NB/SH/T 0811-2023 Determination of oxidation stability of insulating oils

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Card type: Test method

1. Method number / full title / English title / replacement relationship

• Standard number: NB/SH/T 0811—2023
• Chinese title: Determination of oxidation stability of insulating oils
• English title: Test method for evaluating the oxidation stability of insulating liquids
• Replacement relationship: replaces NB/SH/T 0811—2010 "Determination of oxidation stability of unused hydrocarbon insulating oils"; this is the first revision (first published in 2010).
• Adoption relationship: modified adoption of IEC 61125:2018 "Insulating liquids―Test methods for oxidation stability―Test method for evaluating the oxidation stability of insulating liquids in the delivered state, MOD".
• Main technical differences from IEC 61125:2018: a) title modified; b) normative references adopt corresponding Chinese standards; c) added precision of the oxidation test method for mineral oils 500h containing inhibitors; d) deleted the thermometer specification content of IEC 61125:2018 Annex A; e) deleted the oxidation induction period determination content of IEC 61125:2018 Annex B.
• Main technical changes from 2010 edition: a) title modified; b) scope of application modified; c) added technical requirements for "acetone", "potassium hydroxide-isopropanol solution", "concentrated sulfuric acid", "secondary water", and "filter", and deleted technical requirements for "chloroform", "ethanol (95%)", and "titration solvent"; d) modified the technical requirements for "glass filter", added technical requirements for "filter", "oven", "analytical balance", "graduated cylinder", and "volumetric flask", and deleted the technical requirements for "glass filter"; e) added oxidation duration requirements (see 9.2); f) added a "Sampling" clause (see Chapter 7); g) added precision of the oxidation test method for mineral oils 500h containing inhibitors (see Chapter 10); h) added a determination method for oxidation stability of unused ester insulating oils (see 9.7 and Annex A).
• Publication/implementation: published on 2023-12-28, implemented on 2024-06-28.
• Secretariat: Subcommittee on Petroleum Fuels and Lubricants of the National Technical Committee for Standardization of Petroleum Products and Lubricants (SAC/TC280/SC1).
• Drafting organizations: Lubricant Branch of PetroChina Company Limited, Shanghai Branch of Sinopec Lubricant Company Limited, Electric Power Research Institute of Guangdong Power Grid Company Limited, Refining and Chemical Research Institute of PetroChina Karamay Petrochemical Company Limited, Electric Power Research Institute of State Grid Hunan Electric Power Company Limited, Electric Power Research Institute of State Grid Heilongjiang Electric Power Company Limited.
• Main drafters: Zhang Yu, Wang Huijuan, Zhang Qi, Zhang Jinfang, Yu Huimin, Qian Yihua, Ma Shujie, Li Hui, Wang Peng, Wu Junjie, Li Guoxing.

2. Scope of application and non-applicability

• This document describes a method for determining the oxidation stability of insulating oils under accelerated oxidation conditions.
• Applicable to insulating oils, including unused mineral insulating oils and non-mineral insulating oils, with or without inhibitors.
• Not applicable: no explicit non-applicability clause is provided in the available source.

3. Method principle

A solid copper catalyst is placed in the sample to be tested, air is passed in at a constant rate, and after maintaining at a temperature of 120℃ for a certain time, the antioxidant capacity is evaluated by determining the volatile acid value, oil-soluble acid value, sediment content, and dielectric loss factor of the oxidized oil. The test duration may be determined according to the requirements of the corresponding product standards for different types of insulating oils.

4. Reagents and materials (purity / preparation)

4.1 Reagents

• n-Heptane: analytical reagent.
• Isopropanol: analytical reagent.
• Acetone: analytical reagent.
• Alkaline blue 6B indicator: each 100mL of ethanol (95%) contains 2g±0.01g of alkaline blue 6B. Preparation: weigh alkaline blue 2g, add to 100mL of boiled ethanol (95%), reflux in a water bath for 1h, and filter after cooling. If necessary, neutralize the boiled clear filtrate with 0.1mol/L hydrochloric acid solution until adding 1 drop to 2 drops of alkali solution can change the indicator solution from blue to light red and it can return to blue after cooling.
• Phenolphthalein indicator: each 100mL of ethanol (95%) contains 1g of phenolphthalein.
• Potassium hydroxide-isopropanol solution: 0.05mol/L. Weigh 3g±0.01g of potassium hydroxide, add to a flask containing 1L of anhydrous isopropanol solution, install a reflux condenser, heat and shake continuously, slowly boil for 20min, after potassium hydroxide is completely dissolved, cool for a moment, add at least 2g of barium hydroxide (generally 4g to 5g), and then slowly boil for at least 30min. Cool to room temperature, let stand, and after the upper solution becomes clear, carefully pour the upper clear liquid into a brown bottle for storage.
• Concentrated sulfuric acid: sulfuric acid with a mass fraction of ≥98%.
• Oxidation gas: compressed air or air from a pressurized air line, free of hydrocarbons.
• Secondary water: meets the requirements for secondary water in GB/T 6682—2008.

