Dry ice cleaning to improvise dielectric features of high voltage windings in turbine generators

Surface electrical discharges and tracking in service are attributed to contamination of end windings in high voltage turbine generators. Unnoticed over a period of time, it deteriorates winding dielectric, results in low insulation resistance(IR), smaller polarization index(PI) number and higher tanδ. Dirty windings block the ventilation, increases power loss and reduce efficiency. Eventually, machine stalls with sudden earth fault. This paper discusses the novel purification method viz. Dry Ice Cleaning (DIC) to clean the stator end windings of turbine generator in a power plant. Effect of contamination on dielectric parameters of windings is emphasized. Traditional practices and cleaning agents to remove debris on overhangs are presented. New process of solid CO₂ cleaning for dirt removal during overhaul is described. Techno-economic aspects in usage of this cryogenic material and technology are highlighted. Experience on 120MW, 11 KV turbine generator to enhance the life through DIC cleaning is presented. Condition assessment dielectric test results before and after DIC are depicted and compared. Recommendations are given to asset managers of power plants for eco friendly cleaning technology to protect the environment and enhance life of the machine.