Thesis

Liquid water as a dielectric insulator

Creator
Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2012
Thesis identifier
  • T13259
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • Water is used as a dielectric liquid in the pulsed power industry for energy storage, high voltage insulation, acoustic devices and in fast closing switches such as in the Z-machine, the largest X-ray impulse generator in the Sandia National Laboratory in America. The dielectric behaviour of water stressed with high voltage impulse is not fully understood. This project is focused on the investigation of pre-breakdown transient processes in liquid water stressed with high voltage impulses. The tasks in this project were: to develop an experimental test bed, including the high voltage impulse generator, a pulse shaping circuit, a water cell reactor and specialist measurement probes to record current, voltage and electric field under pre-breakdown and post breakdown conditions; to record transient electric fields of a water gap subjected to inhomogeneous, high intensity electric stress from a high voltage impulse; to analyse data using digital signal processing techniques; to use statistical analysis techniques to extract discharge initiation and propagation time lags and to evaluate the velocity of discharge propagation. From literature, a review of the breakdown processes in water has been conducted. Breakdown field (voltage) and time lag to breakdown results have been reviewed and a new characteristic fitting has been discovered. Advanced simulation packages were used to model the experimental test bed. The impulse generator and pulse shaping circuit were modelled in PSpice along with the electric field probe (D-dot probe) and Rogowski coil current measurement probe design. The electrode topology was modelled using Quickfield electrostatic modelling software, simulating the electric field distribution of the water test cell. Electrostatic simulations confirmed the placement of the electric field (D-dot) probe and the impact the probe has on the test cell. A test cell was designed and manufactured to house the point plane electrode topology submerged in water. Breakdown events have been observed and recorded for different inter-electrode distances (2 mm, 5 mm, and 10 mm) in distilled water (~ 0.3 μS/cm); tap water (~ 60 μS/cm); and a water ionic solution (~600 μS/cm). Both polarity voltage impulses were investigated and using Laue statistical analysis the statistical and formative time lags were extracted and discharge velocities calculated. It has been shown using Laue analysis that the speed of streamer propagation in water for positive applied voltage is cm/s in distilled water and ~19 cm/s in tap water. Negative streamers are shown to propagate at ~1 cm/s in both tap and distilled water. The results are consistent with literature data where propagation is measured using optical techniques.
Resource Type
DOI
Date Created
  • 2012
Former identifier
  • 948154

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