The following chapters address routine maintenance, as well as specific testing and diagnostic techniques and tools used to assess the condition of transformers (more detail is included for oil-filled power transformers). Some processes are often above and beyond routine maintenance work to keep the transformer operational. Transformer diagnostics require specialized equipment and training. This expertise is not expected to be maintained in every office. In some cases, it may be necessary to contact diagnostics specialists, either inside or outside, who have the latest equipment and recent experience.
Picture 1 shows the overall transformer condition assessment methodology, linking routine maintenance and diagnostics.
Picture 1: Transformer Diagnostics Flowchart
Standards organizations such as American National Standards Institute/Institute of Electrical and Electronic Engineers (ANSI/IEEE) consider average GSU transformer life to be 20 to 25 years. This estimate is based on continuous operation at rated load and service conditions with an average ambient temperature of 40 ºC (104 degrees Fahrenheit [ºF]) and a temperature rise of 65 ºC. This estimate is also based on the assumption that transformers receive adequate maintenance over their service life.
Current minimum replacement time is around 14 months; a more realistic time may be 18 months to 2 years. In the future, lead times may extend well beyond what they are today. Therefore, high-quality maintenance and accurate diagnostics are important for all transformers, but it is absolutely essential for older ones—especially for critical transformers that would cause loss of generation. It is also very important to consider providing spares for critical transformers.
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