Transformer bushing replacement
Author: Juan Acosta
Power transformers are crucial assets for ensuring a continuous supply of power and must be reliable and rugged as key components of the electrical network. Power Transformers are frequently in use for decades and are therefore subject to the unavoidable effects of aging and wear. Keeping up with recommended maintenance, as well as communication with the original equipment manufacturer (OEM), are fundamental in ensuring long life to these important assets.
One vital component that is commonly replaced during a transformer’s lifetime is the bushing. Bushings are used in transformers as the mean to conduct potential in and out of the tank, and may require replacement due to reasons such as electrical issues often found in power factor and capacitance changes, physical damage, unsatisfactory dissolved gas analysis when oil filled, no oil in reservoir due to leaks, and reasons related to the bushings manufacture and design. A bushing failure could translate into a catastrophic and expensive transformer failure, so keeping all components of a transformer in good operating condition is important in maintaining and extending its life. Safety always comes first, and transformer owners usually decide to replace the bushing when questions arise about the health of these components.
When a decision has been made to replace a bushing, it is critical to ensure that the internal transformer geometry does not change. Figure 1 presents a typical bushing arrangement inside and outside the transformer. Transformers are designed to maintain required electrical clearances to avoid dielectric discharges that could lead to failures. These minimum design clearances from energized parts to ground, or to other phases, must be considered if the internal configuration changes due to bushing replacement. It is always a good practice to contact the manufacturer in order to evaluate risks and recommended practices for these types of jobs.
Risks are lower when bushings are replaced with their exact original type since it is understood that the geometry will remain the same. Other aspects of bushing replacement that should be considered are:
Receiving inspection including testing
When, for any reason, the same type of bushing is no longer available and a replacement needs to be selected, it is strongly recommended to work together with the Original Equipment Manufacturer (OEM) in order to make sure the most suitable bushing in form, fit and function is selected. They will have access to proprietary information, engineering tools and trained personnel that often perform these tasks. Besides the factors already mentioned above, utilizing the OEM will ensure the required engineering, provisioning and installation of new gaskets, turrets (when applicable), and required internal connection adaptors are used to maintain tolerances for clearances inside the individual units. Figure 2 is an example of a simulation done by a factories’ engineer in order to ensure replacement will properly fit. In this case, the new bushing was hitting the bottom of the turret so a flange adaptor was necessary in order to pull the bushing out, and bottom connections were extended.
General requirements needed to be carefully reviewed when selecting new bushings are:
Verifying accuracy of transformer outline drawing with installed equipment and dimensions
Bushing internal length “L”
Maximum internal diameter
CT pocket length (ground sleeve)
Type of internal connection (draw lead, draw bar, or bottom)
External top connection
Flange diameter, bolt circle diameter, number and size of holes.
Nowadays, there are fewer and increasingly shorter windows of replacement opportunities, even for planned service interruptions. It is recommended to assign a project manager in order to develop action plans and risk mitigation before starting a bushing replacement. Having a professional field service vendor with access to the OEM as well as extensive experience that can offer a turnkey solution will facilitate a seamless execution and provide a single point of contact in case problems arise.