Olof Hjortstam

Modeling the Effect of Ionic Dissociation on Charge Transport in Transformer Oil

Transformer oil is one of the most important and widely used of all dielectric liquids. Significant research into the insulating properties of transformer oil has been carried out. Much of this work has focused on the development of streamers, and how they can result in the electrical breakdown of the oil. Due to the complex nature of transformer oil no universally accepted model describing streamer initiation and propagation has emerged. In this paper, a model is developed which explores the role played by electric field dependent ionic dissociation in the initiation and propagation of streamers in transformer oil. This model couples the electrodynamics of charge transport to thermal enhancement in transformer oil. Using the COMSOL Multiphysics software application and the finite element method, the model is solved to show that ionic dissociation alone will not lead to a high enough temperature rise to initiate a streamer in transformer oil.

Measurement of ion mobility in transformer oils for HVDC applications

Conduction process in dielectric liquids usually occurs due to ion migration and is therefore characterized by concentration and mobility of the ionic charge carriers. For this reason accurately determining these parameters and especially ion mobility in insulating oils for HVDC applications needs to be done precisely. In this report various methods for determining ion mobility are described, including single polarity, reversal polarity methods in time domain as well as frequency domain based measurement of dielectric response. The advantages and disadvantages of each of them are presented and illustrated with the results of measurements on various oils using two different test cells. The dependence of ion mobility on measuring voltage level and temperature are analyzed and compared.

Modeling and Measurements of Electric Fields in Composite Oil/Cellulose Insulation

In this paper we report on basic studies of the electric fields under DC stress in a composite insulation consisting of transformer oil and oil-impregnated pressboard, typical for HVDC converter transformers. The time dependent electrical fields in our model geometry are measured using electro-optic Kerr method, and theoretically studied with an ion drift-diffusion conduction model. The measured and calculated electrical fields are compared and analyzed.

Measurements of ion mobility in transformer oil: Evaluation in terms of ion drift

The aim of this paper is to develop a theoretical foundation of the reversed polarity method for measuring the mobility of ions in transformer oil. The results of the measurements of transient currents in oil, showing very distinct transient current peaks appearing after polarity reversal, are evaluated by computer simulations using an ion drift model. It is found from the analysis that the peaks in the measured currents occurred at instants much shorter than the so-called transit time that is the time for an ion to cross the oil gap between electrodes. A relation between the transit time and the current peak position is found that can be used to extract the ion mobility from data obtained with an experimental set up in which the transit time is shorter than the dielectric relaxation time of the liquid. On the other hand, for a setup providing the dielectric relaxation time shorter than the transit time, the current peak position strongly depends on the former and no simple correlation between the current peak position and transit times can be established.

Modelling and measurement of field dependent resistivity of transformer oil

This article studies the apparent resistivity of mineral oil as a function of electric DC stress using theoretical and experimental methods for three different oils. The experimental measurement of the apparent resistivity is done using a cell with two bare Rogowski like electrodes separated by a 2mm gap. The dc current is measured after one hour and the stress is varied in steps. For low DC stress the apparent resistivity rises with increasing voltage, leveling off and dropping slightly for higher voltages. The resistivity is modeled using the ion drift model with equilibrium resistivity measured using dielectric response as input. The charge dynamics and the corresponding resistivity is simulated and the results compared with the measured values. The main features seen in the measurements are captured in the simulation and there is a good overall agreement between the theoretical data and the experimental values.

Modeling streamers in transformer oil: The transitional fast 3 rd mode streamer

This paper presents an electro-thermal hydrodynamic model that explains the development of different streamer modes in transformer oil. The focus is on the difference between the slow 2nd and fast 3rd mode streamers discussed in the literature. Through the use of numerical methods the model demonstrates that streamer modes arise in transformer oil due to the electric field dependent molecular ionization of different families of hydrocarbon molecules (i.e., aromatic, naphthenic, and paraffinic) at increasing electric field levels (or applied voltages). Ionization of the low number density aromatic molecules, that generally have lower ionization energies than naphthenic/paraffinic molecules, leads to the propagation of 2nd mode streamers with velocities on the order of 1 km/s. As the applied voltage is increased, the ionization of the main hydrocarbon molecules in transformer oil, high number density naphthenic/paraffinic molecules, dominates producing high electric field levels and space charge at the streamer tip. This ultimately leads to the propagation of the 3rd mode streamer with velocities on the order of 10 km/s.

Finite Element Analysis of Charge Injection and Transport in a Dielectric Liquid

Unipolar and bipolar space charge injection and propagation is analyzed under simplified linear and Fowler-Nordheim charge injection conditions for parallel plane or needle-sphere electrodes stressed by a step voltage. To verify our finite element numerical method, we first tested parallel plane electrodes with a simplified linear charge injection law for unipolar space charge propagation that showed good agreement with older closed form theoretical analysis. Finally, we analyzed tip-sphere electrodes for bipolar space charge propagation including Langevin recombination using finite element analysis.

Charging insulating barrier by corona in air in large coaxial system

Results of the experimental study and computer simulations of corona charging of an insulating barrier in a large scale coaxial arrangement are reported. A computational model describing transport of charged species generated by corona and their dynamics on dielectric surfaces is presented. The measured and computed voltage-current characteristics of corona discharge are compared and discussed. It is shown that charges deposited on gas-solid dielectric interfaces reach a steady state after few cycles of the applied triangular ac voltage. The electric field induced by the surface charges modifies the field in the entire system and alter corona characteristics.

Measurements and simulations of corona currents due to triangular voltages in large scale coaxial geometry

Corona currents in air measured in a large scale coaxial geometry (douter = 1 m, dinner = 0.26 mm), under both DC and AC voltages of a triangular shape with frequencies in the range (1-50) Hz are analyzed. A computational model describing dynamics of positive and negative ionic charges is employed to examine the results of the measurements. The performed simulations show a good overall agreement with the experimental data.

Ultra high speed camera studies of paper puncture in oil-cellulose insulation systems under LI testing

This paper aims at presenting results related to electrical breakdown of oil paper insulation systems by puncturing of paper insulation. In the investigations the used experimental setup is a needle-plane geometry with sheets of paper inserted in the oil gap. The voltage applied to the needle is a Lightning Impulse (LI) of positive polarity. The propagation of streamers was studied by an ultra-high-speed frame camera. The streamers were always initiated at the needle tip and were classified as 2nd mode streamers. In some cases the streamer spread out along the top of the paper without puncturing it. In other cases the streamer punctures the paper and continues to propagate below the paper. Differences between the two cases are discussed and analyzed. It is found that the paper puncture voltage coincide with the voltage at which the Laplace field below the paper sheet reach the critical field for streamer initiation.