4.2 Materials

• Catalyst: soft electrolytic copper wire, surface area 28.6cm²±0.3cm², diameter 1mm to 2mm.
• Emery cloth: 220 grade silicon carbide emery cloth, meeting the requirements of GB/T 2480—2022.
• Filter: a glass filter with a filter membrane, or a Gooch crucible with a porous glass disc (volume 35mL, porosity 4μm to 10μm) may be used.
• Filter membrane: mixed cellulose filter membrane, pore size 8µm.

5. Instruments and equipment

• Heating equipment (6.1): automatically temperature-controlled aluminum alloy heater or oil bath heater, maintaining the temperature of the insulating oil in the oxidation tube at 120.0℃±0.5℃; the upper surface temperature of the top insulation layer is maintained at 60.0℃±0.5℃; the aluminum block surface is protected with 4mm standard insulation layer. Aluminum alloy heater: total depth of the oxidation tube insertion hole 150mm, heating part hole depth at least 125mm, each oxidation tube is surrounded by a short aluminum alloy ring passing through the insulation layer, ensuring the total length of the heated part of the oxidation tube reaches 150mm. Oil bath heater: the oxidation tube is immersed in oil to 137mm, total length in the heating bath 150mm. Both heating devices: the length of the oxidation tube exposed above the heater surface is 60mm, and the hole diameter just fits the insertion of the oxidation tube; if loose, an inner diameter 25mm O ring may be used for sealing. The heating equipment should be equipped with a support capable of holding the absorption tube. When using the heater, avoid direct sunlight and air circulation. Note: when using an oil bath, it is recommended to place it in a fume hood.
• Oxidation tube (6.2): borosilicate glass or neutral glass test tube, with a 24/29 ground joint, dimensions as in Table 1 and Figure 4. Gas inlet tube with a bent tube and ground stopper, the gas inlet tube extends to within 2.5mm±0.5mm from the bottom of the oxidation tube, and the end forms an angle of 60° with the horizontal axis.
• Table 1 Oxidation tube dimensions (unit: millimeters): total length of the outer tube of the oxidation tube 210±2; outer diameter of the oxidation tube 26.0±0.5; wall thickness of the outer tube of the oxidation tube 1.4±0.2; height of the head of the oxidation tube 28±2; outer diameter of the gas inlet tube of the oxidation tube 5.0±0.4; wall thickness of the gas inlet tube of the oxidation tube 0.8±0.1.
• Absorption tube (6.3): specifications are the same as the oxidation tube. The distance between the axes of the absorption tube and the oxidation tube is 150mm±50mm. The connection between the oxidation tube and the absorption tube should be as short as possible; a short flexible sleeve (silicone tubing is suitable) may be used to connect the absorption tube horizontally to the oxidation tube. To avoid absorption of acidic substances by the silicone tubing, the connecting sleeve should be minimized. The absorption tube should be installed outside the heating device.
• Filter (6.4): a glass filter with a filter membrane, or a Gooch crucible with a porous glass disc of volume 35mL and porosity 4μm to 10μm.
• Crucible or evaporating dish (6.5): volume 50mL.
• Soap bubble flowmeter (6.6): used to measure gas flow rate; a calibrated capillary flowmeter or electronic flowmeter may be used.
• Air flowmeter (6.7): suitable for 0.150L/h±0.015L/h. A calibrated capillary flowmeter or electronic flowmeter may be used.
• Timer (6.8): graduation 0.2s.
• Oven (6.9): temperature can be kept constant at 105℃±2℃.
• Gas source (6.10): supplied from a compressed gas cylinder, passed into a washing bottle containing concentrated sulfuric acid for drying, and then passed into a tower containing spaced glass wool and soda lime layers; a drying tube or commercial gas purifier may also be used. The dried gas enters the oxidation tubes through a flow control system capable of achieving the specified flow rate. The flow control system has a multi-way connector, is connected to the gas purification assembly, and is equipped with multiple air flowmeters or switches, each switch equipped with a precision needle valve to control gas entering each oxidation tube. The gas flow rate can be measured by a flowmeter; the liquid level difference in the two branches of the flowmeter should be sufficiently large to ensure measurement sensitivity.
• Analytical balance (6.11): sensitivity 0.1mg.
• Burette (6.12): volume 10mL, accuracy 0.01mL.
• Stoppered conical flask (6.13): volume 500mL.
• Graduated cylinder (6.14): volume 100mL.
• Volumetric flask (6.15): volume 500mL.
• Thermometer (6.16): meets the requirements for thermometer number GB-59 in GB/T 514.

