Modeling of Partial Discharges from a Spherical Cavity within a Dielectric Material under Variable Frequency Electric Fields
Partial discharge (PD) is a discharge event that does not bridge the electrodes within an electrical insulation system under high voltage stress. In high voltage components, the measurement of partial discharge is used in the performance assessment of an insulation system. Through modeling the discharge process a better understanding of the phenomena may be attained. Previous studies have considered the modeling of PD activity from a cylindrical cavity. However, this is a limited approach as the most common types of cavities found in the insulation materials are either spherical or ellipsoidal. This paper describes the development of a model for a spherical cavity within a homogeneous dielectric. The model developed has been used to study the influence of the applied frequency on partial discharge activity and obtained results are presented.
Modeling partial discharges in a cavity at different applied frequencies
A model of partial discharges (PD) in an insulated disc-shaped cavity is presented. The flat cavity walls are covered with copper foil and each PD is assumed to affect the whole cavity. The discharge process in the cavity is simulated dynamically and the model is charge consistent. The model is used to simulate the sequence of PDs in the cavity at two different applied frequencies: 0.02 Hz and 100 Hz. The simulation results are compared with measurements and good agreement is found. As the applied frequency is increased from 0.02 Hz to 100 Hz, the mean apparent charge increases and the number of PDs per cycle of the applied voltage reduces. This is interpreted as an effect of the statistical time lag of PD.
Effects of reservoir volume on performance of pulse tube cooler.
A pulse tube cooler has the advantages of long-life and low-vibration over conventional cryocoolers such as G-M and Stirling coolers because of the absence of moving parts at low temperature. On the other hand, the combination of a reservoir and orifice is indispensable to optimize the performance of pulse tube coolers. In order to make the pulse tube cooler compact for practical applications, the volume of reservoir should be minimized. This paper analyzes the effects of the reservoir volume on the thermodynamic performance of various components in a simple orifice and a double-inlet pulse tube cooler by combining a linearized model with a thermodynamic analysis. Expressions of entropy production for those components are presented. The results show that the reservoir volume has a significant influence on the entropy production in the various components when the reservoir to pulse tube volume ratio is smaller than about 5. The ratio is important to determine the minimum reservoir volume for a pulse tube cooler. Optimum settings for a double-inlet pulse tube cooler are also discussed.