The paper provides essentially a summary of PD measurements applying the UHF range in order to increase the detection threshold, to improve the localization accuracy and to perform on-line measurements of Partial Discharge (PD) in noisy environments. The electromagnetic UHF technique offers good signal to noise ratios, because external PD signals and disturbances can be shielded effectively. A new developed method allows the localization of PD in gas-insulated substations (GIS) by frequency domain measurements. The basic idea is the displacement law of Fourier transformation. The interference phenomena of superposed signals from two sensors give information about the time delay of the sensor signals. On-site PD measurements are made at cable connectors by means of monopole antennas housed in a barrel sleave, while the cable is in service. Thus a sensitive PD measurement even in noisy environment is possible. PD measurements on several 72 kV cable connectors were performed in an unshielded laboratory. On-site measurements during operation showed the great potential for condition assessment. For decoupling sensitive UHF PD signals from the inner of a power transformer tank UHF sensors applied through drain/oil valves are used. Experimental studies indicate that all relevant types of PD possibly occurring within a transformer emit high frequency spectra to be detected with UHF sensors. Furthermore in laboratory experiments and on-site measurements very moderate UHF signal attenuations have been observed.

Partial discharge measurement in the ultra high frequency (UHF) range

Changes in the dielectric breakdown field of polyimide (PI) films have been studied from 25 to 400°C under dc ramps. Both the area (from 0.0707 to 19.635 mm2) and thickness (from 1.4 to 6.7 ?m) dependences of the dielectric breakdown field have been carried out using the Weibull distribution function. The 63%-breakdown field value (i.e. the ?-scale parameter) of PI shows a decrease with increasing area, thickness and temperature but always remains above 2 MV/cm. The s-scale parameter of the distribution shows a typical decrease with increasing area, however, it exhibits an increase with increasing thickness. This 'curious' behavior is discussed on the basis of the percolation theory. No temperature-dependence is clearly observed. Moreover, physical interpretations are carried out using the pre-breakdown current analysis.

Dielectric breakdown of polyimide films: Area, thickness and temperature dependence

Recent decades have seen the rapid and successful deployment of ester oils as transformer dielectrics. When evaluating the compatibility of ester oils with existing transformer insulation designs, it is essential to not only compare the mean breakdown voltages of the oils, but also to consider the role of dispersion and thus the withstand voltage levels (with the acceptable breakdown likelihood). Insulation designers sometimes estimate the withstand voltage of mineral oil from the dispersion of the data, assuming that the breakdown voltages of the oil follow a parametric distribution. However, there is a lack of discussion as to whether this method would be suitable for estimating the withstand voltages of ester oils. This paper uses samples of breakdown voltages of a synthetic ester, a natural ester and a mineral oil to analyze their distributions, and discusses the applicability of using Weibull and Gaussian distributions to estimate the withstand voltages of the esters. It is found that the distributions of ester breakdown voltages, in particular the lowest breakdown voltage, are similar to those of mineral oil. Consequently there is evidence that from the ac withstand voltage point of view the tested esters are compatible for the use in power transformers.

Statistical analysis of the AC breakdown voltages of ester based transformer oils

Properties of cryogenic insulants

A family of cross-linked polypropylene (x-PP) thin film dielectrics is systematically studied to understand the cross-linking effect on the dielectric properties. Evidently, the butylstyrene (BSt) cross-linkers increase both the dielectric constant (e) and breakdown strength (E), without increasing energy loss. An x-PP dielectric, with 3.65 mol % BSt cross-linkers, exhibits a e∼3, which is independent of a wide range of temperatures and frequencies, slim D-E hysteresis loops, high breakdown strength (E=650 MV/m), narrow breakdown distribution, and reliable energy storage capacity >5 J/cm3 (double that of state-of-the-art biaxially oriented polypropylene capacitors), without showing any increase in energy loss.

Cross-linking effect on dielectric properties of polypropylene thin films and applications in electric energy storage

In order to examine the area and the volume effects on breakdown strength in liquid nitrogen (LN/sub 2/), we measured dc and ac breakdown voltages in LN/sub 2/ with sphere to plane and coaxial cylindrical electrode configurations We also carried out statistical analysis of the experimental results using the Weibull distribution and discussed with the statistical stressed electrode area (SSEA) and/or liquid volume (SSLV) The dc and ac breakdown strength in LN/sub 2/ decreased to 1/5 with increasing SSEA and SSLV over the wide range from 10/sup 0/to 10/sup 5/ mm/sup 2/ and from 10/sup -1/ to 10/sup 5/ mm/sup 3/, respectively The Weibull shape parameter m for the sphere to plane and the coaxial cylindrical electrodes was estimated to be 6 to 8 and 11 to 13, respectively; the breakdown strength in LN/sub 2/ saturated with increasing SSEA and SSLV From these results, it was verified that the area and the volume effects definitely determined the breakdown strength in LN/sub 2/ The results of the Weibull statistics were compared with the conventional concept of the stressed electrode area (SEA) and liquid volume (SLV) Consequently, SSEA or SSLV was found to be nearly equal to 80 to 85% of SEA or SLV >

Weibull statistical analysis of area and volume effects on the breakdown strength in liquid nitrogen

This paper reports the results of a basic study of testing methods to replace the DC after laying test that is presently most frequently used as an after laying test for XLPE power cable lines. Chosen as the alternative waveforms for DC voltage were damped oscillating wave (OSW) and very-low-frequency wave (VLF) voltages. Experiments were conducted on the ability to detect various types of line defects under these voltages. It was found that these alternative waveforms are promising as after laying test voltage waveforms to replace DC and that greater defect detection ability can be achieved by combining them when conducting tests.

Study on detection for the defects of XLPE cable lines

We measured dc and ac breakdown voltages in liquid nitrogen (LN/sub 2/) with a sphere-to-plane electrode configuration. Experimental results revealed that the breakdown voltage in LN/sub 2/ did not increase monotonously but partially decreased as the sphere diameter increased at a constant gap length. Thus, the existence of the area and the volume effects on the breakdown voltage in LN/sub 2/ was verified quantitatively; the breakdown strength decreased when increasing the. >

Area and volume effects on breakdown strength in liquid nitrogen

Quench-induced breakdown characteristics of liquid helium and optical observation of thermal bubbles

Until now many diagnosing techniques for water-tree degraded MV class XLPE cables have been proposed. However, although they can detect the average degree of degradation throughout the cable, the degraded part can hardly be located. The maintenance cost will be significantly reduced if the degraded part is located by repairing only the degraded part of the cable. This presentation will introduce a new method to locate the water-tree degraded part along a cable line by applying DC voltage. While DC leakage current measurement is carried out for diagnosing cables degraded by water-tree, it is often observed that the leakage current varies unstably. This phenomenon, called as ldquokickingrdquo, is considered to have a relation with partial discharges accompanied by water-tree. We tried to locate this highly degraded part along the cable length. Using a short size cable, it was proved that partial discharges, accompanied by an electrical tree generated from a water-tree, were taking place. The point was precisely located by using timedomain waveform measurement. Based on the above achievement, an equipment to locate highly degraded part along a cable line was developed. It was proved using a full-scale cable line that the equipment could locate the degraded part with a sufficient precision.

Katsumi Uchida

Papers

Weibull statistical analysis of area and volume effects on the breakdown strength in liquid nitrogen

Study on detection for the defects of XLPE cable lines

Area and volume effects on breakdown strength in liquid nitrogen

Quench-induced breakdown characteristics of liquid helium and optical observation of thermal bubbles

Location of water tree degraded point along XLPE cable line using DC voltage