A transformer has several components that must work together to deliver power reliably. When a transformer problem is suspected, oil testing is often the first step. The results can reveal signs of developing faults and help guide further investigation. Electrical testing and field inspections can then help pinpoint the affected component and determine the appropriate repair. Windings, bushings, and tap changers are common areas to evaluate, along with connections, insulation, and oil containment systems.
In September 2024, CIGRE A2.62 published a Technical Brochure analyzing data from more than 1,200 transformer failures reported by 66 utilities across 27 countries. The data revealed 19% of transformer failures come from tap changers, 25% of transformer failures are caused by bushings, and 37% of failures come from the windings.
Windings
Transformer windings are exposed to electrical, thermal, and mechanical stress. Over time, overheating, insulation deterioration, or movement caused by a fault can affect their condition.
A winding concern is rarely evaluated with one test alone. Technicians may review operating history and oil diagnostic results alongside electrical tests such as winding resistance, turns ratio, excitation current and sweep frequency response analysis (SFRA). Comparing results with previous tests can help reveal changes that need further investigation.
Bushings
Bushings allow energized conductors to pass through the transformer tank while remaining insulated from it. A deteriorating bushing can pose a serious reliability concern, making visual inspections and appropriate electrical diagnostic testing important parts of a maintenance program.
Signs that warrant investigation may include leaks, visible damage, a change in the C1 and C2 power factor or capacitance test results. Depending on the bushing and the concern, the next step may be closer monitoring, additional testing, or replacement during a planned outage.
Tap Changers
Tap changers adjust a transformer’s voltage ratio. Their mechanical and electrical components can wear with operation, particularly in load tap changers that change tap positions while the transformer is energized.
Operating history, inspection findings, and diagnostic results help determine whether a tap changer needs maintenance or repair. Dissolved Gas Analysis test results are very insightful when analyzing the various gas ratios to identify potential issues to make the repairs. In some cases, a retrofit or replacement may be the appropriate long-term solution.
Other Areas to Watch
Lead exits and internal connections, winding insulation, and the tank and piping can also develop problems. Leaks, changes in oil level, unusual temperatures, or abnormal diagnostic results should be evaluated in the context of the transformer’s design and operating history. Even a concern that appears minor at first may call for further investigation.
Finding the Cause and Planning the Repair
A useful transformer assessment brings together routine inspections, oil diagnostics, electrical testing, and knowledge of the unit’s history. Each provides a different piece of information. Once the cause and extent of a problem are better understood, asset owners can make informed decisions about monitoring, maintenance, or repair.
MVA provides transformer electrical testing, inspections, and field repairs, supported by oil diagnostics. Contact MVA to discuss a transformer concern or plan an upcoming project.
