Electrical properties of mineral oil and oil/impregnated pressboard for HVDC converter transformers

Abstract

Modern power industry requires higher performance dielectric liquids. Mineral oil is one of those most important and widely used insulating materials. Recently, research on the dielectric properties of mineral oil insulation reveal that oil resistivity can greatly influence the field distribution within an oil-pressboard insulation system in a DC field environment, especially during polarity reversals. Basic test methods such as dielectric spectroscopy and polarization and depolarization measurement have already been used to test mineral oil and pressboard conductivity. However, the knowledge about the mechanism of electrical conduction ininsulating oil and pressboard is still limited. Therefore, the goal of this thesis is to gain a better understanding of the mechanism of electrical conduction in mineral oil and oil impregnated pressboard.

Polarization and depolarization current method (PDC) has gained huge popularity for insulation diagnosis. This time-dependent measurement may provide sufficient information about the dielectric properties of mineral oil and its electrical performance. Here, the dielectric characteristics of three types of mineral oils with different ageing times have been studied using the PDC method. A new polarization theory involving two kinds of charge carriers have been proposed to explain the dielectric behaviour observed in our measurements.

Dielectric spectroscopy is a powerful tool to study dipole relaxation, electrical conduction and structure of molecules. Electrode polarization, as a parasitic effect due to the blocking of charge carriers in the vicinity of an electrode, can make the frequency response at low frequency difficult to understand. Since charge carriers in mineral oil are not only generated from dissociation but also from injection at electrodes, the current induced by the motion of the injected charge carriers should also be taken into consideration. The polarization caused by the injection current has been studied in this thesis. When the electric field is not intense, the injection current is proportional to the field and only contributes to the imaginary part of the complex permittivity. A new computer based calculation method and a modified space charge polarization theory have been proposed with this injection current being involved. The frequency responses of three different kinds of mineral oils have been measured and the experimental results have been compared with the simulation using the modified model. It seems the density of the injected charge carriers increases with the aging period. This new model enables one to gain a better understanding of electrical conduction in mineral oil.

The design and choice of an electrode system is important in DC conductivity measurement of insulating liquid. In this thesis, the electric field distribution of an electrode system which consists of two parallel circular metallic electrodes and a guard electrode has been studied using Comsol Multiphysics software. A new parameter which is not yet involved in current standards, the edge radius, has been investigated by means of field calculation. It has been found out that there are regions in the vicinity of the edges of the guard and measuring electrode at which the field is dramatically distorted. If the edges of these two electrodes are sharp, the maximum electric field in the test cell will be much higher than the average field between the measuring electrode and the high voltage electrode. An empirical equation has been proposed to calculate this maximum field. The classic correction expression of effective radius has been re-evaluated with the edge radius being taken into account. Experimental work has been performed to confirm this conclusion. Three kinds of mineral oils with different aging times have been tested under the DC field using a guarded electrode system and the electric strengths of these oils have been estimated. A recommendation has been made to current standards in insulating liquid measurement.

The dielectric properties of the oil impregnated pressboard sample have been evaluated with the PDC measurement under different temperatures and electric fields. The classic R-C equivalent model has used to explain the dielectric behaviour of the oil/pressboard sample in our PDC measurement. As the electrode effect should be taken into consideration in a DC field, a modified R-C equivalent model has been proposed and used to fit the experimental results and good fitting has been obtained.

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