IEC 60247 Insulating liquids - Measurement of relative permittivity, dielectric dissipation factor (tan δ) and d.c. resistivity
4 General
Permittivity, tan δ and resistivity, either separately or together, are important indicators of the intrinsic quality and degree of contamination of an insulating fluid. These parameters may be used to interpret the deviation from desired dielectric characteristics and the potential influence on performance of equipment in which the fluid is used.
4.1 Permittivity and dielectric dissipation factor (tan δ)
The permittivity and the dielectric dissipation factor (tan δ) of electrical insulating liquids depend to a considerable extent on the test conditions under which they are measured, in particular on the temperature and on the frequency of the applied voltage. Permittivity and dissipation factor are the measurements of dielectric polarization and conductivity of the material.
At power frequency and sufficiently high temperature, as recommended in these methods, the losses may be attributable exclusively to the conductivity of the liquid, that is, to the presence of free charge carriers in the liquid. Measurements of the dielectric properties of high purity insulating liquids are therefore of value as an indication of the presence of contaminants.
The dielectric loss factor is usually inversely proportional to the measuring frequency and varies with the viscosity of the medium. The value of the test voltage when measuring the dissipation factor is less important and often governed by the sensitivity of the measuring bridge. However, it must be borne in mind that too high a voltage stress results in secondary phenomena at the electrodes, dielectric heating, discharges etc.
While relatively large amounts of impurities produce a comparatively small change in permittivity, the tan δ of insulating liquids may be strongly affected by traces of dissolved contaminants or colloidal particles. Some liquids are much more sensitive to contamination than hydrocarbon liquids due to their higher polarity, which results in turn in higher solvent power and dissociation capability. Therefore, they require comparatively greater care in handling than hydrocarbon liquids.
Since the initial value is thought to be representative of the actual conditions of the liquid, it appears most desirable that tan δ should be measured as soon as temperature equilibrium has been reached. Tan δ is very sensitive to changes of temperature. Its increase, with increases in temperature is generally exponential. It is therefore, necessary to carry out measurements, under sufficiently precise temperature conditions. The procedure described below allows the test specimen to attain temperature equilibrium with the test cell in minimum time.
4.2 Resistivity
The conventional resistivity as measured by this standard is generally not the true resistivity. Application of a d.c. voltage will change the initial characteristics of the liquid with time, due to charge migration. The true resistivity can only be obtained at low voltage, immediately after application of the voltage. This standard uses a relatively high voltage for an extended time and the result will generally be different from that from IEC 61620.
Measurements of resistivity of liquids to this standard, depends on a number of test conditions, namely:
a) Temperature
Resistivity is very sensitive to changes of temperature, its dependence on the inverse of the temperature, expressed in Kelvin, (1/K) is generally exponential. It is therefore necessary to carry out measurements under sufficiently precise temperature conditions.
b) Magnitude of the electrical field
The resistivity of a given specimen may be influenced by the applied stress. For results to be comparable, measurements shall be made with approximately equal voltage gradients and with the same polarity. The gradients and the polarity shall be noted.
c) Time of electrification
Upon the application of d.c. voltage, the current flow through the specimen decreases due to the sweep of charge carriers to the electrodes. The conventional arbitrary time of electrification is 1 min. Variation in the time of electrification can result in appreciable variation in the test results. (Some high viscosity fluids may require considerably longer electrification time (see 1 4.2).)
4.3 Sequence of determinations
Application of d.c. voltage to a specimen can modify the results of a subsequent a.c. determination of tan δ.
When permittivity, tan δ and resistivity measurements have to be made consecutively on the same specimen, the a.c. determination shall always be made before applying the d.c. voltage to the specimen. The cell electrodes should be short-circuited for a minimum of 1 min after the a.c. tests, before beginning the resistivity measurements.
4.4 Factors leading to erroneous results
Only gross contamination is likely to affect permittivity. However, DDF and resistivity can be strongly affected by minute amounts of contamination.
Unreliable results usually originate from contamination due to improper sampling or handling of liquid specimens, from incomplete cleaning of the cells or from the absorption of water, and especially from the presence of undissolved water.
Extended exposure to light during storage may lead to deterioration of dielectric characteristics. Standardized procedures for the storage and transfer of the liquid samples and for the construction and cleaning of test cells are recommended so that errors caused by contamination are minimized.
