Active High Voltage Insulation - A New Hybrid Insulation Concept with Dynamic and Active Features

Doctoral thesis, 2000

In this thesis, the concept of a new environmental-friendly and high-performance high voltage insulation is presented. Basically, it is an electric field steering technique where the dynamic process of charge accumulation, actively and advantageously, re-distributes the electric field within an insulation system. The concept is theoretically and experimentally demonstrated on a one-dimensional plane-parallel insulating structure consisting of an air-gap bounded by two dielectrically covered electrodes. The vital process of charge deposition and relaxation was experimentally studied in a highly lightning impulse (LI) voltage stressed air-gap. The temporal and spatial development of dielectric barrier discharges, studied through current measurements correlated with sequential high speed imaging, revealed fundamental properties of the highly stressed system. In general, an intense discharge was found at the front of the LI followed by a pulse train of significantly smaller discharges, but of opposite polarity, at the tail of the LI. During both a slowly rising dc-voltage and during a sequence of LI- voltages; voltage levels remarkably above the level which, according to the capacitive field distribution gives critical field strength in the air-gap, were reached. The improvements ranged from about 50% for a 27 mm dc-stressed gap, up to 350% for a 3 mm LI-stressed gap. Actual maximum withstand levels were not reached. It can be concluded that a significant feature is that the system adapts its insulation level with respect to what the applied voltage level requires by an accumulation of the appropriate quantity of charge. If the demand of charge exceeds the supply, additional free charge is automatically created through ionization. Although, ionization is not a pre-requisite behind the concept. Under ideal conditions, such a system will in equilibrium exhibit a zero air-gap field, e.g., the electric breakdown strength is only determined by the coatings. Thereby, the breakdown strength could be considerably improved in comparison with a conventional uncovered air-gap.