Power Transformer Maintenance Guide: From Fault Diagnosis to Reconnection to the Grid

If the equipment is forced to shut down due to protective action, abnormal gas, pressure release, obvious deformation or shell damage, etc., do not blindly try to transmit power before the specific reason is found out, otherwise it is easy to cause secondary damage. The correct approach is to isolate the equipment at the first time, keep the protection action, load condition, temperature data, alarm information and relevant test records intact at that time, and then investigate them according to the specific fault mechanism. Although abnormal temperature rise, peculiar smell, abnormal noise, abnormal vibration, oil leakage or discoloration all indicate risks, it is impossible to accurately find out the specific location and severity of internal damage by these external manifestations alone.

A qualified plan for power transformer repair services must connect the technical scope, fault diagnosis, repair process, test verification and re-service conditions in series into a complete closed loop. If the contract merely mentions in passing that "normal operation will be restored after overhaul," then there is far too much room for maneuver. The customer has no way of determining whether this quotation actually includes the core work packages that are critical to the equipment’s reliability.

First, it is essential to accurately determine the true extent of the fault.

The diagnostic procedure must align with the specific structure of the equipment and the observed fault symptoms. For example, the local overheating of terminals or joints can indeed be found out by infrared temperature measurement, but when drawing conclusions, the load situation, surface emissivity and observation conditions at that time must be comprehensively considered. For oil-immersed transformers, it is necessary to make a comprehensive study and judgment in combination with the state of insulating oil, dissolved gas analysis (DGA) in oil, casing, tap-changer condition and historical operation record. In contrast, for dry-type transformers, the focus should be on the exterior appearance, cleanliness, connection points, insulation condition, partial discharge, and the cooling control system. Routine test results such as winding direct resistance, variable ratio, frequency response, insulation and dielectric loss must not be viewed in isolation and must be cross-verified with historical baseline data, phase-to-phase comparison results and other circumstantial evidence.

In the event of an external short circuit, please exercise extreme caution. A short-circuit current, in an instant, generates not only thermal stress but also substantial axial and radial electromagnetic forces. Even if the DC resistance measured after the fault removal looks normal, the winding may actually have been displaced, deformed, or the supporting parts are loose. For added reliability, frequency response analysis, short-circuit impedance testing, or other specialized tests are often performed as well. Remember, a single test result can never replace a comprehensive injury assessment.

Secondly, the scope of repair must be strictly aligned with the failure mechanism.

The term "power transformer repair" in English contracts is too general. It may cover a lot of miscellaneous work such as seals, accessories, bushings, cooling systems, tap switches, leads, insulation and even windings. What you need is a 1 reliable statement of work (SOW) that clearly defines inspection boundaries, diagnostic methods, repair and replacement parts, material and process standards, re-inspection items, on-site operating specifications, and final delivery documents. Only in this way can the technical scope and quotation be truly aligned, thus preventing individual manufacturers from cutting corners in diagnosis, transportation, testing or quality assurance and playing the routine of "winning the bid at a low price.

Localized repair must also remain under control. In order to save trouble, repairing on site, changing the tap connection mode without authorization, or using non-same grade insulation materials may completely disrupt the original electric field distribution, change the hot spot location, or even destroy the protection coordination. When performing internal recompaction or mechanical repairs, consideration must be given to the original structural design, dry condition, and previous diagnostic evidence. If the winding and insulation are subjected to compression and tightening without prior design verification, they may easily be rendered completely unusable.

Furthermore, the quotation for winding re-winding takes into account meticulous design details.

When the work involves power transformer rewinding services, don’t focus solely on whether new wire has been used. Winding rewinding is a work that affects the whole body. It is directly related to the number of turns, wire specifications, insulation performance, winding geometry, impedance, temperature rise and short-circuit mechanical strength. This scheme must be led by a unit with real talent and practical design and testing capabilities, and must be tight to conductor tension, dimensional deviation, joint treatment, turn-to-turn insulation, cooling oil channels and geometric tolerances. Any changes to materials or processes must be documented in writing and approved; the final as-built documentation must accurately reflect the actual condition of the facility after the overhaul.

When screening a power transformer repairer, we have to break the casserole and ask to the end: let the other party explain clearly the failure mechanism, how to divide the design responsibility, how to control the key processes, how to trace the materials, how to test the ability, and how to do closed-loop treatment in case of unqualified items. The hard power of the manufacturer must cover all aspects such as winding, insulation treatment, body assembly, lifting and loading, packaging and shipping, etc., and must clearly explain which processes are done by itself and which are outsourced. Regardless of the loud shouting of those marketing gimmicks, and regardless of how many management system certificates they took out, these fancy shelves will never replace a set of targeted repair plans, original test data, and hard project acceptance evidence.

Finally, completing the repairs does not mean that power can be immediately restored and the equipment put back into service.

After the overhaul, simply saying "the resistance is normal" is by no means sufficient to prove that the equipment has been restored to its rated capacity level. The scope of verification depends on how extensive the previous fault’s impact was. Under normal circumstances, winding direct resistance, variable ratio and group, short-circuit impedance, insulation and dielectric loss, partial discharge test, as well as sealing performance, cooling system and protection control function verification, are the same. If the mechanical structure is moved, the test results such as frequency response must be carefully compared with the original baseline data under the same wiring mode. All temporary wiring and short grounding wiring on site shall be completely restored or removed. Drawings, protection setting values, nameplate data and new baseline data for future trend analysis must also be updated in place simultaneously.

At the end of the day, the final decision has to be weighed comprehensively in 1 big game: whether to strengthen monitoring of sick operation, do partial maintenance, rewind windings, completely renovate, or simply change a new machine. Maintenance costs are just one item in this account. The loss of production capacity caused by shutdown, the cost of temporary power supply, the cost of transportation back and forth, the remaining life of the old accessories, the operating efficiency, the serious consequences in case of failure again, and even the degree of toss and toss of the interface transformation of the old and new equipment will influence the final decision. Only by writing the diagnostic basis, scope of work, re-examination standards and re-service conditions in the same document in black and white can we ensure that every penny spent corresponds to a real and testable recovery goal.