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 . Reclamation, Bonneville Power Administration, and Western Area Power Administration conduct regular studies to determine statistical equipment life. These studies show that average life of a Reclamation transformer is 40 years. Reclamation gets longer service than IEEE estimates because of operating at lower ambient temperatures and with lower loads. A significant number of transformers were purchased in the 1940s, 1950s, and into the 1970s. Several have been replaced, but Reclamation has many transformers that are nearing, or are already well past, their anticipated service life. We should expect transformer replacement and failures to increase, due to this age factor.
Figure 25 – Transformer Diagnostics Flowchart
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.