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Transformers are indispensable assets within transmission and distribution networks, serving as the silent workhorses that maintain electrical stability. Yet, due to natural aging, external stresses, or unforeseen failures, these devices occasionally demand attention. Maintenance activities are broadly divided into two categories: major overhauls and minor repairs. Traditionally, oil-immersed transformers requiring oil drainage and core lifting (or bell cover removal) fall under major overhaul, while external work such as topping up insulating oil, minor leakage treatment, or oil processing are classified as minor repairs.
Routine inspections form the foundation of reliable transformer operation. A systematic checklist includes:
Cleaning bushings and checking porcelain for discharge marks or cracks.
Inspecting bushing bolts for looseness and overheating.
Cleaning the oil tank, conservator, safety piping, and tap-changer assembly.
Removing dust deposits inside pressure-relief valves.
Verifying conservator oil levels and gauge clarity, draining sludge from the collection basin.
Checking breathers and replacing exhausted silica gel.
Supplementing insulating oil in the tank and oil-filled bushings.
Inspecting radiators for seepage and ensuring cooling fans operate correctly.
Checking thermometers that measure top oil temperature.
Examining Buchholz relays for leakage, verifying valve flexibility and cable insulation.
Inspecting grounding systems for the tank shell and neutral points.
Servicing the tap-changer control circuit, linkage system, and contacts; cleaning its operating box.
Taking oil samples for simplified and chromatographic analysis.
Rectifying localized oil leaks and defects in situ.
Performing standard electrical diagnostic tests.
If diagnostic testing or oil analysis indicates abnormality, a major overhaul may be mandated. Furthermore, core lifting becomes essential under specific conditions.
Within the first five years of commissioning, a bell cover inspection is advisable.
Power transformers that have withstood severe short-circuit events often require early overhaul.
Units displaying internal faults or alarming test results must undergo complete disassembly and inspection.
A comprehensive overhaul encompasses both mechanical and electrical rejuvenation:
Removing the bell cover or core for inspection.
Repairing windings, leads, and shielding systems.
Checking the core, clamping bolts, tie rods, and grounding strips.
Servicing the oil tank and accessories such as bushings and breathers.
Overhauling cooling assemblies, pumps, fans, valves, and piping.
Testing and calibrating protection devices, thermometers, and Buchholz relays.
Inspecting control panels and verifying switching mechanisms.
Servicing off-circuit and on-load tap changers.
Replacing gaskets and resealing components.
Drying insulation if moisture ingress is detected.
Filtering or replacing transformer oil.
Conducting post-repair testing and trial runs.
Incorporating upgrades such as reinforced tanks, externalized grounding leads, or improved conservator sealing systems.
Cleaning and repainting the tank exterior.
Maintaining cleanliness and environmental control during core inspection is paramount:
Preferably conducted indoors; if outdoors, work must proceed under dry, clear skies.
Exposure time of the active part is strictly limited based on relative humidity: ≤16 hours below 65% RH, ≤12 hours below 75% RH.
Core temperature must remain above ambient to prevent condensation, often achieved with vacuum oil heating circulation.
Personnel must use insulated tools, protective clothing, and safety-approved low-voltage lighting.
Disassembled parts are cleaned, categorized, and stored with care.
A strict dismantling sequence ensures precision reassembly: small instruments and bushings first, followed by larger components.
Sealing of removed accessories prevents moisture ingress.
Fragile items like bushings, gauges, and thermometers require careful handling.
Reassembly includes thorough venting of radiators and piping to expel trapped gases.
Tap changer disassembly requires positional markings to maintain correct reassembly.
The technical process unfolds as follows:
Disconnect leads.
Perform preliminary electrical and oil tests.
Drain oil partially and dismantle accessories.
Remove tap-changer connections.
Lift bell cover or active part for internal inspection.
Test insulation and dry if necessary.
Refill oil under vacuum.
Reinstall bushings, coolers, and accessories.
Conduct leak and seal integrity tests.
Adjust oil levels and re-test oil quality.
Carry out final electrical testing and commission the unit.
Inspect inter-phase spacers and electrostatic shields for deformation, overheating, or discharge traces.
Ensure winding alignment is correct and free from tilt or displacement.
Check insulation blocks for tightness, even spacing, and secure fastening.
Clean oil ducts and remove insulating paper or contaminants.
Reinforce damaged insulation with appropriate wrapping.
Examine leads and stress cones for brittleness, damage, or poor solder joints.
Verify adequate clearance between leads and structural parts.
Ensure brackets are tight, bolts are secured with anti-loosening measures, and added insulation prevents abrasion.
Confirm clearance of heavy-current leads (copper or aluminum bars) to tank walls, typically ≥100 mm, to avoid eddy-current heating.
Transformer repairs—whether minor or comprehensive—safeguard electrical reliability and prolong service life. Minor repairs address surface-level deficiencies, while major overhauls penetrate deep into the core and windings, revitalizing the heart of the transformer. By adhering to precise procedures and environmental controls, utilities ensure that these critical assets remain steadfast guardians of power networks.
