Determining whether a transformer tank requires a major overhaul requires a comprehensive assessment considering long-term maintenance cycles, abnormal thresholds of key indicators, and sudden fault signals. A major overhaul is not simply a periodic "parts replacement," but a deep repair based on the equipment's condition, typically involving significant operations such as core inspection, seal replacement, and oil treatment.
Overhaul Trigger Cycle: Basic Reference, Not an Absolute Standard
General Recommendation: Oil-immersed transformers should undergo their first major overhaul within 5 years of commissioning, followed by overhauls every 10 years.
Special Circumstances May Be Adjusted: For fully sealed tanks with welded edges or those with special manufacturer specifications, and after successful testing and operational evaluation, the overhaul cycle can be appropriately extended.
However, if the following situations occur, a major overhaul should be arranged immediately, regardless of whether the cycle has expired.
Key Indicator Threshold Exceeds Standards: The Core Basis for Major Overhaul
When periodic test data shows severe deterioration, it indicates substantial internal damage:
Severe Oil Deterioration
Breakdown Voltage < 25kV (Operating Oil), and Oil Filtration Cannot Recover → High Risk of Insulation Performance Loss.
Moisture Content > 50ppm, Long-Term Exceeding Standard → Accelerates Insulating Paper Aging and Sludge Formation.
Acid Value > 0.1mgKOH/g, Continuously Rising → Oil Oxidized, Corroding Metals and Insulating Materials.
Dielectric Loss Factor (tanδ) > 0.01 (90℃) → High Level of Polar Impurities in the Oil, Significant Energy Loss.
Abnormal Chromatographic Analysis: Acetylene (C₂H₂) > 5μL/L → Arc Discharge Presence; Rapid Increase in Total Hydrocarbons → Internal Overheating.
Severe Degradation of Electrical Performance
Winding insulation resistance drops below 50% of its initial value, or absorption ratio < 1.3 → Insulation is severely damp or aged.
Winding DC resistance three-phase imbalance > 4% (inter-line > 2%) → Inter-turn short circuit or loose connection.
Abnormally increased no-load/load losses → Deterioration of core or winding structure.
Frequent Tripping of Protection Devices
Repeated alarms for light gas or tripping for heavy gas → Internal gas generation, indicating discharge or overheating faults.
Differential protection trips; after eliminating external faults → Severe defects in windings or core.
