Effect of direct fluorination on surface charge of polyimide films using repetitive pulsed power

As one of the basic insulating materials, polyimide films have been widely used in turn to turn insulation and turn to ground insulation in wind turbine generators. However, the pulse voltage from the converter often causes overvoltage and localized high electrical field in the insulation system. Surface charge could affect the dielectric property and cause the breakdown of insulation. Direct fluorination is a method to modify the surface layer of the polymers without changing the bulk properties. As a result of the fluorination, hydrogen atoms are replaced by the fluorine atoms and the component of the polymer surface is changed, which can influence the surface charge accumulation and affect the electrical properties. The experiment in this paper was focused on the effect of direct fluorination on surface charge of polyimide films under pulse voltage. The polyimide films with the fluorination time of 15, 30, 45 and 60 min were prepared, together with the unfluorinated one as a comparison. Surface charging was performed by corona discharging using repetitive pulse power at the temperature of 25 °C with the relative humidity of ~45%. The rising time of the pulse was 25 μs and the frequency was set at 200, 400, 600 and 1000 Hz, respectively. The surface charge decay was measured using a specific probe coupled with an electrostatic voltmeter. The results showed that the surface charge behavior was affected by the fluorination, corona discharging time, pulse voltage amplitude and frequency.