Polyimide film is widely used in different areas due to its brilliant chemical and electrical properties. This chapter discusses effect of direct fluorination on surface charge and surface partial discharge of polyimide films. The influences of different fluorination time and voltage frequency are studied. The trap properties of polyimide film are analyzed. It is clear that the fluorination has changed the micro structure of the sample so that the trap property and the dissipation time are different. Further research focuses on the effects of water absorption on surface charge and dielectric breakdown of nanocomposite. The polyimide/Al2O3 film is selected as sample and the relation between water absorption and surface charge and breakdown voltage is discussed.
TopIntroduction
Polyimdie is one type of universal engineering materials. Owing to its excellent chemical and physical properties, polyimide films have been widely used in turn to turn insulation and turn to ground insulation in wind turbine generators. PI has been used as an important insulation material (Du,B.X.,2013;Xin,H.,2005). Researches show that PI exhibits excellent combination properties: good dielectric property and heat-resistance, excellent chemical stability, high flame retardancy and mechanical strength, as well as radiation and weather resistance(Du,B.X.,2013).
However, the pulse voltage from the converter, which is imposed on the double-fed induction generator insulation, has a fast rising time, high repetition frequency and amplitude compared with the common sinusoidal voltage(Sun,P.,2012). It often causes overvoltage and localized high electrical field in the insulation system, leading to the premature failure of the insulations, partial discharge and acceleration of insulation degradation (Wheeler, J. C. G.,2005). Partial discharge will occur in insulation systems of high voltage power equipment. Surface discharge can occur in liquid, vacuum or gas near to the surface of solid dielectric (Preetha, P.,2011). Insulating system usually contains various types of defects. These defects will be the root of partial discharge(Florkowska, B.,2012). Due to the discharge plasma close to the surface of solid dielectric, the surface of insulation material will be degraded gradually. The longtime of partial discharge can cause the breakdown of electrical insulation system which will result in apparatus failures. Because of the existence of carbonyl and amine groups, the PI surface is hydrophilic which results in the absorption of water. Its insulating properties are vulnerable to be degraded after absorbing water, thus leading to the increasing risk of electrical breakdown and the appearance of malfunction or irremediable damage in the operating system.