Kenneth J. Thomas

Surface flashover of oil-immersed dielectric materials in uniform and non-uniform fields

The applied electrical fields required to initiate surface flashover of different types of dielectric material immersed in insulating oil have been investigated, by applying impulses of increasing peak voltage until surface flashover occurred. The behavior of the materials in repeatedly over-volted gaps was also analyzed in terms of breakdown mode (some bulk sample breakdown behaviour was witnessed in this regime), time to breakdown, and breakdown voltage. Cylindrical samples of polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, and Rexolite, were held between two electrodes immersed in insulating oil, and subjected to average applied electrical fields up to 870 kV/cm. Tests were performed in both uniform- and nonuniform- fields, and with different sample topologies. In applied field measurements, polypropylene required the highest levels of average applied field to initiate flashover in all electrode configurations tested, settling at ~600 kV/cm in uniform fields, and ~325 kV/cm in non-uniform fields. In over-volted point-plane gaps, ultra-high molecular weight polyethylene exhibited the longest pre-breakdown delay times. The results will provide comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage, pulsed-power machines.

Effect of applied field and rate of voltage rise on surface breakdown of oil-immersed polymers

In sub-systems of high-voltage, pulsed-power machines, the introduction of a solid into bulk liquid insulation located between two conductors is often necessary to provide mechanical support. Breakdown events on or around the surface of the solid can result in permanent damage to the insulation system. Described in the present paper are experimental results pertaining to surface breakdown of five different solid dielectrics held between plane-parallel electrodes immersed in mineral oil. The effect of varying level of peak applied field from 200 kV/cm (dV/dt 70 kV/μs) to 1 MV/cm (dV/dt 350 kV/μs) is investigated, and the breakdown voltages and times to breakdown are compared to those for an open oil gap. The time to breakdown is shown to be reduced by the introduction of a solid spacer into the gap. Rexolite and Torlon samples suffered significant mechanical damage, and consistently showed lower breakdown voltage than the other materials ¿ average streamer propagation velocity up to 125 km/s was implied by the short times to breakdown. Although ultra-high molecular weight polyethylene yielded the longest times to breakdown of the five types of liquid-solid gap, breakdown events could be initiated at lower levels of applied field for spacers of this material than those with permittivity closely matched to that of the surrounding mineral oil. Polypropylene and low-density polyethylene are concluded to provide the most stable performance in mineral oil. Due to the similarity of the applied voltage wave-shape (1/6.5 μs) to short-tail lightning impulses, the results may also be of interest to high-voltage system designers in the power industry.

Impulse-driven Surface Breakdown Data: A Weibull Statistical Analysis

Surface breakdown of oil-immersed solids chosen to insulate high-voltage, pulsed-power systems is a problem that can lead to catastrophic failure. Statistical analysis of the breakdown voltages, or times, associated with such liquid-solid interfaces can reveal useful information to aid system designers in the selection of solid materials. Described in this paper are the results of a Weibull statistical analysis, applied to both breakdown-voltage data and time-to-breakdown data generated in gaps consisting of five different solid polymers immersed in mineral oil. Values of the location parameter γ provide an estimate of the applied voltage below which breakdown will not occur, and under uniform-field conditions, γ varied from 192 kV for polypropylene (PP) to zero for ultrahigh-molecular-weight polyethylene (UHMWPE). Longer times to breakdown were measured for UHMWPE when compared with the other materials. However, high values of the shape parameter β reported in the present paper suggest greater sensitivity to an increase in applied voltage-that is, the probability of breakdown increases more sharply with increasing applied voltage for UHMWPE compared to the other materials. Analyzing peak-applied-voltage data, only PP consistently reflected a low value of β across the different sets of test conditions. In general, longer mean times to breakdown were found for solid materials with permittivity more closely matched to that of the surrounding mineral oil.

Impulse-Breakdown Characteristics of Polymers Immersed in Insulating Oil

Surface discharges along oil-immersed solids used as insulators and supports in high-voltage pulsed-power equipment can lead to catastrophic system failures. To achieve reliable compact pulsed-power systems, it is important to quantify the electrical fields at which surface flashover, or other types of breakdown event, will occur for different dielectric materials. This paper reports the observed behavior of samples of polypropylene, lowdensity polyethylene, ultrahigh-molecular-weight polyethylene, Rexolite, and Torlon, which were subjected to impulse voltages of peak amplitude of 350 kV and a rise time of 1 μs. The cylindrical samples were located between pairs of electrodes immersed in insulating oil. Breakdown events were studied under both nonuniformand uniform-field conditions, with sample lengths being chosen so that the breakdown events occurred on the rising edge of the impulse. Ultrahigh-molecular-weight polyethylene showed the highest average breakdown field, which is 645 kV/cm, in uniform fields, and the corresponding breakdown field was reduced to ~400 kV/cm in the nonuniform fields. Weibull plots of the various sets of results are presented, providing comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage pulsed-power machines.

