The Reconditioning and Rewinding Process


The Reconditioning and Rewinding Process

Published under Technical Articles
There are many reasons why you may need a new transformer. This could be due to a failing unit or increased power demands. UTB understands that the task of obtaining a new transformer can be daunting. Lead times on new units can occasionally be several months out. Rewinding or reconditioning a transformer can solve most of your issues. This can be beneficial to you financially and save you time in the end.

When a transformer is designated to be rewound or reconditioned, it is initially brought into the shop for a visual inspection and tested for any electrical issues. The bushings, radiators, gauges, cabinets and main tank are all checked for any damage that requires immediate attention and repair. All electrical and mechanical components are then tested for proper operation.

The next step of the process is to cut the top off the transformer and pull out the core and coils. Once the internals of the transformer are removed, they are inspected for defects. The decision is then made on whether it can be reconditioned or if needs to be rewound. If there are any issues with the internals, the transformer will be designated to be rewound (e.g. structural integrity, insulation deterioration, grounding issues, winding discontinuity). If the transformer internals pass, it will continue on in the reconditioning process to be repaired.

The rewinding process is more extensive and time consuming when compared to reconditioning, however, it will extend the life of your transformer by a large factor. First, data will be collected on the internals of the transformer to assist in the designing phase. The core and coils along with all of the leads will then be disassembled. All parts will be checked throughout the disassembly process for any damage. The coils will be unwound from the core, and the core will be un-stacked.

Once the transformer has been completely disassembled, it will be redesigned to meet both ANSI and IEEE standards as well as any customer requests. New materials will then be sourced for the transformer. This includes new copper windings (unless aluminum has been requested), new insulation, and any materials required for rebuilding the tap changers, gauges, cabinets and all other parts of the transformer.

Once all necessary parts have been procured, the transformer will begin to be reassembled. The coils will be wound based on the designed method. This could be disc, layered, helical, or any other type of winding design. The coils will be baked to eliminate any moisture inside of the insulation. This will help extend the life of the transformer as well as improve the power factor. The insulation resistance will then be tested on the coils. If they have passed, the coils will be placed onto the re-stacked core and the voltages will be tested. A new lead structure will be manufactured and installed. The insulation resistance will be checked again between each component to verify the absence of any grounding or arcing issues.

Any transformers that only required reconditioning will continue onto this step after being untanked and tested.

The tank and radiators will then be checked for any leaks or blockages to oil flow. Any damage or leaks will then be corrected. The internals are then dropped back into the tank and fixed in place. Once the tank has been sealed back up, the transformer is vacuum filled with either new mineral oil or FR3 fluid.

The testing process will then take place. Tests include but are not limited to the following: Quality Tests, Performance/Losses Tests, Thermals Tests, Dielectric Tests, and Mechanical Tests, along with any others that have been specified and requested by the customer. Once the testing has been completed and all parts of the transformer have been reassembled, the transformer is sandblasted and painted to the customers specifications.

Rewinding and reconditioning may be the best process for you. It will save you money and time as well as meeting your exact specifications. Contact UTB today for a quote on reconditioned and rewound transformers.

Quinn Alleman
Utility Transformer Brokers