C. Volat

Three-dimensional modeling of potential and electric-field distributions along an EHV ceramic post insulator covered with ice - Part I: Simulations of a melting period

The main objective of this paper is to determine the potential and electric-field distributions along a typical ceramic extremely-high-voltage post insulator covered with atmospheric ice during a melting period. Commercial software, Coulomb 3D, based on the boundary element method (BEM), was used for all of the three-dimensional modeling and simulations. It was demonstrated that the BEM is well suited for evaluating the effect of ice shedding on the potential and electric-field distributions along an ice-covered insulator during a melting period. The results obtained show that the length and number of ice free zones, also called air gaps, are the major parameters that affect the applied voltage distribution along an ice-covered insulator. The mean electric field per arcing distance, affected mainly by the air-gap lengths, can provide a good indication of the presence of partial arcing along the different air gaps.

Three-dimensional modeling of potential and electric-field distributions along an EHV ceramic post insulator covered with ice-Part II: effect of air gaps and partial arcs

The main objective of this paper is to investigate the effect of air-gap position, length, and number on the phenomena preceding flashover along a typical 735-kV porcelain station post insulator unit under heavy ice accretion. Under these conditions, this type of insulator seems to be the most likely subject to flashover. The numerical simulations were performed using Coulomb 3D, a commercial software based on the boundary element method (BEM). Computer simulations were performed during a melting period. Special attention was paid to the presence of a conductive water film at the ice surface and to a partial arc along an air gap. The results obtained showed that the number of air gaps, with or without the presence of a partial arc along them, has a direct effect on the distribution of potential along the iced insulator. Moreover, for the same total arcing distance, the number of air gaps and their position did not affect the average electric field per arcing distance that was used to determine the partial arc presence. Based on these results, suggestions for improving station insulator geometry for icing conditions were proposed.

Single Shot and Vectorial Characterization of Intense Electric Field in Various Environments With Pigtailed Electrooptic Probe

In this paper we illustrate the ability of electrooptic sensors to perform electric (E)-field vectorial measurements. Thanks to their frequency response spreading over nine decades and to their measurement dynamics reaching 120 dB, these sensors are of high interest for some applications (near field mapping, energy line monitoring, electromagnetic compatibility, and so on). Furthermore, due to their fully dielectric structure and millimetric size, almost no perturbation is induced on the E-field to be measured, even in the near field region. This paper is focused on high-intensity pulsed E-field characterization in different environments such as air, water (bioelectromagnetism applications), or plasmas (in situ assessment of the E-field associated to an electric discharge and to the induced plasma). The use of such a technology for electrical equipment and energy line monitoring is also investigated.

Numerical simulations of ice-covered EHV post station insulator performance equipped with booster sheds

The main objective of this paper is to study numerically the influence of addition of 6 booster sheds (BS) to 2 units of EHV porcelain post insulator on their electrical performance under severe wet-grown ice accumulation based on experimental results. The numerical investigations have been carried out during melting period in order to determine the potential and electric field distributions and the voltage drop along the different air gaps resulting from the addition of BS. Numerical simulations were done using the finite element method (FEM). Adding BS helps to created artificially air gaps which length depends on their position along the ice-covered insulator. Numerical simulations have helped to demonstrate that voltage drop repartition along the different air gap is not uniform. In particularly, it was shown that more that 53% of the applied voltage is concentrated along the first air gap closed to the HV electrode, which also the longest. Also, it was demonstrated that numerical simulations can be useful and an interesting alternative to experimental test in order to improve and optimize the use of BS for improving electrical performance of post insulator under severe icing conditions.

Detection of AC corona discharges using an electro-optic E-field sensor

This paper present some preliminary results concerning the detection of partial corona or partial discharges in air using a new electro-optic (EO) E-field sensor. The E-field sensor used an ultra-compact single-ended EO probe allowing non-invasive, remote and unidirectional measurement of the E-field component. These characteristics were used to validate the sensitivity of the EO sensor to corona discharge activity. A series of experimental tests were performed using a rod-to-plane configuration with the EO probe position along the axis of the air gap. The EO probe orientation was kept constant to measure the vertical E-field component in the air-gap. The tests were carried out under AC applied voltage for different air gap lengths, EO probe positions and HV rod electrode radius. The experimental results obtained show that the output voltage of the EO sensor undergoes a sudden increase from at a given applied voltage threshold. It was demonstrated that this threshold is dependant of the air-gap length and HV electrode radius. The experimental threshold voltages were compared to corona onset voltage calculated using Peeks empirical law. The good agreement between experimental and theoretical values demonstrated that the EO sensor was able to detect ignition of corona discharges.

