All dielectric measurements involve the determination of the electrical polarization and conduction properties of a sample subjected to a time-varying electric field. Section 2 addresses dielectric measurement methods, the various instruments and electrodes, and their calibrations. Section 3 examines the microscopic mechanisms giving rise in the observed microscopic dielectric properties, and Section 4 explores in detail the effects of temperature and cure on these properties. Finally, Section 5 contains a selected bibliography of applications of dielectric analysis to the study of thermoset cure.

Dielectric Analysis of Thermoset Cure.

The effect of particles and moisture on the 60 Hertz breakdown strength of tranformer oil was investigated. Different sizes and concentrations of iron, copper and cellulose particles were used. The breakdown voltage was determined according to ASTM D1816 using VDE electrodes with an 80 mil (2 mm) gap. The breakdown strength of oil decreased approximately 25 percent when the moisture content was increased from 5 ppm to 20 ppm. A further reduction in the breakdown strength of oil occurred when iron, copper or cellulose particles were added to the oil. The transformer oil showed very low values of 60 Hertz breakdown strength as the result of the combined effects of moisture and particles' type, size and concentration.

Particles and Moisture Effect on Dielectric Strength of Transformer Oil Using VDE Electrodes

The inclined plane tracking and erosion test IEC-60587 is not specified for DC testing. A dc test has been developed from the current ac standard and three formulations of silicone rubber tested. These materials were tested under three voltage levels (2.3, 2.7 and 3.2 kV) for both polarities. Positive dc tests have the highest average and peak leakage current and exhibit a higher degree of surface damage. The observed surface degradation pattern is heavily dependant on polarity. Consistently higher levels of erosion have been observed in the higher voltage positive cases. Erosion of the surface always starts at the bottom electrode, and spreads toward the top electrode. Puncturing of the 6 mm thick samples or deep erosion over more than half the distance between electrodes has only been observed under 2.7 and 3.2 kV positive polarity tests. Results have been analysed using a variety of leakage current analysis techniques going beyond the criteria specified in the original ac standard. The low-frequency behaviour of the leakage current was monitored using a 15 sample per second current recorder. The leakage current magnitude is investigated in a case study and is shown to follow a normal distribution. Reversing the polarity of tests shows the leakage current seen in a test is largely independent of the surface degradation pattern present, but the surface degradation pattern over the first three hours may dictate the morphology of ensuing deep erosion.

/pdf/performance-of-silicone-rubber-in-dc-inclined-plane-tracking-1bsdxzttll.pdf

Performance of silicone rubber in DC inclined plane tracking tests

Both the dust/fog and IEC methods have been used to investigate the resistance to surface tracking and erosion of a range of synthetic insulating materials under polluted conditions The relative merits of these test methods are discussed, and reasons given for preferring the dust/fog test Consideration is also given to methods of accelerating the dust/fog erosion testResults of such tests have been considered from a theoretical aspect, and a relationship between chemical structure and resistance to surface tracking is proposed Using this approach, it was deduced that the alicyclic epoxy resins would be superior to the conventional bisphenol-based resins; this has been confirmed by dust/fog tests Tracking resistance can also be increased by use of fillers, and this is illustrated for silica and alumina trihydrate in some epoxy casting systemsThe application of track- and erosion-resistant materials to service use is considered, with special reference to casting resins It is shown that the use of fillers to improve track resistance may cause deterioration in other electrical and mechanical properties of the material

Performance of synthetic insulating materials under polluted conditions

https://iopscience.iop.org/article/10.1088/2053-1591/ab3f0d/pdf

Aging of silicone rubber-based composite insulators under multi-stressed conditions: an overview

Electrical failure of the surface of insulating materials can occur in several ways. These include: 1. Air breakdown (flashover) which transfers to the adjacent insulation surface. 2. Thermal degradation of the surface (from arc interruption nearby, etc.) which temporarily or permanently damages the surface so that electrical failure occurs. 3. Partial volume breakdown, particularly of alminates, just beneath the surface which produces thermal or other degradation so that final failure ultimately occurs on the surface. 4. Surface degradation from electrical corona starting initially in limited surface areas exposed to voltage gradients high enough to produce localized air breakdown. 5. Progressive failure across the surface resulting from tiny arcs (scintillation) caused by conducting, or partially conducting, surface contamination.

Surface electrical failure in the presence of contaminants: The inclined-plane liquid-contaminant test

Particles influence both AC and DC voltage breakdown in insulating oil. Larger and conducting particles decrease the breakdown voltage most. For both DC and AC the ratio of breakdown voltage for clean oil to oil with non-conducting particles is nearly the same. For conducting particles the ratio for DC is generally larger than for AC. Particle concentration and motion is influenced by DC and AC stress in different ways. Particles may be drawn into or expelled from high stress regions. Fine particles may agglomerate or be dispersed. Particle chains may develop. These complex physical factors have not been related directly to voltage breakdown in the oil. Particles may cause partial discharges to develop in oil but these discharges also have not been related directly to short time breakdown of the oil.

/pdf/influence-of-particles-on-partial-discharges-and-breakdown-55b0tdfq4m.pdf

Influence of particles on partial discharges and breakdown in oil

In the search for cable designs capable of transmitting larger blocks of power, consideration has been given to replacing oil in the oil/paper cable with liquified gases. This paper reports the evaluation of liquid hydrogen as the substitute for oil. The system is not operated at the superconducting region but at higher temperatures. Not only cellulosic paper but synthetic papers and films were evaluated. The results were very encouraging.

Cryogenic Cable Dielectrics

The electrical failure of contaminated insulation surfaces is caused by surface scintillation — tiny arcs. It is often characterized by progressive carbonization in a number of branching and sometimes interconnected channels often called "tracks." The appearance and nature of tracks is dependent upon the concentration of the contaminant and the voltage stress as well as the composition of the insulating material1. With some track resistant material formulations, tracking as described above does not occur, particularly at lower voltage stress and higher contaminant concentrations. Instead, a loss of volume — erosion of the insulation — occurs under the influence of the surface electrical scintillation.

Erosion — Inclined plane, liquid contaminant tracking test

Voltage breakdown is perhaps the most important factor in the performance of electrical equipment in service At high voltages corona* may lead to voltage breakdown especially over a period of time In liquid impregnated insulation systems, corona often varies tremendously with time in both magnitude and repetition rate, which complicates both its measurement and the understanding of its influence

K. N. Mathes

Papers

Surface electrical failure in the presence of contaminants: The inclined-plane liquid-contaminant test

Influence of particles on partial discharges and breakdown in oil

Cryogenic Cable Dielectrics

Erosion — Inclined plane, liquid contaminant tracking test

Corona in insulating liquids and in combinations of paper and liquid