6. Recommended operating conditions

• Oxidation temperature: 120.0℃±0.5℃.
• Air flow rate: 0.150L/h±0.015L/h.
• Oxidation duration: determined by the corresponding product standard; usually 164h for mineral insulating oils without inhibitors, and 500h for mineral insulating oils containing inhibitors.
• Upper surface temperature of the top insulation layer: 60.0℃±0.5℃.
• Check air flow rate and temperature every day.
• Annex A (ester insulating oils): oven temperature 150℃±2℃, oxidation 164h; if the glass bottle is placed in an unheated oven, add an additional 0.5h of oxidation time.

7. Sampling and sample preparation

• Sampling shall be carried out in accordance with the requirements of GB/T 4756 (Chapter 7).
• Cleaning of test containers (8.1): the oxidation tube and absorption tube are first cleaned with acetone, then rinsed with secondary water; after draining, soak in concentrated sulfuric acid for at least 16h, then rinse with tap water until free of acid, then rinse clean with secondary water; place in an oven at 105℃ and dry for at least 3h, then place in a desiccator or drying oven to cool to room temperature for later use.
• Preparation of insulating oil sample (8.2): filter the sample with a previously dried (at 105℃ for 1h) glass filter to remove residual sediment and water in the sample; discard the first 25mL of filtrate.
• Catalyst preparation (8.3): Use soft electrolytic copper wire; before use, polish the copper wire of the specified length with 220 silicon carbide abrasive cloth meeting the requirements of GB/T 2480—2022; wind the copper wire into a spiral with outer diameter 2cm and length 5cm; immerse the copper coil in n-heptane for thorough cleaning, dry in air, and immediately place into the oxidation tube; the prepared copper coil can only be picked up with tweezers, and the copper wire must not be reused.
• Appendix A Sample preparation: Place the oil sample in an open beaker exposed to dry air (desiccator or similar equipment), keep in the dark for 24h, then determine the water content according to NB/SH/T 0207 (should not be greater than 200mg/kg); if the water content exceeds 200mg/kg, the sample cannot be used for the test; check the drying equipment and repeat the pretreatment until the water content meets the requirement. Pour 37.5mL of the pretreated oil sample into a 50mL glass headspace vial; polish both sides of the copper sheet with abrasive cloth, bend both ends and place into the headspace vial, ensuring the copper sheet does not touch the glass vial wall; seal tightly with a silicone/polytetrafluoroethylene cap using a crimping tool.

8. Analysis procedure

8.1 Installation and commissioning (9.1)

• Adjust the heater so that the temperature of the insulating oil sample in the oxidation tube is maintained at 120.0℃±0.5℃.
• Weigh 25g±0.1g of insulating oil each time and load into three oxidation tubes, and place copper coils inside. At least three oxidation tubes are required to ensure sufficient measurement of DDF (at least two oxidation tubes), sludge, and oil-soluble acid value (one oxidation tube). Insert the gas inlet tube with a ground stopper, place the oxidation tubes into the heating device; if necessary, use O-shaped rubber rings to seal between the oxidation tubes and the insulation layer.
• Place 25mL of secondary water into each absorption tube. Insert the gas inlet tube with a ground stopper and connect to the oxidation tube.
• Adjust the air flow rate so that when a flow meter is connected to the outlet of the absorption tube, the flow rate is 0.150L/h±0.015L/h.

8.2 Oxidation (9.2)

Under conditions of maintaining a temperature of 120.0℃±0.5℃ and an air flow rate of 0.150L/h±0.015L/h, oxidize the insulating oil for a certain duration. This duration is determined by the corresponding product standard; typically, for mineral insulating oil without inhibitor 164h, and for mineral insulating oil with inhibitor 500h. Check the air flow rate and temperature daily.

