Experience on a 362 MVA Turbogenerator pressplates cracks evolution at underexcited conditions

Abstract

The operation of turbogenerators at underexcited conditions increases the heating in the end-core region, especially in the pressplates. The pressplates should have excellent mechanical properties but low magnetic permeability to avoid high losses due to magnetic flux, so exotic steels and irons are used for this application. This paper presents the experience on a 362 MVA turbogenerator which operates at underexcited conditions for more than ten years, always in the allowable area according to the capacity curve. In the pressplates of this machine several cracks appeared, with the risk of catastrophic failure. Although the manufacturer recommendation was to replace the cracked pressplates, the material and the cause of the cracks have been analyzed and it has been determined than in this particular case, changing the machine operation, the risk of the growth of the cracks was very low. After one year of operation under our specified operating conditions, the size of the cracks has remained the same, avoiding some months of machine overhaul.