Overvoltage protection of large power transformers:a real life study case

This paper demonstrates the results from a detailed study of the overvoltage protection of a particular 400/150 kV 400 MVA power transformer. The work presented here is based on real-life power system substation design and data and initiated by Danish TSO Energinet.dk as a consequence of a serious transformer overvoltage damage. A simulation model for the entire system consisting of overhead line, transformer, surge arrester and earth grid has been created in PSCAD/EMTDC. Main focus has been put on the earth grid, which has been submodeled in details in MATLAB using an electromagnetic transient approach based on the thin-wire program made by J.H.Richmond in 1974 for NASA. The earth grid model is verified with excellent agreement compared to already published results. The overvoltage performance of the particular case is analyzed, and it is showed that the transformers LIWL have probably been exceeded. It is clearly illustrated that the transient performance of the earth grid plays an important role in the overall overvoltage protection system desig