8.3 Determination of sediment (9.2.1～9.2.6)

• Place the oxidation tube containing 25g oxidized insulating oil in a cold, dark place for 1h, then transfer to a stoppered conical flask.
• Use 300mL n-heptane to wash the insulating oil adhering to the oxidation tube, copper coil, and gas inlet tube in portions until no oil traces remain, and recover the washed n-heptane-insulating oil mixture into the conical flask containing the sample. Let the mixture stand in the dark at 20℃±5℃ for 24h, then filter through a pre-weighed filter into a suction flask. At the beginning of filtration, use slight vacuum to prevent sediment from passing through. If the filtrate is turbid, it can be filtered again.
• Repeatedly wash the sediment with n-heptane to remove all traces of insulating oil. The total volume of n-heptane used for washing the sediment should be 150mL. Dry the filter containing the sediment at 105℃ to constant weight to obtain the sediment.
• Sediment adhering to the copper coil, oxidation tube wall, and gas conduit can be dissolved with acetone (total amount 30mL), transferred to a porcelain dish (or evaporating dish), and then dried at 105℃ to constant weight to obtain the sediment.
• The mass of sediment obtained by acetone dissolution plus the mass of sediment obtained by n-heptane washing and filtration is the total sediment mass, expressed as a percentage of the initial mass of the insulating oil.

8.4 Determination of oil-soluble acid (SA) (9.3)

• Transfer the n-heptane-insulating oil mixed solution after sediment filtration to a 500mL volumetric flask and dilute to the mark. Take 100mL of the n-heptane-insulating oil mixed solution and determine the acid value according to NB/SH/T 0836.2. Perform three determinations.
• Note: If the mixed solution is dark in color and the color change is difficult to judge, the oil-soluble acid value can be determined by potentiometric titration according to NB/SH/T 0836.

8.5 Determination of volatile acid (VA) (9.4)

• Remove the absorption tube, add 4 drops of phenolphthalein indicator, and directly titrate the acid value in the absorption tube with potassium hydroxide-isopropanol solution. During titration, add the potassium hydroxide-isopropanol solution dropwise while shaking the aqueous solution in the absorption tube; when the color of the aqueous solution in the absorption tube just turns pink and does not fade within 15s, record the titration volume as V.

8.6 Determination of dielectric dissipation factor (DDF) (9.6)

Remove the oxidation tube from the oxidation bath, store at room temperature 20℃±5℃ for 24h, during which the sample cools naturally and insoluble sediment settles to the bottom. Then transfer the insulating oil to a clean test cup without stirring, ensuring that no sediment enters the test cup. Only about 80% of the insulating oil is transferred to the test cup; the remaining insulating oil and sediment are not used for the dielectric dissipation factor determination. The test method is carried out according to GB/T 5654.

8.7 Determination of oxidation stability of synthetic ester transformer oil (9.7 and Appendix A)

For the oxidation stability of synthetic ester transformer oil, in addition to the above method, the method in Appendix A can also be used.

• Appendix A Oxidation: Oxidize in an oven at 150℃±2℃ for 164h. If the glass bottle is placed in an unheated oven, add an additional 0.5h oxidation time. One sample to be tested requires placing 4 bottles of insulating oil to be tested in the oven; for duplicate tests, place 8 bottles (300mL oil samples) of insulating oil to be tested.
• Appendix A Determination of oil-soluble acid value: Directly determine the acid value of the oxidized ester insulating oil according to NB/SH/T 0836.2. If the sample color is too dark, determine its acid value according to NB/SH/T 0836.
• Appendix A Determination of dielectric dissipation factor (DDF, 90℃): See 9.6 (at least three headspace vial oil samples are required).
• Appendix A Appearance: Place the headspace vial containing the oxidized synthetic ester insulating oil in the dark for 24h, evaluate its turbidity according to GB/T 14542, usually expressed as whether it is turbid.
• Appendix A Kinematic viscosity: Determine the kinematic viscosity of the sample according to GB/T 265.

9. Qualitative and quantitative and calculation formulas

9.1 Oil-soluble acid value (Formula 1)

X_SA = (V₂ − V₁) × M × 56.1 / m

• X_SA — oil-soluble acid value of the sample, unit mg/g (calculated as KOH);
• M — concentration of potassium hydroxide-isopropanol solution, unit mol/L;
• V₂ — volume of potassium hydroxide-isopropanol solution required to titrate the n-heptane-insulating oil mixed solution, unit mL;
• V₁ — volume of potassium hydroxide-isopropanol solution required to titrate 100mL n-heptane solution according to NB/SH/T 0836.2, unit mL;
• m — mass of the sample, unit g.