Breakdown of mineral oil: Effect of electrode geometry and rate of voltage rise

Experimental data on the propagation of streamers in mineral oil is important for the design of high-voltage systems in the power and pulsed-power industries. In the present study, breakdown voltages and pre-breakdown delay times were measured for plane-parallel electrodes, and for two non-uniform electrode arrangements. For each geometry, the breakdown characteristics were determined for impulses of rise-time 100 ns, and also rise-time 1 μs. The maximum rate of voltage rise (dV/dt) was 4 MV/μs. For the non-uniform geometries with inter-electrode gap length of 8.5 mm, the time to breakdown was 2.5-3 times longer for impulses of rise-time 1 μs than for 100 ns risetime. The time-to-breakdown data suggest that streamer propagation velocity increases with higher values of dV/dt. For example, the estimated propagation velocity for pinplane geometry with a 1 μs rise-time is 10-12 km/s. At 100 ns rise-time for the same electrode geometry, the average propagation velocity exceeds 40 km/s. The results are compared with data previously generated in parallel liquid-solid gaps, and it is concluded that the time to breakdown is longer, and that higher applied fields are required to initiate breakdown, in open oil gaps compared to the case when a solid spacer is present. The results presented are intended to provide reference data for designers of oil-immersed, high-voltage systems such as power transformers and pulsed-power supplies.

Prototype IVA module

A prospective module has been designed for a future large inductive voltage adder (IVA) machine that would be suitable for the most demanding radiography carried out at AWE Aldermaston, Berkshire, UK. This machine would operate at 13-15 MV. The prototype IVA module (PIM) machine has been built to test a single such module and may also provide the basis for other, lower voltage machines in the future. It consists of a single inductive cavity pulsed by a 10-/spl Omega/ water dielectric Blumlein pulse forming line which is charged by a Marx generator. The Blumlein switch initially installed was a coaxial two stage Rimfire and Trigatron switch. A laser triggering configuration utilizing two radial switches, designed by Titan PSI, is also to be tested. The Blumlein PFL and its switch were tested using a copper sulphate resistive load and achieved a peak output voltage of 1.7 MV. The inductive cavity has been added and a pulse of 1.5-MV peak voltage has been successfully applied to it.

Experiments on PIM in support of the development of IVA technology for radiography at AWE

The PIM machine has been designed and constructed at AWE as part of a program to investigate IVA technology for radiographic applications. PIM, as originally constructed, was a prospective single module of a 14 MV, 100 kA, ten module machine. The design of such a machine is a primary goal of the program as several are required to provide multi-axis radiography in a new Hydrodynamics Research Facility (HRF). Another goal is to design lower voltage machines (ranging from 1 to 5 MV) utilizing PIM style components. The original PIM machine consisted of a single inductive cavity pulsed by a 10 ohm water dielectric Blumlein pulse forming line (PFL) charged by a Marx generator. These components successfully achieved their design voltages and data on the prepulse was obtained showing it to be worse than expected. This information provided a basis for design work on the 14 MV HRF IVA, carried out by Titan-PSD, resulting in a proposal for a prepulse switch, a prototype of which should be installed on PIM by the end of this year. The original single, coaxial switch used to initiate the Blumlein has been replaced by a prototype laser triggered switching arrangement, also designed by Titan-PSD, which it was desired to test prior to its eventual use in the HRF. Despite problems with the laser, which will necessitate further experiments, it was determined that laser triggering with low jitter was occurring. A split oil co-ax feed has now been used to install a second cavity, in parallel with the first, on the PIM Blumlein. This two cavity configuration provides a prototype for future radiographic machines operating at up to 3 MV and a test facility for diode research.

Results of the Installation of a Prepulse Reduction System on the PIM Blumlein

In the PIM pulsed power machine a 1.5 MV, 100 ns duration pulse is generated by a 10 Ohm water Blumlein A prepulse reduction system based on a gas switch has been designed and built by Titan PSD. This has been fitted and reduced the prepulse to less than 3 kV. Prepulse of this order is necessary to drive the focused e-beam diodes used for flash radiography. Details of the prepulse reduction system, the experimental data obtained and comparison with computer models of the machine will be presented.

Surface Flashover of Dielectric Materials Used in Pulsed Power Research

Summary form only given. Insulator technology has evolved steadily for power transmission and distribution applications over the past 30 years, allowing for reliable design rules and testing procedures to be developed. These rules and procedures have been validated for applications under DC. and power frequency conditions (50/60 Hz and 400 Hz in aircraft applications), allowing for the appropriate choice of insulation to be made. Selection procedures for insulators to be used under high voltage/high-current pulsed power conditions are however, not well defined, and insulator failure has occurred in large pulsed power facilities when the materials were selected based on experience with low frequency systems. This work is being undertaken to investigate dielectric material behavior under high dE/dt conditions. A 10-stage Marx generator, capable of producing output voltages in the region of 750 kV, is used in conjunction with an adaptable test assembly to determine the flashover properties of different materials. Results on the dielectric behavior and ageing of solid insulators are reported, along with hold-off voltage performance and degradation due to surface discharges.

The statistical and formative times for breakdown at a polymer-oil interface

The average statistical time and formative time associated with breakdown across a polymer-oil interface have been extracted from experimental time-to-breakdown data, gathered using a non-uniform electrode geometry designed to promote surface discharges. Five polymer materials of relevance to the designers of pulsed-power systems have been considered: polypropylene; LDPE; UHMWPE; Rexolite; and Torlon. It has been assumed that the temporal distribution of breakdowns can be described using the approach developed by von Laue, and the validity of this approach is discussed within the paper. The materials were clearly separated in terms of the statistical component of the overall time to breakdown, with Torlon showing the shortest average statistical time (~17 ns), and Rexolite the longest (~54 ns). However, the derived formative times were shorter for Torlon and Rexolite than for the other materials; PP, LDPE, and UHMWPE have similar formative times, at ~350-400 ns.