Measuring Transformer Oil Resistivity:

Standard Methods May Not Be Completely Reliable

by Kjell sundqvist

Resistivity is a very fundamental property of transformer oils. Ideally the oil should not conduct an electric current at all and therefore it should have a very high resistivity. It is desirable that this property, or its antithesis, conductivity (the ability to conduct electricity) is measured and recorded. Currently, two standard methods are used (IEC 247): The measurement of resistivity and the measurement of dielectric dissipation factor, tan ð.

There is good reason to question the use of resistivity measurement, especially when it comes to alternating current applications. A working group within the International Electrochemical Commission is currently working on a review of IEC 247.

A prerequisite for a substance being able to conduct an electric current is that charges can be transported through the substance. In a fluid, it is ions that are transported. Oil has a very low conductivity, since ions, or compounds that are ionizable, do not normally occur in it. The more impurities oil contains, the greater the risk that there are ionizable impurities, and therefore that conductivity will increase.

A measure of conductivity thus provides information about a property that is of direct interest, while at the same time indicating the degree of impurities.

The three methods of measurement in IEC 247 are for permittivity, dielectric dissipation factor, and resistivity. In all three methods, oil is placed in a cell and an electric voltage is applied between two surfaces in the cell. This apart, the methods have some essential differences.

The measurement of permittivity assumes that the cell forms a capacitor with a certain capacitance. Accordingly, alternating voltage is used. If, instead of oil, there is air, or even better, a vacuum, the capacitance will be higher than if there is any conductive material between the surfaces. Thus, capacitance is measured first with oil, then with air in the cell. The ratio between the capacitance with oil and the capacitance with air is called permittivity. The theoretical minimum value is 1.

When measuring the dielectric dissipation factor, tan ð, alternating voltage is also used. Here, however, it is the phase displacement, or loss angle, that is measured. An alternating voltage can be described as a sine shaped curve. When the alternating voltage is applied over a capacitator, a current arises in the cords connected to both of the surfaces. This current can also be described as a curve. If the impedance in the circuit only consists of capacitance, the curve describing the current deviates by 90 degrees compared with the voltage curve. If, however, there is a current going through the oil, the current curve deviates. The more current, the greater the deviation. It is the tangent on the deviation that gives a measurement of conductivity: the lower the value, the lower the conductivity (=higher resistivity).

The Usefulness of Methods
The resistivity measurement according to IEC 247 means that a continuous voltage that gives an electric field of 250 volt/mm is applied to the cell containing oil. The current is measured after 60 seconds. Resistivity is the ratio between the strength of the electrical field and the current (unit Wm).

The three methods essentially measure the same property, but the different methods of measuring means that their practical useability and relevance varies.

When it comes to new transformer oil used in a modern transformer for alternating current, conductivity is so low that it is not in itself of any significance. The reason for measuring it, in spite of this, is because an increased value is an indication that the oil is contaminated. Contamination by an ionizable substance can lead to other properties of the oil being affected, e.g. its life expectancy.

It is therefore important to use a method that correlates well with the content of ionizable impurities.

Permittivity measurement is seldom carried out, since it gives only small variations, regardless of degree of contamination.

Resistivity measurements as such have the potential for supplying relevant data. Unfortunately this does not apply to the resistivity measurement according to lEC 247. One problem is that the values from resistivity measurements are dependent on time. At the precise moment when the voltage is applied, the current is considerably stronger than after 60 seconds, which is when the measurement is carried out.

What happens is that as sonn as the current is applied, the few ions that are in the oil start to move towards the surface that has the opposite charge to their own. When positive ions start to collect at the negative pole, an excess of positive charges arises there. This repels other positive ions and thus impedes the current. Since there were already from the start very few ions, there are soon none left that can move towards the negative pole.

A corresponding process occurs at the positive pole. Thus, the current lessens and settles at a constant level. According to lEC 247, the current should be measured after 60 seconds, i.e. long after the initial peak current. Depending on the viscosity of the oil, and the character of the ionizable substances, the point in time when the current becomes constant can be displaced.

New Methods of Resistivity Measurements
There are other methods of resistivity measurements, which measure the current after a very short time using low voltage. These methods have been shown to correlate well with the dielectric dissipation method, tan ð.

A project is currently in progress within Cigre Task Force 15.02.04 (which is part of lEC) to develop a method that will measure resistivity and capacitance at the same time (from capacitance, tan ð can be calculated). This work has not yet been completed, but one fact was established at an early stage: the conductivity of a fluid must be determined when the fluid is in thermodynamic balance. In order for it to be in this state, high electrical load and/or electrical load over a long period has to be avoided.

When measuring the dielectric dissipation factor, alternating current is used, which means that the phenomenon whereby charges accumulate on the surface never materialise. It could be said that the method is more relevant just because it measures alternating voltage, since it is the characteristics in the case of alternating voltage that are of interest. However, this argument is weak, since it is not actually conductivity/resistivity that is of interest, but their correlation with the level of impurities.

The conclusion is that resistivity might as well be measured, but not using the present standard method. Since there is a standard measure for the measurement of the dielectric dissipation factor and this gives relevant results, there is today no reason to use any other method. It will only be when an equally good method for measuring resistivity is in existence that it will be of interest to discuss which method is best. The work in lEC also involves a review of tan ð measurement. There is also scope for improvement here. For the moment, however, it is the best tool in the quality control of transformer oil.