Laurentiu Marius Dumitran

Corona Charging and Charge Decay Characteristics of Non-Woven Filter Media

The surface-potential-decay measurement techniques are commonly employed for the adjustment of Xerox photography processes, development of electrets, and assessment of polyethylene films for cable insulation. In all these situations, it is important to evaluate the surface charge density and compare it to the limit imposed by the dielectric rigidity of atmospheric air. The aim of the experiments reported in this paper is to enhance the efficiency of the corona charging of nonwoven filter media for heat, ventilation, and air-conditioning applications. Samples of nonwoven polypropylene fibrous media were exposed to positive corona discharges from a wire electrode, as the efficiency of the filter can be enhanced when the insulating fibers are electrically charged. The observed surface potential levels and the potential decay characteristics indicate that the charge of the filter is limited by the local discharges that occur inside the fibrous dielectric. The performance of the filter can be further improved by using a triode-type electrode system that assures a uniform distribution of the electric charge at the surface of the filtering media.

Correlation between current density and layer structure for fine particle deposition in a laboratory electrostatic precipitator

On the basis of the occurrence of a geometrical pattern for the dust layer deposited in an electrostatic precipitator, the physical phenomena leading to such a deposit are characterized. The measurement of the current-density distribution on the collecting plate shows that the dust layer pattern is strongly correlated with this distribution, leading to two kinds of deposition: zones of dense packing and zones of dendritic packing. Some explanations about the observed pattern are proposed. The size distribution of the collected powder is determined for the two zones of collection and as a function of the distance downstream from the precipitator entrance. This analysis reveals the existence of a clear difference in the size distributions of deposited powder in the two collection zones. A Deutsch-like model fairly well accounts for the observed decrease of mean diameter of the collected particles as a function of the distance downstream from the precipitator entrance. In the light of some considerations on re-entrainment and electrohydrodynamic secondary flows, possible mechanisms leading to such phenomena are briefly discussed.

Computational and experimental study of ionic space charge generated by combined corona-electrostatic electrode systems

Numerical computation of the electric field intensity and space charge density in electrode systems consisting of ionizing and nonionizing elements, connected at a same DC high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. The aim of the present paper is to demonstrate the effectiveness of an original method of field computation in the analysis of the factors that influence the distribution of the ionic space charge in such combined corona-electrostatic electrode systems. The computations and the experiments were carried out for an ionizing wire of diameter 0.3 mm, located at different distances h (10 to 30 mm) from a tubular support of diameter 25 mm. Several inter-electrode distances (20 to 45 mm) were simulated. The extension of the zone at the surface of the grounded electrode which is affected by the space charge diminishes when reducing the intervals between these elements of the electrode system, and - at similar applied voltage - the density of the corresponding corona current increases. The experimental data were in good agreement with the computed results, validating the accuracy of the numerical method of space-charge calculation in this special electrode configuration.

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%.

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.

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.

Comparative study on power transformers vegetable and mineral oil ageing

Due to the increasing need to replace the petroleum based products, theres an interest in replacing the mineral insulating oil for electrical equipment with biodegradable vegetable oil. This paper presents a comparative experimental study concerning the ageing of mineral and vegetable oils used in power transformers. In this view, samples of mineral and vegetable oil were accelerated thermally aged, and their dielectric properties (real part of the complex relative permittivity, loss factor and resistivity) were determined at different ageing times. The evolution of the degradation process in time was studied by determining the dielectric properties variation rate. The results show that the values of the relative permittivity and loss factor determined for new oil are higher in the case of vegetable oil than for the mineral oil, and vegetable oils resistivity is lower than that of mineral oil. It has been determined that during the ageing process the color of the mineral oil samples visibly changes with the ageing time, while in the case of vegetable oil samples, these changes are not significant. Also, the variation rate of vegetable oils real part of relative permittivity, loss factor and resistivity is decreasing in time, while that of mineral oil has an increasing tendency.

Lifetime estimation of vegetable oil for transformers

In this paper the results of an experimental study concerning the vegetable oils (subjected to thermal ageing) lifetime estimation based on the activation energy and IEC 60216-1/2001 methods are presented. Three accelerated thermal ageing tests were performed on transformer vegetable oil samples at 135, 155 and 175 °C. The lifetime curves were drawn for three diagnostic factors, namely loss factor, conductivity and total acid number. Using DSC (Differential Scanning Calorimetry) method, the activation energy corresponding to the oxidation reaction has been determined. Subsequently, using the results of thermal ageing at 175 °C, the lifetime curves have been drawn. It is concluded that the activation energy method is quicker and less expensive therefore it could be useful to obtain new information about a new vegetable oil for paper/oil insulations.

Interpretation of surface potential measurements performed with the vibrating capacitive probe of an electrostatic voltmeter

In case that the vibrating capacitive probe of an electrostatic voltmeter is facing a metallic plate of known electric potential V, which can be easily imposed by connecting it to a DC high-voltage supply, the value displayed by the instrument is equal to V. The same surface potential measurement technique is employed to characterize the charging state of insulating materials, such as films or non-woven media. The aim of the paper is to give some elements of answer to the following question: What significance can be attributed to the value displayed by the electrostatic voltmeter when the charge is non-uniformly distributed at the surface of the bodies examined by the probe and no constant surface potential can be defined? The measurements were performed for some simple experimental models that simulate surface potential non-uniformity. Thus, a copper wire, a copper rod, and two aluminium strips, connected to a high-voltage supply (V = 875 V) were located at 5 mm above a grounded metal plate. The vibrating capacitive probe was placed at various distances h above the grounded plate and at various distances x from the axial plan of the high-voltage strip/rod/wire - grounded plate system. Different V(x) curves obtained for each of the situations under study, pointing out that the value displayed by the instrument depends on the distribution of the potential across the surface examined by the probe. A commercial computer program based on the superficial charge simulation method was employed for the numerical analysis of the electric field in a simplified model of the geometric system formed by the probe, the high-voltage strip/rod/wire, and the grounded plate. The potential that anneals the electric field at the surface of the probe (to simulate its principle of operation) was calculated by an iterative method. A good agreement was found between the results of the numerical simulations and the experimental data.

Electric field distribution in dual dielectric DC cable joints

This paper is focused on the electric field computation in a cylindrical joint model whose insulation consists of two insulating layers (XLPE and EPR). The quasi-stationary regime was considered and the volume charge density (ρv) and the superficial one at the XLPE/EPR interface (ρs) were taken into account. The electrical conductivity was measured on XLPE (A), EPR (B) and XLPE/EPR (C) flat samples for electric field values E =1 ... 10 kV/mm and the permittivity for frequencies f = 10-3 - 106 Hz. Using the conductivities on flat samples A and B, the equivalent conductivity σCc of sample C was computed. The presence of the space charge leads to the modification of the electric field E. The most significant influence is given by the presence of both space and superficial charge, E increasing with 100 % on the outer surface (EPR).