*5.1. Experiment Description*

The internal fault mode of power transformer is mainly mechanical fault, thermal fault and electrical fault, the latter two types of faults is the major issues, and mechanical fault is often shown in the form of thermal fault or electrical fault [18]. General power transformers are often subdivided into fault modes according to the degree of heating or arcing.

Due to safety culture of nuclear power plants, maintenance strategy for the main transformers tends to be conservative and strict; The failure of nuclear power transformers is rare to happen, and the failure data that can be accessed to publicly is very few. Available fault data cannot cover all the modes. Therefore, the condition of the main transformers are divided into the following three types in this paper, and only two summative failure modes are reserved and the corresponding as illustrated in Table 1.

**Table 1.** Code of power transformer operation condition.


The fault data used in the experiment in this paper are reactor-related transformer data obtained from IEC TC database, and the feature selection data and normal data are the monitoring values of a nuclear power main transformer under normal operation. The data can be obtained in the Supplementary Material. A nuclear power plant generator, 24 kV voltage, is stepped up to 500 kV and connected to the 500 KV power grid through the main transformer. The main transformer is a three single-phase transformer, each phase capacity 410 MVA. The neutral points on the high voltage side are connected and directly grounded. Oil is regularly sampled and analyzed once every 3 months manually. Sampling intervals can sometimes be uneven, depending on special focus judged by sampling staff or adjustment by work schedules.
