Petru V. Notingher

Thermal step method for space charge measurements under applied dc field

The thermal step method (TSM) is a nondestructive technique which measures the distribution of the electric field and space charge density across solid insulating materials. This work first reviews the principle and the basic equations of the TSM when used on a short circuited flat insulating structure (film or plate). An evolution of the method, which allows the determination of space charge distribution in an insulator while submitted to an external dc field, is then described. The fundamentals, the experimental setup and the validation of this technique on flat samples and on power cables are presented and discussed.

The effect of water treeing on the electric field distribution of XLPE. Consequences for the dielectric strength

This paper describes the influence of water trees on dielectric properties of flat samples cut from a HV crosslinked polyethylene (XLPE) cable insulation. In order to obtain experimental results in a short period of time, we used accelerated aging conditions of voltage and frequency as well as different techniques for producing a multitude of artificial water tree inception points. The average value of the water tree permittivity and a law for the time-dependence of permittivity inside the treed degraded areas were deduced from measurements of capacitance and water tree lengths. On the basis of the experimental knowledge of permittivity and water tree length, we determined with analytical numerical methods, the distribution of the electric field for different shapes of treed regions. Because of the increase in length and permittivity of the water tree, the electric field in front of the degraded area is amplified, which is an important factor of risk leading to the electric breakdown.

Water treeing in underground power cables: modelling of the trees and calculation of the electric field perturbation

Abstract In order to explain the development of different types of water trees and the related dielectric breakdowns in extruded power cables, it is necessary to analyse the dielectric properties of the corresponding treed regions and their influence on the distribution of electric field. The study presented in this paper is both experimental and theoretical. Experimentally, we performed the laboratory ageing of a power cable for accelerated conditions of applied voltage and frequency: we inspected the different shapes, orientations and dimensions of vented, bow-tie, single-bow-tie and global water trees produced in the polymeric insulation. From a theoretical point of view we proposed adequate models for obtaining the field distribution analytically and using a suitable finite element method. A local electric field amplification is calculated according to the different characteristics of the water trees: length, shape, permittivity and law of variation for permittivity. From our results, a compared analysis of the risk of electric breakdown is performed for the different types of water trees according to their configurations.

Electric field computation in water treed polyethylene with space charge accumulation

Electrical degradation of the insulating systems of energy cables is close related to the water treeing and space charge development. In this paper the results of the calculations of the electric field repartition in the insulation of a medium voltage cable affected by water trees and their related space charge are presented. Continuous and individual water trees, growing from outer and/or inner semiconductor have been considered. The space charge density has been determined by measurements performed on plane samples of polyethylene by using the thermal step method. The repartition of the space charge in the water treed regions has been estimated from FTIR analysis on the repartition of the ions associated to water trees. The results show that the presence of water trees and of space charge determines an important increase of the electric field strength near to the semiconductor layers, which facilitate the inception of electrical trees in insulations and consequently the breakdown at lower voltage

Surface potential decay characterization of non-woven electret filter media

Electric charging of the non-woven electret filter media leads to increased particle collection efficiency. Estimation of the charge state of the media can be done by measuring its surface potential. The aim of this paper is to evaluate the effects of the factors that influence the surface potential decay characteristics of these media: the grid potential of the triode-type corona electrode system employed for the electric charging, the active carbon, the ambient relative humidity and the temperature at which the media are thermally pre-conditioned. The experiments were carried out on two types of samples, one of which includes an active carbon layer between two 400 μm thick layers of non - woven polypropylene fibers, the diameter of each fiber being approximately 20 μm. The results show that the surface potential values increase with the increasing of the electric potential of the grid. Thus, if the grid potential Vg increases from 3 to 10 kV, the surface potential value increases over 3 times. A 20% increase of the surface potential values is also recorded in the case of thermally conditioned samples. The surface potential decreases by roughly 10% in the case of the samples with carbon layer, and by about 20%, and in the case of relative humidity increase from 21% to 80%.

