Power transformer end-of-life assessment-pracana case study*

Rede Electrica Nacional (REN) is the operator of the Portuguese National Electricity Transmission Grid (RNT), which currently has about 170 large power transformers and autotransformers on the 400-, 220-, and 150-kV networks. Rede Eléctrica Nacional's network expansion and reinforcement program over the last few years has resulted in a significant increase in the number of installed units, number of substations, and the total mega volt amperes, as shown in Figure 1. Presently, about 24% of the units installed are more than 30 years old and 39% are up to 10 years old. The group of more recent units represents 47% of the total power installed, and most have a rated power above or equal to 120 MVA (84%), which is presently the minimum power rating specified for new transformers. Older units are still in service with power ratings of 60 to 90 MVA.