The process of reconditioning transformers is started when we first pick up the units. When they are unloaded into our shop, we perform a visual inspection. A visual inspection is done to check for any failed components (bushings, radiators, etc.) that may require immediate attention. After the visual inspection we do some preliminary testing. In the case that it is unknown whether a unit needs to be repaired or undergo a complete re-wind, preliminary testing is done to help determine the initial condition of the transformer. The preliminary tests performed will depend on information gathered from the customer about the exposure/operation of the transformer and its condition. After these tests it will be determined whether the reconditioning will be enough or if a rewind is necessary.
Reconditioning Units
If reconditioning is all that is needed, the oil will be drained from the transformer. Then the core/coils will be removed. The core, coils, and lead structure will then be inspected for any defects, such as: insulation breakdown, core ground issues, structural failure, and clearance issues. After these inspections it is common for us to make recommendations of any repair work that is needed. If approved, we will go through with our repair work.
After that work is done, the core/coils will be re-installed. Since this is the reconditioning process, we will use the same core and coils that were in the unit originally. New mineral oil replaces the old used oil after the core and coils are installed. Now the unit should be ready to be sanded and re-painted, but before that we will perform testing to make sure that the unit is in fully working condition. If the customer has any special testing requirements, then those tests will be added to the schedule as well. After the testing is passed the unit will either be sanded or sand blasted, then painted the correct color for the application.
Rewinding Units
Some units are in worse condition and need a full rewind. The steps are similar but go further in depth for the core and coils. We start off by draining the oil and cutting off the lid to take out the old core and coils. After that is done, we must collect data, for example we must have the correct measurements for the design of the process. Now the disassembly starts, we have to break it down to the bare components for a rewind. The core, coils, and lead structure will be disassembled. The frame and tap changer will be removed and the core steel will be unstacked if necessary. The coils will be removed from the core steel. Now the transformer will be designed using the most recent ANSI/IEEE design standards and will account for any specific requirements as requested by the customer.
Most of our power transformer rewinding services use copper windings and insulation, unless specified by the customer. For rewinding the coils, we have highly experienced winders that specialize in all types of coil designs (layer, helical, disc, etc.). Throughout the entire process quality checks are made to ensure the accuracy and workmanship of the windings. When the winding complete, the coils are sized accordingly and prepared for the oven. The coils are put in the oven to remove any moisture and improve the overall power factor of the transformer.
The tank of the transformer that holds the oil is then leak checked to make sure that it doesn’t leak after it is rebuilt. The core is then cleaned and baked if necessary. The coils are landed onto the core and the steel is re-stacked. A TTR test is done at “half-stack” to verify accuracy of all voltages. A new lead structure is put into place using all new materials and utilizing the most recent clearance standards pertaining to the appropriate BIL of the transformer. During the assembly process, the core insulation resistance is checked several times to ensure that it is correct throughout the process.
The core and coils are then placed back into the tank and the rest of the steps we take are the same as reconditioning transformers. Although the testing can be slightly different for rewound units.
The Conclusion
As you can tell we put forth all our effort in to making sure these units are as good as new when they are delivered to you. While the reconditioned units don’t have a new core and windings. That is because they didn’t need that repair done. Our reconditioned units come in at a great cost and an even better lead time, while not sacrificing workmanship nor quality. All our reconditioned and rewound units come with a 3-year warranty because we stand by our work.
