Joachim Schiessling

Dielectric properties of transformer oils for HVDC applications

The knowledge of the behavior of electric conductivity in mineral oils for possible applications in HVDC converter transformers is of paramount importance for proper design of their insulation system. This study presents the results of measurements of dielectric properties of various transformer oils by means of frequency response technique as well as time domain measurements, including measurements of ion mobility using the reversal polarity method. Influences imposed by varying the measurement voltage level and temperature are investigated. Some important parameters of the investigated oils, e.g. their conductivities, ion mobilities, ionic concentrations and effective ionic radiuses are compared and discussed. The results show that despite of similarities in various physical parameters of insulating mineral oils available on the market, the dielectric behavior and especially ionic conduction vary greatly between different oil types. Changes of these properties with temperature are characterized by different activation energies.

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.

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.

DC field measurements around a cable end

Even under corona-free conditions, insulating structures (materials) may be charged in DC fields. This is due to the fact that air has a finite ionic conductivity. The finite conductivity of air results from charges generated by background radio activity and cosmic radiation. Typical ion concentrations on ground are 107 to 1010 ions/m3 which leads to conductivities ranging from 10-16 to 10-13 S/m. An experiment has been conducted where the end of a terminated high voltage XLPE cable was used as an insulating structure. The field distribution around the stripped cable end was measured with an applied DC voltage of -140 kV using a rotating field probe. The measurements are compared to simulations, which do not specifically take into account space charge effects. A big discrepancy is observed, showing the relevance of taking into account space charge effects when modeling HVDC insulation components subjected to air.

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.

Evolution of the electric field due to forced flow in oil pressboard ducts

The electrical insulation system of power transformers is a combination of mineral oil and pressboard materials. We have measured the influence of forced oil flow on the electric field stress in oil pressboard insulation system with applied DC potential using Kerr electro-optic effect. The data show a variation in the electric field strength in flow direction, depending on velocity, time and applied voltage. The observations are explained by variation in space charge distribution on the pressboard.