The paper presents studies on the local damage dynamics in various designs of semi-conductive slot coatings of the turbogenerator stator windings under operating conditions. The authors used combined effects of electric voltage, temperature and ozone to imitate the operating conditions of a real air-cooled turbogenerator in a laboratory. They developed a methodology for studying and evaluating the local damage dynamics in semi-conductive enamel and tape slot coatings. Then, the coatings were tested in laboratory conditions on mock-ups for comparison. No significant differences were found in the growth rate of local defects at the selected level of impacts for enamel or tape coatings and their combinations. However, the use of protective dielectric varnishes reduced the degradation rate of coatings.