Determination of Interfacial Tension of Transformer Oil

I. Overview

  Transformer oil is a mineral insulating oil obtained by deep refining of petroleum fractions. It is the core liquid insulating medium in the power industry, widely used in power transformers, reactors, instrument transformers, high-voltage switchgear and other electrical equipment, undertaking the key functions of insulation, cooling and heat dissipation, arc extinguishing, and equipment condition monitoring. The quality of transformer oil directly determines the operation safety and service life of high-voltage electrical equipment, and the oil-water interfacial tension is the core quality control indicator for evaluating the purity, oxidative aging degree and contamination status of transformer oil.

II. Experimental Purpose

  During long-term operation, transformer oil will undergo oxidative aging under the action of high temperature, electric field and oxygen, producing polar impurities such as organic acids and colloidal substances, which will significantly reduce the oil-water interfacial tension. Therefore, the change of interfacial tension is an early warning signal for the deterioration of transformer oil quality, which can effectively guide the equipment maintenance and oil replacement.

  This test is carried out in strict accordance with GB 6541-86 *Standard Test Method for Interfacial Tension of Oil Against Water by the Ring Method for Petroleum Products*, and the whole process of detection is completed by SH107 Automatic Interfacial Tensiometer developed by Shandong Shengtai Instrument Co., Ltd. The instrument fully meets the requirements of the standard method, and can accurately and efficiently complete the determination of liquid-liquid interfacial tension and liquid-gas surface tension of various liquids.

III. Experimental Samples, Instruments and Equipment

1. Experimental samples: No.25 mineral insulating transformer oil (1 group of new oil sample and 1 group of in-service oil sample from on-site operation)

2. Core instrument: SH107 Automatic Interfacial Tensiometer (supporting special platinum ring, sample test cup, 1g standard calibration weight)

3. Auxiliary appliances and reagents: double distilled water, petroleum ether, acetone, chromic acid cleaning solution, anhydrous ethanol, alcohol lamp, dust-free lens cleaning paper

IV. Experimental Operation Procedures

1. Cleaning and Pretreatment of Appliances

  In strict accordance with the requirements of GB 6541, the test appliances are thoroughly cleaned: first, rinse the sample cup and platinum ring with petroleum ether to remove oil residue; then rinse repeatedly with acetone and double distilled water in turn; the platinum ring shall be fully burned in the outer flame of the alcohol lamp until red-hot to completely remove surface organic matter; after being immersed and washed with hot chromic acid cleaning solution to remove oil, the sample cup shall be rinsed with double distilled water until neutral, and stored upside down in a dust-free environment for later use to avoid secondary contamination.

2. Instrument Installation and Debugging

  Place the instrument on a stable platform free of wind, severe vibration and strong air flow interference, install the platinum ring and supporting hanger, adjust the three feet at the bottom of the instrument to make the level bubble on the sample pan in the center position, and ensure that the instrument is in a horizontal state. Connect the AC220V power supply, power on and preheat for 15 minutes to stabilize the instrument system.

3. Instrument Calibration

  Enter the color touch screen operation interface of the instrument, and complete the weight calibration first: hang the matched 1g standard weight on the platinum ring, confirm that the weight indication error of the instrument is within ±0.010g, and re-calibrate through the calibration function of the interface if the deviation exceeds the limit.

Then carry out pure water calibration: pour double distilled water into the clean sample cup to the specified scale line, place the sample cup in the center of the sample pan, and the instrument will automatically complete the test process. In the 25℃ constant temperature environment, the calibrated interfacial tension value of pure water shall be stable within the range of 71~72mN/m, and sample testing can be carried out only after the calibration is qualified.

4. Test Parameter Setting

  Enter the parameter setting interface, confirm the core parameters such as platinum wire radius, platinum ring radius, density of heavy liquid (water), and density of light liquid (oil sample to be tested) according to the test sample and appliance parameters, click Save after the setting is completed. The default parameter settings of the instrument have been completed before delivery, and no modification is required under normal circumstances.

5. Sample Testing

  First pour double distilled water into the clean sample cup to the lower scale line, place the sample cup in the center of the sample pan, click to enter the sample test interface, and the instrument will automatically adjust the height of the lifting platform to make the platinum ring accurately immerse 5mm into the aqueous phase; then slowly pour the transformer oil sample to be tested along the cup wall to the upper scale line to avoid air bubbles, let stand for 30 seconds and click "Continue", the instrument will automatically complete the whole test process, and display the dynamic change curve of tension in real time. After the test, the interfacial tension result will be automatically calculated and stored, and the test report can be printed directly as needed.

6. Parallel Testing

  Complete 3 parallel tests on the same sample to verify the repeatability and stability of the test results.

V. Data Analysis and Result Evaluation

  In this test, the SH107 Automatic Interfacial Tensiometer was used to test the interfacial tension of new transformer oil and in-service transformer oil samples respectively in a 25℃ constant temperature environment. The results of 3 parallel tests are as follows:

| Sample Type | 1Test Value (mN/m) | 2 Test Value (mN/m) | 3 Test Value (mN/m) | Average Value (mN/m) | Relative Deviation |

|-------------|-----------------------|-----------------------|-----------------------|-----------------------|--------------------|

| New Transformer Oil | 43.52 | 43.68 | 43.59 | 43.60 | ≤0.2% |

| In-service Transformer Oil | 22.05 | 22.17 | 22.08 | 22.10 | ≤0.3% |

According to the requirements of GB/T 7595-2017 *Quality of In-service Transformer Oils*, the interfacial tension of new transformer oil before commissioning shall be ≥40mN/m at 25℃; the interfacial tension of in-service transformer oil shall be ≥18mN/m at 25℃.

  The test results show that the average interfacial tension of the new oil sample is 43.60mN/m, which meets the standard requirements for new oil, with pure oil quality and no obvious oxidation and contamination; the average interfacial tension of the in-service oil sample is 22.10mN/m, which is still within the qualified limit of the standard, the oil quality is stable and can continue to be used. It is recommended to regularly track and monitor the change of this index in the follow-up.

  This test verifies that the SH107 Automatic Interfacial Tensiometer has a test accuracy up to 0.1mN/m and repeatability ≤0.3%, which fully meets the technical requirements of GB 6541 standard. The instrument adopts color touch screen human-computer interaction, with simple and convenient operation, and can automatically complete the whole process of calibration, testing, result calculation, storage and printing, which greatly reduces manual operation errors, with high test efficiency and stable and reliable results. It is suitable for liquid interfacial tension and surface tension detection in electric power, petroleum, chemical industry, commodity inspection, scientific research and many other industries.