The dc breakdown strengths of 4-15 mu m thick capacitor grade polypropylene (PP) films were determined. The measurements were performed with an automatic measurement system using five electrodes with areas between 0.045 and 9.3 cm(2). Some samples were studied with larger electrodes. Samples were not aged but were measured as received. The measurements were performed in air at room temperature and typically 40-80 breakdowns per sample per electrode area were collected. The Weibull distribution parameters alpha and beta were fitted to the data. Depending on the electrode area and the film grade, the alpha-parameter values obtained were between 450 and 850 Wpm. The breakdown strength values showed decreasing area dependence with decreasing electrode area and the Weibull scaling law was not valid for the smallest electrode areas. The alpha-values for the large sample areas were extrapolated from the small area data by area and Weibull extrapolation. The area extrapolation overestimated the breakdown strength at 4 m(2) by 40-50% whereas the Weibull extrapolation gave an accuracy of +/- 15%. The extrapolated breakdown strengths for the full-scale power capacitors deviated from the capacitor manufacturers test data. This indicated that different defect distributions were responsible for the breakdown strengths in the oil impregnated power capacitors than in the small dry film samples.

An automatic method for determining the breakdown strength of thin polymer films was evaluated. The equipment consists of a scanning electrode arm with which the breakdown voltage over the film sample is measured. The measurement electrodes were of solid brass and the ground electrode was of thin aluminum foil. Five different electrode areas between 0.045 and 9.3 cm 2 were used and 40-100 individual measurements per electrode area per sample were performed. All measurements were performed in air. The Weibull function parameters alpha and beta were fitted to the obtained data. Different features concerning the measurement system and conditions, e.g. the criteria for the automatic detection of the breakdowns, the effect of the electrode edge design, the DC ramp speed, partial discharges and humidity were analyzed. In order to control the reproducibility of the results, also another type of measurement setup was tested. It was concluded that the measured alpha-parameter values were stable and repeatable over a period of several years. The average beta-parameter values varied plusmn10-30%, more for the large than the small electrodes, and were also sensitive to changes in the sample, in the measurement conditions and the choice of the electrode system

DC breakdown voltages of 8-20 /spl mu/m thick poly(ethylene terephthalate) (PET) films were measured as a function of electrode area. The area of the measurement electrodes were between 0.045-9.3 cm/sup 2/. Also 13-15 m/sup 2/ capacitor elements were tested. Weibull distribution parameters, scale parameter /spl alpha/ and shape parameter /spl beta/, were fitted to the obtained breakdown strength data. Based on the small area results, short term breakdown strength of large area capacitor elements was predicted. Area extrapolation based on the scale parameter /spl alpha/ was compared with Weibull extrapolation, in which also the shape of the distribution is taken into account. It was concluded that the Weibull extrapolation better represented the large area breakdown strength than the area extrapolation.

For an improved design of high voltage apparatus a material with a low dielectric constant and a relatively high breakdown strength is advantageous. Therefore the insulating properties of polymer foam with closed pores are investigated. It was found, that for a foam with closed gas filled spheres the breakdown voltage is close to the partial discharge inception voltage and it depends on the void size and the degree of foaming. For practical applications a void diameter lower than 50 /spl mu/m in combination with a high gas content is desirable. The tested foam had a breakdown strength of 6 kV/mm and a relative dielectric constant of 1.1. Using this material a decrease of 30% of the size of high voltage electrodes can be achieved.

In laboratory set-ups, polymer covering of the electrodes has resulted in the reduction of the insulating distances by a factor two. One of the problems with covered conductors is however that of creeping discharges on the surface of the covering. The aim of this work was to investigate how creeping discharges develop under lightning impulse (1.2/50 /spl mu/s) on two covered conductors at a 90-degree angle to each other. Furthermore, the aim was to investigate how to stop the propagation of a creeping discharge. This was achieved by inserting at least one cable termination, e.g. the type of termination used for high voltage cables, into the discharge path. The experiments showed that a set-up with two crossing cables can withstand very high voltages without any shed or termination of the creeping discharges. Further tests show a considerable increase of the breakdown voltage in other arrangements comprising covered electrodes. This was accomplished by using two cable terminations at each cable end to stop the propagation of creeping discharges.

Dielectric sample combinations were tested in a Pulse Electro Acoustic measurement set-up. It was seen that charge accumulates in a predictable way due to a difference in resistivity and permittivity of the materials. Furthermore it was seen that flame-oxidation of the surface of a specific material diminishes the charge accumulation at the insulator/insulator interface. No relation could be found between different grades of surface roughness and charge accumulation at the interface

A single drop on an insulating surface stressed by an electric field parallel to it has been studied for a range of drop volumes for different insulating materials, under both DC and AC stress. The results show that on hydrophobic surfaces, the electrohydrodynamic effects are quantitatively the same as for a free drop. The less hydrophobic the surface, the larger is the deviation from free drop behavior. For DC, the drop elongation occurs somewhat stepwise due to contact angle hysteresis. For AC, the elongation is smooth and flashover due to instability occurs at a lower field strength. When the AC frequency is increased above the power frequency, the flashover voltage increases. Vibration modes for drops on hydrophobic surfaces are very similar to free drop modes

The commenters suggest that the low average field strength to start partial-discharge (PD) activity on a hydrophobic insulator covered with condensed water and the time delay observed by the authors of the above-titled paper (ibid., vol.24, p.229-37, 1989) can be explained satisfactorily in terms of electrodynamic effects. The authors reply that the commenter's approach is very interesting but may be limited to parameters other than those applied in their experiments.<>

The behavior of partial discharges (PD) along a polymer foam insulator in air is investigated at 50 Hz AC voltage. The results were compared to the PD behavior along other solid/air interfaces. The PD inception voltage depends on the dielectric constant of the solid material. It was highest for the foam/air interface. Further, the effect of the dielectric constant on the PD mechanism is investigated. A qualitative explanation is provided.