9.2 Volatile acid value (Formula 2)

X_VA = V × M × 56.1 / m

• X_VA — volatile acid value of the sample, unit mg/g (calculated as KOH);
• M — concentration of potassium hydroxide-isopropanol solution, unit mol/L;
• V — volume of potassium hydroxide-isopropanol solution required during titration, unit mL;
• m — mass of the sample, unit g.

9.3 Total acid value (Formula 3)

X_TA = X_SA + X_VA

• X_TA — total acid value of the sample, unit mg/g (calculated as KOH);
• X_SA — oil-soluble acid value of the sample, unit mg/g (calculated as KOH);
• X_VA — volatile acid value of the sample, unit mg/g (calculated as KOH).

9.4 Sediment

Total sediment mass = mass of sediment obtained by acetone dissolution + mass of sediment obtained by n-heptane washing and filtration, expressed as a percentage of the initial mass of the insulating oil.

10. Precision

10.1 Overview

The precision for mineral insulating oil samples without inhibitor was obtained by statistical calculation after 164h test. The precision for mineral insulating oil samples with inhibitor was obtained from an interlaboratory study involving 6 laboratories, which examined 8 samples after oxidation for 500h with total acid value less than 0.1mg/g (calculated as KOH), sediment less than 0.1%, and dielectric dissipation factor (90℃) less than 0.3; determined by statistical analysis of the interlaboratory study results according to GB/T 6683.1. The reliability of test results is judged according to the following provisions (95% confidence level).

10.2 Repeatability (r)

The difference between two consecutive test results obtained by the same operator, in the same laboratory, using the same apparatus, following the same method, on the same sample shall not exceed the values specified in Table 2 and Table 3.

10.3 Reproducibility (R)

The difference between two single, independent test results obtained by different operators, in different laboratories, using different apparatus, following the same method, on the same sample shall not exceed the values specified in Table 2 and Table 3.

Table 2 Repeatability and reproducibility of oxidation stability test for mineral oil without inhibitor under conditions of 120℃, 164h

Note: X₁—average of two replicate test results; X₂—average of two single, independent test results.

Table 3 Repeatability and reproducibility of oxidation stability test for mineral oil with inhibitor under conditions of 120℃, 500h

Note: The precision applies to mineral oils whose total acid value after oxidation is less than 0.1mg/g (expressed as KOH), sediment less than 0.1%, and dielectric dissipation factor (90℃) less than 0.3.

Table A.1 Precision of oxidation stability test for additive-containing synthetic ester insulating oils

Note: X₁—the average of two replicate test results; X₂—the average of two single, independent test results. The precision in Table A.1 was obtained by statistical calculation from ester insulating oil samples after the 164h test.

11. Report

The report shall include the following:

• a) this document NB/SH/T 0811;
• b) test duration;
• c) test temperature;
• d) total acid value (TA), accurate to 0.01 (expressed as KOH) mg/g;
• e) sediment, accurate to 0.01% (mass fraction);
• f) dielectric dissipation factor, accurate to 0.001.

Appendix A The result report shall include:

• a) this document NB/SH/T 0811, according to the oxidation method in Appendix A;
• b) oil-soluble acid (SA);
• c) dielectric dissipation factor of 90℃. The report may also include: a) appearance; b) kinematic viscosity.

12. Normative References

Normative References

• GB/T 265 Petroleum products—Determination of kinematic viscosity and calculation of dynamic viscosity
• GB/T 514 Specification for glass liquid thermometers for petroleum product testing
• GB/T 2480—2022 Conventional abrasives—Silicon carbide
• GB/T 4016—2019 Petroleum products—Terminology
• GB/T 4756 Petroleum liquids—Manual sampling
• GB/T 5654 Liquid insulating materials—Measurement of relative permittivity, dielectric dissipation factor and DC resistivity
• GB/T 6682—2008 Water for analytical laboratory use—Specification and test methods
• GB/T 14542 Guidelines for maintenance and management of transformer oil
• NB/SH/T 0836.2 Determination of acid value of insulating oil—Part 2: Color indicator method

Bibliography

• [1] GB/T 6683.1 Petroleum and related products—Determination of precision of measurement methods and results—Part 1: Determination of precision data of test methods
• [2] NB/SH/T 0207 Determination of water content in insulating liquids—Karl Fischer coulometric titration method
• [3] NB/SH/T 0836 Determination of acid value of insulating oil—Automatic potentiometric titration method

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

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