Comparison between the use of surface and volume conductivity to compute potential distribution along an insulator in presence of a thin conductive layer

This paper describes a comparative study on the modeling of a thin conductive dielectric layer usually found on polluted or ice-covered insulators. Two different approaches such as those offered by most FEM commercial software were studied. The first one is a volume approach which takes into account the thickness of the thin layer. The second is the surface approach where the thin layer is treated as a specific boundary condition. Simulations were performed using the FEM commercial software Comsol Multiphysics® that allows both volume and surface approaches. Parameters such as conductivity and permittivity of the thin layer as well as the number of elements used are studied both in 2D axisymmetric and 3D modeling. The results obtained demonstrated that the surface approach is the best solution as it provides the same results as the volume approach but with 3 times less elements required for the mesh. The surface approach should then be considered for 3D complex problems where thin conductive dielectric layer is present.

Aging effect on oil cooling capacity of a non-guided disc windings power transformer

Many transformers around the world are serving close to or beyond their design life. In these important asset, cores and coils losses cause heating and energy losses within the core decreasing their efficiency and shortening their life expectancy. The aim of this contribution was to study the aging effect on mineral oil cooling capacity of power transformers. The oil aging effect on its viscosity and its physicochemical properties is also reported. A 2D axisymmetric thermo-fluid numerical model was developed with COMSOL Multiphysics to study the cooling capability of oil aged at different levels. The results show that the oil viscosity increase considerably with aging and its physicochemical properties are also affected. It was found that sludge as oil oxidation byproduct contributes to the widening of hot spots in the transformers with non-guided disc windings.

3D numerical investigation of internal defects in a 28 kV composite insulator

This paper presents a numerical investigation based on 3D finite element method modeling of a dead-end 28 kV non-ceramic insulator having different internal semi-conductive defects in terms of size and position. All the internal defects were modeled as cylinders of 1.5 mm of radius having different lengths: 15 mm and 30 mm which correspond to 3.5 % and 7 % of the insulator length respectively. The internal defect was positioned close to the HV electrode and in the middle of the insulator (floating potential) at equal distance between the sheds. The simulations have been focused on the modification of the axial and radial E-field components closed to the insulator shank surface as well as at the vicinity of the insulator shed extremity. The results obtained demonstrated that small conductive internal defects (lower than 3.5 % of the insulator length) led to a significant distortion of the axial and radial E-field components at the shank surface. The E-field component distortion obtained for a small defect (floating potential) positioned in the middle of insulator between sheds is detectable at the shank surface but invisible at the shed extremity.

Single shot measurements of electric field vector in various environments with pigtailed electro-optic probes

We here illustrate the ability of electro-optic sensors to perform electric field vectorial measurements. Thanks to their frequency response that remains flat over nine decades and to their measurement dynamics reaching 120 dB, these sensors are of key interest for many applications. Furthermore, due to their fully dielectric structure and to their millimetric size, almost no perturbation is induced on the electric field to be measured, even in the near field region. This paper is focused on high intensity pulsed electric field characterization in different environments such as water (bioelectromagnetism applications) or plasma (in-situ assessment of the electric-field associated to an electric discharge). The use of such technology for electrical equipments and energy lines monitoring is also investigated.

A new single-arc AC dynamic FEM model of arc propagation on ice surfaces

This paper presents an improved dynamic model based on the finite element method (FEM) for predicting AC critical flashover voltage (FOV) of ice-covered insulators during the melting period. The proposed model is implemented on the COMSOL Multiphysics™ FEM commercial software which is coupled with Matlab® for iterative calculation. The proposed dynamic model is based on the Obenaus/Rizk model, and deals with the case of a single arc established on an ice surface. The arc is modelled by its root which is considered to be an equipotential surface for which a voltage boundary condition is used, and whose propagation condition and arc propagation velocity are provided by the Hampton and Bondiou-Galimberti criteria. This new dynamic numerical model was validated by comparing the results obtained for the arc velocities and FOV with the corresponding experimental values available in the literature as well as with those obtained from actual mathematical dynamic models. A good agreement between the different results was obtained which confirms that this new dynamic numerical model is an effective tool for insulator dimensioning under severe ice accumulation.