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

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

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

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

Investigation of the Static Breakdown Voltage of the Lubricating Film in a Mechanical Ball Bearing

In this paper, the current pulse (discharge) activity in a small bearing is investigated when current-limited DC voltages of different levels are applied to the shaft under varying rotational speeds and mechanical loads. The measurements show that the inception voltage of discharges in the bearing is higher than the extinction voltage. However, when the discharge activity has been intense the bearing exhibits difficulties to recover its insulating properties when the voltage is decreased to a low voltage level and thus the discharge activity continues. The measurements also show that at low voltages the bearing acts as an insulator but if the voltage is increased the bearing starts to act as a conductor. This transition occurs during a narrow voltage interval. The time interval between two consecutive current pulses decreases significantly with increasing shaft voltage

Investigations of Particle-Initiated Insulation Breakdowns in Bearings

Early failures in bearings of wind turbine drivetrains have increased after introduction of power electronic switches, which leads to shaft voltages and bearing currents. In presence of voltage, a rupture of bearing insulation could occur due to several plausible electro-physical mechanisms viz., asperities, electric breakdowns, particles, etc. The flow of high amperage current through the bearing during a breakdown mechanism could lead to early failures. Our aim is to understand the electrical behaviour of a bearing and elaborate by an equivalent electric circuit model, emphasizing on particle-initiated breakdowns. In presence of a shaft voltage, the particles form a path of low resistance through the bearing and results in flow of shaft or bearing currents, which could cause pre-mature failure of the bearing. Particles such as Arizona Test Dust (ATD), carbon black, aluminium powder and fine iron powder were mixed in lubricant at particle concentrations ranging between 7.5 mg/L and 150 mg/L. The breakdown characteristics of electrical insulation of the bearing during a given test is quantified as time of conduction, which is expressed as a percentage of the time the bearing is in the conducting state during a test. An investigation of time of conduction for different lubricant samples was conducted along with studying the effects of start and stops of the rotating shaft. The electrical conductive nature of the particle played no role in breakdown of bearing voltage. At a fixed concentration of 150 mg/L, the insulation breakdown events were highest in lubricant with ATD, followed by iron powder, aluminium power and carbon black particles. The time of conduction increases up to 24 times for the same test lubricant, as the particle concentration was increased from 7.5 mg/L to 150 mg/L. The current activity reduced to almost half in the test after stopping the shaft rotation. The resistance of bearing during insulation breakdown events is highest for aluminium powder, followed by fine iron powder, carbon black and ATD