New high voltage insulation system for air-cooled turbine generators

For competitive power generation all over the world, air-cooled turbine generators tend to be used with large capacity up to a few hundred MVA, while hydrogen-cooling was adopted ten years ago. At the same time, the customers require compact, cost-effective generators. To meet such a new trend of power market, Mitsubishi Electric has developed a new high-voltage global impregnation insulation system that is applicable up to 250 MVA class air-cooled generators and started to apply it in 1998. In large-scale power generators, thermal fatigue due to the different coefficient of thermal expansion of conductor, insulation and core is supposed to be the biggest problem. Therefore, thermal and stress and strain analyses in stator windings were carried out by making full use of the latest numerical simulation techniques and were verified by thermal cycle tests of full-size models. Dielectric and thermo-mechanical properties of the insulation were also evaluated by experimental techniques such as electrical lifetime tests and mechanical fatigue tests with model coils. From the comparison between the analytical and the experimental results, it was shown that the new insulation system has sufficient dielectric and thermo-mechanical properties through the lifetime of turbine generators. Furthermore, the dielectric and mechanical reliabilities of the insulation are improved by the newly developed nonadhesive plane.