Power Transformer Repair: When to Rewind or Replace

After the fault, stop first to judge

From power plant boost and power grid transmission interconnection to large-capacity industrial power supply and terminal distribution, transformers appear at almost every key node in the power supply chain. It is one of those devices that is expected to operate stably for a long time, but long-term operation does not mean that there will never be any problems. Long-term overload, excessive ambient temperature, cooling failure, ventilation blockage and harmonic current will gradually raise the operating temperature of the winding and accelerate insulation aging; once the insulation degrades, the dielectric and mechanical strength will decrease, and the equipment will be more likely to fail under subsequent overvoltage or short-circuit shock.

When protection action, abnormal odor, pressure release or obvious deformation occurs, the first action is to isolate the equipment and keep a record of the event. Repeatedly sending electricity to test whether the equipment can still be rotated will not determine the true state, and may also expand the originally controllable damage into irreversible failure. The restraint at this stage often directly determines the cost and results of subsequent repairs.

Diagnosis: First find the fault mechanism

Repairs are never taken down right after they come up. The first step in the professional power transformer repair process is to figure out which layer the fault occurred on. Abnormal temperature rise, odor, discoloration, noise, vibration, leakage, discharge traces and changes in trend data are just signals that require further diagnosis and cannot be considered failure conclusions in themselves.

There are divisions of labor in each diagnostic method. Infrared imaging is suitable for detecting relative hot spots caused by uneven terminals, connections, and cooling; dissolved gas analysis of liquid-immersed products can identify potential thermal faults and discharge activity, but must be viewed in conjunction with gas production rate, gas combination, and historical data; winding DC resistance, ratio, frequency response, and insulation and medium testing are used to locate changes in connections, windings, or insulation These data need to be compared with benchmark values and phase data to be meaningful. Any single test anomaly is not enough to judge the entire transformer individually.

The scope of repair is determined by the fault

After figuring out the mechanism, the scope of maintenance is basically clear. It may be a seal, an accessory, a sleeve, a cooling, a tapping device, a lead, or it may be insulation or the winding itself. Treating transformer repair as a standard process is not feasible. The processing depth and risks corresponding to different faults are completely different.

For example, an external short circuit will generate a lot of electric power, and the winding may undergo axial or radial displacement, deformation or support loosening. In this case, even if the DC resistance reading is normal after the fault is cleared, frequency response analysis or impedance inspection may be required to assess whether the winding is shifted. For example, loose conductive connections will cause contact resistance and local overheating, further causing oxidation, insulation carbonization or terminal damage. Although this type of problem may not seem big, it is a common source of thermal failure.

What does winding rewinding mean

Of all the repair operations, winding rewinding is the heaviest. Rewinding changes the number of turns, conductors, insulation, geometric dimensions, impedance, temperature rise and short-circuit mechanical strength at the same time, affecting the entire body. Therefore, services such as power transformer rewinding services must essentially form a controlled solution by units with design and trial capabilities, rather than simply going back as is.

Replacing wires, changing tap connections, or replacing non-equivalent insulation materials on site may cause electric field concentration, hot spots, and protection mismatch, and is an unacceptable temporary recovery method. This holds true at any voltage level, and rewinding is never a process where experience can replace design.

How to choose a repairer

With such a high barrier to entry for repair and rewinding, choosing who will do it becomes crucial. A qualified power transformer repairer must at least be able to clearly explain the failure mechanism, repair scope and subsequent verification plan, and be able to provide traceable design and test records. After repairs or modifications, electrical, mechanical, sealing, cooling and protection verifications shall be repeated by area of influence, and nameplates, drawings, values and baseline records shall be updated, instead of leaving only one sentence that has been repaired.

For users seeking power transformer repair services, the focus should be on whether the other party can form a controlled solution and whether they have the corresponding experimental capabilities. The price of the quotation cannot be determined by the numbers alone. The low price may be due to the supply range, wear and tear, testing or lack of accessories.

Repair, refurbish, rewound or replace

The final decision to face is: repair, refurbishment, rewinding or replacement. This decision cannot be made based solely on a fixed number of years, nor can repainting be used as a substitute for internal condition assessment. It shall be weighed against the scope of failure, the state of remaining insulation, system importance, downtime window, spare parts situation, testing capacity and the consequences of another failure.

Normal resistance after repair is not sufficient to prove that the equipment has regained its rated capacity. It is also necessary to complete the prescribed tests based on the impact range and evaluate whether derating operation or enhanced monitoring is required. If the equipment has reached the point of being scrapped or replaced, the insulation fluid and possible presence of regulated substances must be identified before disposal, and testing, storage, transportation and disposal must be completed in accordance with the regulations of the equipment location.

Conclusion

The repair of an electric transformer is never just about taking it apart and putting it back together. It is a superposition of a series of judgments: first judge the fault mechanism, then judge the scope of repair, then judge who should do it, how to verify it after it is done, and finally judge whether it is worth repairing. Building every step on diagnosis and verification is the responsible approach to this type of long-term asset.