IEC 60247-2004 pdf – Insulating liquids – Measurement of relative permittivity, dielectric dissipation factor (tan δ δ ) and d.c. resistivity.
1 Scope This International standard describes methods for the determination of the dielectric dissipation factor (tan δ ), relative permittivity and d.c. resistivity of any insulating liquid material at the test temperature. The methods are primarily intended for making reference tests on unused liquids. They can also be applied to liquids in service in transformers, cables and other electrical apparatus. However the method is applicable to a single phase liquid only. When it is desired to make routine determinations, simplified procedures, as described in Annex C, may be adopted. With insulating liquids other than hydrocarbons, alternative cleaning procedures may be required. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60093, Methods of test for volume resistivity and surface resistivity of solid electrical insulating materials IEC 60250, Recommended methods for the determination of the permittivity and dielectric dissipation factor of electrical insulating materials at power, audio and radio frequencies including metre wavelengths IEC 60475, Method of sampling liquid dielectrics IEC 61620, Insulating liquids – Determination of the dielectric dissipation factor by measure- ment of the conductance and capacitance – Test method 3 Terms and definitions For the purposes of this document, the following terms and definitions apply.
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 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.
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.