winding coatings or high-IP enclosures, suitable for underground environments; prioritise these during selection.

3. Substation Civil Works & Ventilation

• Seal Moisture Ingress Paths: Seal cable wall-penetrations with fire-rated sealant to block underground moisture entering along cable routes; fit cabinet doors with sealing gaskets.
• Reasoned Ventilation: Maintain normal ventilation system operation; however, when outdoor humidity is extremely high, blindly introducing outside air worsens condensation — prioritise dehumidification equipment instead.
• Drainage System Maintenance: Regularly inspect cable-trench drainage pumps and floor drains to prevent water backflow.

4. Routine O&M & Testing

• Increased Inspection Frequency: During humid seasons, focus on checking winding surfaces for condensation and discharge traces, listening for abnormal discharge sounds, and using infrared thermometers to check joint temperatures.
• Regular Cleaning: Periodically blow dust off winding surfaces with dry compressed air; washing with water is strictly prohibited.
• Insulation Resistance Trending: Maintain a "temperature-humidity vs. insulation resistance" correlation log; increase testing frequency during humid seasons, monitor trends, and issue early warnings.
• Preventive Testing: Conduct comprehensive insulation testing at least annually; record absorption ratio (R60/R15 ≥ 1.3 qualifies) to assess moisture ingress severity.
• Grounding System Inspection: Humid environments increase grounding system corrosion risk; regularly check grounding resistance (recommended ≤ 4 Ω) to ensure reliable earthing.