Fluid-filled transformer refurbishment: points to monitor

Refurbishing a transformer

When considering refurbishment, numerous evaluations and protocols become necessary. Operating within the guidelines of EHS and WHMIS directives, initial considerations include:

• The loading history of the transformer
• The dissolved gas trend analysis over time
• Presence of any monitoring equipment and data
• Whether internal insulation paper samples can be extracted for degree of polymerization (DP) analysis
• Whether furan samples can be analyzed (where DP is not an option)
• Selected low-voltage tests, such as SFRA or NBDFR
• Visual inspection of the transformer condition

Cosmetic external and ancillary items

The refurbishment process involves the systematic evaluation of existing equipment including:

• Tap-changer
• Cooling equipment (fans, radiators, pumps, etc.)
• Bushings
• Protection devices (including current transformers)
• Tank/fabrications and gaskets
• Condition of welds, bolted joints and surface finish
• Accessories, such as valves and monitoring devices
• Control cabinet and wiring

A general refurbishment may address several or all these items to varying degrees.

However, each component must be given the respect and attention it deserves, particularly if parts require replacement to comply with local standards, including interchangeability checks to avoid operational problems.

Partial or full replacement of the internal active part

When the internal active part of the transformer is deemed to require repair or replacement of one or several parts, the process becomes far more complex.

Initially, it may be necessary to perform a series of more detailed tests, including fluid characteristics (PCB, ASTM-D-1275, miscibility) or an internal inspection of the transformer to identify areas that may require intervention. This is followed by a detailed logistics and transportation plan to remove and return the equipment within the window of opportunity.

Some companies offer reverse engineering services on such winding assemblies. This is a cost-effective and time-efficient option, as the price is often lower than for a new unit and the lead times are shorter.

Once refurbished, transformers are subjected to factory acceptance testing, often at a lower voltage than a new unit (depending on which parts have been replaced). There may be restrictions with coolers for thermal performance validation, if these were not sent for refurbishment with the transformer. It is important to bear in mind all other ancillary components when preparing the logistics plan for off-site refurbishment.

During shipping, it is best to install at least one impact recording device to ensure that transportation damage is effectively ruled out. If the unit is shipped without the fluid (under dry gas), the internal dew point must be recorded and a positive gas pressure maintained throughout the process, with a suitable warning sign.

Preparing with diligence

Regardless of the transformer refurbishment project, the greater the effort put into preparing for the refurbishment at the beginning of the project (clearly defining specifications and standards, scope of work, procedures, milestones and communication protocols), the less the risk of the transformer performing outside the client’s expected parameters.

Transformer refurbishments may offer a valuable economic and logistical alternative to purchasing a new unit under operational and financial constraints. However, properly supporting the process from the start and constant vigilance to avoid pitfalls is the key to successful refurbishment. Ultimately, the process depends on clear instructions and good communication with the various stakeholders.

If you have a transformer refurbishment project, contact us to discuss it.