The role of electrical insulation is critical for the proper operation of electrical equipment. Power equipment cannot operate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating certain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1]-[3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12]. In addition to their main functions of protecting solid insulation, quenching arc discharges, and dissipating heat, insulating liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equipment, liquid samples can be obtained without service interruption.

50 years in the development of insulating liquids

Over the past several years, a new surge of interest in paper electronics has arisen due to the numerous merits of simple micro/nanostructured substrates. Herein, the latest advances and principal issues in the design and fabrication of paper-based flexible electronics are highlighted. Following an introduction of the fascinating properties of paper matrixes, the construction of paper substrates from diverse functional materials for flexible electronics and their underlying principles are described. Then, notable progress related to the development of versatile electronic devices is discussed. Finally, future opportunities and the remaining challenges are examined. It is envisioned that more design concepts, working principles, and advanced papermaking techniques will be developed in the near future for the advanced functionalization of paper, paving the way for the mass production and commercial applications of flexible paper-based electronic devices.

Flexible Electronics Based on Micro/Nanostructured Paper.

This paper presents an experimental study of positive streamer propagation in mineral oil, in large point-plane gaps under impulse voltage. A systematic investigation was done concerning the influence of gap distance (/spl les/35 cm), and voltage from streamer inception up to large overvoltages. The measurements presented concern breakdown voltage measurements, time to breakdown, visualization of streamers (streak and still photographs), electrical measurements (transient current, charge), light emission intensity. Streamers are characterized and then classified into different modes (2nd, 3rd and 4th modes) according to their propagation velocities up to more than 100 km/s. According to the applied voltage, transitions between modes are observed, and correlations are established between charge, shape, and streamer velocity. The physical mechanisms as well as practical consequences are then discussed.

Positive streamer propagation in large oil gaps: experimental characterization of propagation modes

Interests on ester liquids as replacement of mineral oil in large power transformers are growing in recent years. This paper presents experimental studies on streamer and breakdown in synthetic and natural esters. Lightning impulse voltage was chosen when considering the practicality of standard tests of transformer industry. First, streamer length, velocity, shape and mode were analyzed based on optical images obtained by a multi-channel ultra high speed camera. Under the same voltage level, streamer in both natural and synthetic esters propagates faster and further, with more branches, than in mineral oil. Breakdown tests at various gaps from 15 mm to 100 mm were conducted, and it was found that fast streamer led breakdown becomes statistically dominant in a large gap under lightning impulse and ester liquids have lower breakdown voltage than mineral oil due to their low tolerance to fast streamer. Therefore a relationship between the results under lightning impulse and previously published results under step voltage was built up and an empirical formula was obtained to predict the breakdown voltage of esters at very large gaps.

Streamer characteristic and breakdown in synthetic and natural ester transformer liquids under standard lightning impulse voltage

Dielectric response is a non-invasive diagnostic method with a potential to give information about the status of the solid insulation in a power transformer. To investigate what conditions the method can and cannot reveal, frequency domain spectroscopy has been applied to oil-impregnated paper and pressboard with and without moisture and/or acids added, and to paper at various stages of aging. Water and carboxylic acids are important aging indicators. It is found that moisture is clearly detectable. Also low molecular weight carboxylic acids will affect the dielectric response in a way similar to moisture, but high concentrations are needed for a contribution comparable to that of moisture. It is difficult to tell moisture and acids apart from the responses obtained. High molecular weight carboxylic acids make little or no contribution to the dielectric response of the solid insulation in a paper/pressboard - oil system. It was also observed that keeping paper or pressboard at elevated temperatures had a conditioning effect on the response

Dielectric response of mineral oil impregnated cellulose and the impact of aging

Inception and propagation of streamers in a point-plane gap, with and without pressboard interface parallel to the field, have been studied. Current and light emission have been recorded. Positive streamers propagate at /spl sim/2 to 4 mm//spl mu/s at voltages to 2/spl times/ the minimum breakdown voltage. Above this voltage, a fast event (>100 mm//spl mu/s) may occur, preceded by streamers propagating with velocities in the 10 to 20 mm//spl mu/s range. The negative streamers have /spl sim/2/spl times/ the breakdown voltage as the positive ones. Even negative streamers may develop into a slower kind of fast event. Solid pressboard parallel to the field does not change the breakdown voltage, but makes inception of fast events easier. The breakdown process in oil shows several similarities to a gas breakdown.

Propagation of positive and negative streamers in oil with and without pressboard interfaces

A 67 mm point-plane gap immersed in transformer oil, at atmospheric pressure, stressed with a 1/180 /spl mu/s impulse, has been studied by recording gap current and light emission. Time-resolved images were obtained by use of an image converter camera. Minimum breakdown voltages were twice as high for negative as for positive points. All streamers at these and higher voltages were supersonic, with negative streamer velocities /spl les/50 km/s. The speed of the positive streamer gradually increased to 19 km/s with increasing voltage, where it saturated. At high stresses the propagating positive streamer has been observed to switch to a faster mode which propagates with speeds in the 65 to 200 km/s range. Positive streamers were bush shaped to 2/spl times/ the minimum breakdown voltage, the branching diminishing with increased voltage. Negative streamers were coarsely bush shaped or usually tree shaped. For both polarities, light emission consisted of a background light, with superposed pulses corresponding to 15 ns wide current pulses of up to 10 A. The pulses are caused by very bright, brief re-illuminations of single streamer channels. The tips of positive streamers are considerably more luminous than the channels, indicating that electron multiplication may take place at the tips. A model of the streamers as being plasma filled channels may explain the re-illuminations. >

Streamer propagation under impulse voltage in long point-plane oil gaps

Hydrolysis of transformer insulating Kraft paper is experimentally investigated. Paper moisture content was varied and five different acids representative of what may be found in an aging transformer - were added to the oil. The low molecular weight acids were to a large degree absorbed by the paper, contrary to the high molecular acids that remained in the oil. The three low molecular acids accelerated the aging of the paper, and a clear synergy between these acids and water content was found. The high molecular acids did not influence the paper aging significantly. The results support a theory explaining the hydrolysis as governed by acid catalysis. Examples from measurements on real transformers are shown. It is concluded that low molecular acids constitute an important aging factor for transformer windings and should be included in diagnostic schemes, and when estimating the pay-back from maintenance actions.

Aging of Kraft paper by acid catalyzed hydrolysis

In this letter, we experimentally demonstrate that the lightning impulse breakdown characteristics of natural ester liquids can be significantly enhanced by dispersing molecular additives possessing lower values of both the ionization potential and 1st excitation energies as compared to the base liquid One such additive contributed to an increase in the breakdown and acceleration voltage of the base liquid by 32% and 90%, respectively Apart from the expected influence of the low ionization potential of the additives, results also indicate a positive effect of lower value of 1st excitation energy

D. Linhjell

Papers

Dielectric response of mineral oil impregnated cellulose and the impact of aging

Propagation of positive and negative streamers in oil with and without pressboard interfaces

Streamer propagation under impulse voltage in long point-plane oil gaps

Aging of Kraft paper by acid catalyzed hydrolysis

Enhancements in the lightning impulse breakdown characteristics of natural ester dielectric liquids