Dielectric properties of LDPE-SiO 2 nanocomposites

The dielectric behaviour of nano-SiO2 filled low density polyethylene is investigated over a frequency range of 10 mHz–10 MHz and for different temperatures from 250 K to 350 K. It is shown that the presence of nanoparticles change significantly the dielectric behaviour of the polymer system. The frequency variations of the permittivity and of the tan delta emphasize a α-relaxation process, for each of the nanocomposite samples. The relaxation is more important and occurs at higher frequencies with the increase of the filler content. The increase of the temperature leads to a shift of the relaxation frequency to higher values. Results from chemiluminescence measurements and from X-ray diffraction analysis are discussed in connection with the dielectric behaviour.

Assessment of transformer mineral oil condition based on dc and ac conductivity

Due to the increase of electrical energy consumption, the power transformers are loaded beyond nameplate rating. The degradation of their insulation systems is intensified and the dielectric properties are worsened. Therefore, the interest of asset managers, regarding the condition of their transformers is a very important topic. Electrical conductivity is strongly influenced by the high operating temperatures and catalysis products (water, gases, acids) that appear as a result of oil and paper degradation reactions during transformer operation. The present paper is focused on an experimental study regarding the influence of water, acids and gases (hydrogen, acetylene, ethylene, methane etc.) resulted from degradation processes on electrical conduction phenomena of mineral oil. For that, the frequency/time variation curves of ac/dc conductivity for different concentrations of water, acids and gases are drawn. Finally, the correlation between ac and dc conductivity and the possibility to assess the ageing state of the mineral oil based on these quantities are analyzed.

Calculation of the remaining lifetime of power transformers paper insulation

This paper presents the remaining lifetime calculation of power transformers paper insulation and consequently of power transformers. The calculations are performed based on two models, which are related to the thermal degradation of the cellulose winding paper insulation: the common IEC loading guide and a paper degradation model. The paper insulation models prediction can be improved by involving data from furfural analysis. The remaining lifetime is extracted from the fault probability (reliability) of the paper insulation. The two models are brought together, to aid the asset manager in the decision making process. A probabilistic approach is used, which can be coupled to analysis in terms of risks, benefits, costs, and availability by the asset manager.

Mineral oil lifetime estimation using activation energy

Lifetime estimation of mineral oil is based on IEC 60216-1/2001, using accelerated thermal ageing at three temperatures), method which requires a long experimental time. In order to reduce the experimental times, this paper proposes a simpler method to draw the lifetime curve by determining an experimental point (carrying out only a thermal ageing at the highest temperature - which requires the least time) and curve slope (based on the activation energy of oxidation reaction). Assuming the volume resistivity and the loss factor as diagnostic factors, their variation curves with ageing time were drawn, and the lifetime values were calculated. Using differential calorimetry measurements (DSC curves) for four heating rates, the activation energy value Wa of oxidation reactions was obtained. Knowing the Wa value and the ageing time τ to achieve the end of life criterion for thermal ageing at the highest temperature (155 °C), the D3 lifetime value was determined and compared with D1 and D2 values obtained by accelerated thermal method at three temperatures. Finally is showed that, the differences between D3 and D1,2 values are relatively reduced and the method based on activation energy obtained by DSC, can be used to assess the lifetime of mineral oil.

Influence of water trees on the electric field distribution and breakdown in the point–point geometry

Abstract A study on the influence of water trees in polymeric insulations for a point–point geometry is presented. Using an adequate numerical method we were able to calculate the electric field distribution: the variation of the maximum electric field is considered as a function of the water tree length and permittivity. In laboratory, we obtained the partial-discharge inception voltage (PDIV) as well as the pre-breakdown voltage (PV) and we observed the alteration of the breakdown paths induced by the PE degradation. We also found a good correlation between these experimental results and the calculated distribution of electric field. Both experimental and computational results provide interesting knowledge about the effect of water treeing in polymeric insulations.