IEC 61125 Insulating liquids - Test methods for oxidation stability - Test method for evaluating the oxidation stability of insulating liquids in the delivered state
1 Scope
This document describes a test method for evaluating the oxidation stability of insulating liquids in the delivered state under accelerated conditions regardless of whether or not antioxidant additives are present. The duration of the test can be different depending on the insulating liquid type and is defined in the corresponding standards (e.g. in IEC 60296, IEC 61099, IEC 62770). The method can be used for measuring the induction period, the test being continued until the volatile acidity significantly exceeds 0,10 mg KOH/g in the case of mineral oils. This value can be significantly higher in the case of ester liquids.

The insulating liquid sample is maintained at 120 °C in the presence of a solid copper catalyst whilst bubbling air at a constant flow. The degree of oxidation stability is estimated by measurement of volatile acidity, soluble acidity, sludge, dielectric dissipation factor, or from the time to develop a given amount of volatile acidity (induction period with air).

In informative Annex B, a test method for evaluating the oxidation stability of inhibited mineral insulating oils in the delivered state by measurement of the induction period with oxygen is described. The method is only intended for quality control purposes. The results do not necessarily provide information on the performance in service. The oil sample is maintained at 120 °C in the presence of a solid copper catalyst whilst bubbling through a constant flow of oxygen. The degree of oxidation stability is estimated by the time taken by the oil to develop a determined amount of volatile acidity (induction period with oxygen). Additional criteria such as soluble and volatile acidities, sludge and dielectric dissipation factor can also be determined after a specified duration.

In informative Annex C, a test method intended to simulate the thermo-oxidative behaviour of ester insulating liquids (headspace of air at 150 °C for 164 h) is described.

Additional test methods such as those described in IEC TR 62036 based on differential scanning calorimetry can also be used as screening tests, but are out of the scope of this document.

2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes requirements 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 60247, Insulating liquids - Measurement of relative permittivity, dielectric dissipation factor (tan δ) and d.c. resistivity
IEC 62021-2, Insulating liquids - Determination of acidity - Part 2: Colorimetric titration
IEC 62021-3, Insulating liquids - Determination of acidity - Part 3: Test methods for non-mineral insulating oils
IEC 60422:2013, Mineral insulating oils in electrical equipment - Supervision and maintenance guidance
ISO 383, Laboratory glassware - Interchangeable conical ground joints
ISO 4793, Laboratory sintered (fritted) filters - Porosity grading, classification and designation
ISO 6344-1, Coated abrasives - Grain size analysis - Part 1: Grain size distribution test
ISO 3104, Petroleum products - Transparent and opaque liquids - Determination of kinematic viscosity and calculation of dynamic viscosity
ASTM E287, Standard specification for laboratory glass graduated burets

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:
IEC Electropedia
ISO Online browsing platform

3.1 unused insulating liquid
insulating liquid that has not been used in, or been in contact with electrical equipment or other equipment not required for manufacture, storage or transport.

Note 1 to entry: See also IEC 60296, IEC 61099 and IEC 62770.

3.2 recycled insulating liquid
insulating liquid previously used in electrical equipment that has been subjected to re-refining or reclaiming (regeneration) off-site.

Note 1 to entry: Any blend of unused and recycled oils is to be considered as recycled.

3.3 oxidation stability
ability of an insulating liquid to withstand oxidation under thermal stress and in the presence of oxygen and a copper catalyst.

Note 1 to entry: Oxidation stability gives general information about the stability of the insulating liquid under service conditions in electrical equipment. The property is defined as resistance to formation of acidic compounds, sludge and compounds influencing the dielectric dissipation factor (DDF) under given conditions. Test durations for insulating liquids are described in the corresponding standards.

3.4 induction period with air
graphical representation of the oxidation rate over the entire period which can be obtained by titrating volatile acidity daily (or at other suitable time interval) and plotting the cumulated results against time.

Note 1 to entry: The induction period with air is determined by reading the time corresponding to 0,10 mg KOH/g volatile acidity in the case of mineral oil. In the case of ester liquids a higher value needs to be established.

3.5 volatile acidity
measurement of the amount of oxidation products collected in the water phase in the absorption tube.

3.6 soluble acidity
acidity (neutralization value) of oil as a measure of the acidic degradation products in the insulating liquid.

Note 1 to entry: The acidity of an oxidized oil is due to the formation of acidic oxidation products. Acids and other oxidation products will, in conjunction with water and solid contaminants, affect the dielectric and other properties of the oil. Acids have an impact on the degradation of cellulosic materials and may also be responsible for the corrosion of metal parts in a transformer.

3.7 total acidity
sum of volatile and soluble acidity.

3.8 sludge
polymerized degradation product of solid and liquid insulating material.

Note 1 to entry: Sludge is soluble in oil up to a certain limit, depending on the oil solubility characteristics and temperature.

3.9 dielectric dissipation factor (DDF)
measure for dielectric losses within the oil.

Note 1 to entry: High DDF values can indicate contamination of the oil by polar contaminants or poor refining quality.

Note 2 to entry: DDF shall be measured at 90 °C, and in accordance with IEC 60247.