Power Transformer Overhaul and Reconstruction: Fault Assessment, Winding Rewinding, and Post-Repair Verification

When on-site duty personnel encounter transformer tripping or abnormality, their first reaction is often “try a strong transmission once” to see if power can be restored directly. However, from the perspective of equipment evaluation, once protective action, gas abnormality, pressure release, or visible deformation and casing rupture occur, the most correct approach is to immediately isolate the equipment and preserve the event record. Don’t expect to test whether the equipment can withstand repeated power supply tests. Blind and arbitrary test power supply not only fails to yield reliable conclusions, but also easily amplifies local defects that could otherwise be repaired into irreversible damage to the windings or insulation. Therefore, before starting any transformer overhaul, the most crucial step is to stop and find out the true “bottom line” of the equipment.

In-depth investigation after failure

An external short circuit will instantly apply huge axial and radial electric forces to the winding, which can easily cause winding displacement, deformation or loosening of the support. A common pitfall on site is: after the fault was removed, the DC resistance was measured and the data was found to be normal, so the winding was mistakenly thought to be not off. In reality, deeper detection methods such as winding frequency response analysis (FRA) and short-circuit impedance measurement are usually needed to truly probe the health of the bottom winding. Abnormal temperature rise, odors, discoloration, increased noise, vibrations, oil leaks, traces of electrical discharge, and even changes in trends in protection actions and historical data are all things that need to be explored “alarm signals”. They are mutually reinforcing relationships, and we must never draw conclusions from any single point in isolation. Infrared temperature measurement can indeed catch relative hot spots caused by terminals, connectors or uneven cooling, but don’t forget that temperature measurement results will be affected by equipment load, surface emissivity and field environment. When making a conclusion based on the graph, these variables must be taken into full consideration.

How should the scope of maintenance be defined?

Treating transformer maintenance as a standardized operation on the assembly line is an extremely common cognitive misconception. The true scope of repair must follow the fault mechanism and investigation results. It may just be replacing seals, accessories, sleeves, coolers or tap switches, or it may involve leads, insulation, or even penetrate deep into the winding “major surgery”. Winding rewinding must be done with extreme caution, because this action completely changes the number of turns, conductors, insulation structure, and geometric dimensions, and the associated impedance, temperature rise, and short-circuit resistance will all change accordingly. This work must be handed over to a unit with formal design and testing capabilities, and a rigorous and controlled plan must be developed before it can begin. As for randomly changing wiring, randomly adjusting taps, or confusingly replacing them with insulation materials of different specifications on site, these “wild roads” can easily lead to electric field distortion, local overheating, and misalignment of protection and coordination, and are absolutely intolerable as a temporary recovery method.

Transformer transformation and upgrading

Equipment refurbishment and upgrading does not mean that the windings must be removed and rewound. Often, the focus of the renovation can be entirely on casing, instruments, cooling systems, tap-in accessories, or monitoring and protection devices. However, every change has to be re-evaluated, affecting the entire body, including insulation fit, mechanical interface, load-bearing capacity, control logic, and the consistency of drawings and data. If the device body needs to be re-clamped or internally mechanically repaired, it must be performed strictly against the factory structure, drying condition, and diagnostic data. Blindly pressurizing or tightening without a design square will only push the windings and insulation into a more dangerous situation.

Replacement and treatment of insulating oil

For oil-immersed equipment, how to handle insulating oil (liquid) is an unavoidable part of the overhaul decision. Before changing or filtering oil, you must first find out the bottom of the old oil, especially whether it contains controlled hazardous substances. Subsequent testing, storage, transportation, and waste disposal must all be carried out in accordance with the environmental compliance requirements of the location of the equipment. Never use the regulations of location A to apply to projects in location B. When handling sensitive substances, you must keep a close eye on the current regulations of the country and region where the project is located.

How to carry out the repair “acceptance and closure”

Just because the machine is repaired and the resistance is qualified does not mean that the equipment will be fully revived and can run at the rated capacity. The correct routine is to complete a complete set of preventive and handover tests based on the scope of this maintenance, and then have professionals evaluate whether the transformer needs to be reduced in capacity operation in the future, or whether the frequency of online monitoring needs to be increased. Similarly, when the equipment should be overhauled or refurbished must be calculated by spreading out its operating status, failure history, design life, spare parts inventory, and economic accounts, rather than blindly disassembling it after staring at it rigidly “after several years of operation”. In other words, repainting the casing is called vanity engineering, and it cannot replace a hard-core assessment of the true condition of the equipment interior.

Minor repairs, major repairs, rewinding or just replacing?

This is actually the ultimate choice for a multi-dimensional game. You have to put the fault failure surface, the degree of insulation aging, the importance of the node in the power grid, the power outage time window, the speed of spare parts allocation, the field test conditions, and the consequences “in case of repeated accidents” all on the balance. If the decision is made to scrap or recycle, the insulation oil and toxic and hazardous substances must be identified in advance and the compliant storage, transportation and consumption process must be completed. In short, whether it is a modification or an overhaul, the only secret to keeping dark diseases after the equipment is repaired is to make every step of the operation based on solid diagnostic evidence and controlled plans, rather than blindly trying and failing based on so-called “empirical intuition”.