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 breakdown voltage induced by single metallic cylinders of millimeter lengths in a quasi-uniform ac field are reported. A particular electrode arrangement, including forced oil circulation, has been designed in order to avoid particle ejection out of the gap. Breakdown occurred in the range 25 to 80 kV/cm depending on the particle dimensions, and always when the particle was in contact with the positive electrode. These breakdown levels are clearly lower than those measured with a fixed particle subjected to a positive step voltage. All these measurements were sensitive to the influence of carbon contaminants, especially under step impulse voltage. As concerns free particles, the breakdown is thought to be controlled in our experimental conditions by the initiation of positive streamers. Calculations showed that the initiation field at the particle tip is not constant but mainly depends on the particle diameter.

http://ieeexplore.ieee.org/iel4/94/10237/00484307.pdf

Particle-initiated breakdown in a quasi-uniform field in transformer oil

The insulating performance of oil is vulnerable to particles especially the conductive particles. This paper investigated the influence of copper particles of micrometer size on the breakdown strength and frequency-dependent properties of vegetable oil. The AC breakdown voltage of contaminated vegetable oil with copper particles of different numbers (ranging from 103 to 106) was measured. The frequency-dependent dielectric properties including volume resistivity, relative permittivity and dissipation factor of the contaminated vegetable oil with copper particles of various volumetric concentrations (0.011%, 0.056% and 0.112%) were measured. Identical experiments were conducted with mineral oil for comparison. Results show that the AC breakdown voltage of vegetable oil decreases with the increase of copper particle concentration. The mineral oil exhibits a similar trend, but the influence of copper particles on the AC breakdown voltage of vegetable oil is less significant than that on mineral oil because of its greater viscosity. The relative permittivity and dissipation factor increase with increasing copper volumetric concentration while the volume resistivity decreases. The influence of copper particles on them is significant at low frequencies and this influence becomes inconspicuous as the frequency increases. Similar results were observed for mineral oil.

Influence of Copper Particles on Breakdown Voltage and Frequency-Dependent Dielectric Property of Vegetable Insulating Oil

Probability distributions of partial discharge (PD) and dielectric breakdown voltage were investigated for the typical components of oil-filled transformer insulation.

PD and BD Probability Distribution and Equi-Probabilistic V-t Characteristics of Oil-Filled Transformer Insulation

A study of particle levels in transformer oil was conducted, covering about 200 samples taken from field and factory units. Factory samples were included to determine a relative base condition on which to compare field data. Particle distribution curves show size and count relationship in transformers. The particles were studied also by Atomic Absorption Spectroscopy to measure the content of iron, copper and other common metals expected. Particle analysis can supplement more standard laboratory techniques to judge the condition of oil in operating power transformers. The diagnostic value of such analysis, especially in determining pump performance problems, is illustrated with examples.

Particle Contamination Levels in Oil-Filled Large Power Transformers

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

New concepts for a gas-insulated power transformer include a very compact winding construction, comprised of sheet aluminum conductor insulated with polymer film and impregnated with compressed SF 6 gas. 60 Hz partial discharge and breakdown studies on models built with this construction have been made at 25, 100, and 125°C. A comparison has been made between models constructed with Mylar film, the material of choice to date, and with Kapton film, a backup material. For both materials, the measured performance at elevated temperature was found to be equal to or better than that at 25°C. A very similar performance was exhibited by the two polymer films.

http://ieeexplore.ieee.org/iel7/7465133/7465134/07465217.pdf

J.M. Atkins

Papers

Influence of particles on partial discharges and breakdown in oil

The influence of temperature on partial discharge and breakdown behavior in model gas transformer windings