Determination of Water‑Soluble Acids in In‑Service Transformer Oil

Overview

Transformer oil serves as a critical insulating medium for power transformers. During long‑term equipment operation, it undergoes oxidative aging under the combined effects of temperature, oxygen, moisture and other factors, generating organic acids known as water‑soluble acids. Increased content of water‑soluble acids in oil degrades the insulating performance of transformer oil, corrodes internal metal components and insulating paper of transformers, accelerates aging of insulating materials, and poses direct risks to the safe operation of power transformers. Therefore, water‑soluble acid is a key testing indicator in power equipment maintenance. This test is carried out in accordance with GB/T7598‑2008 Determination of Water‑Soluble Acids in In‑Service Transformer Oil, and analysis of samples is completed using the SH259B Automatic Water‑Soluble Acid Tester.

Test Purpose

Determine the pH value of water‑soluble acids in in‑service transformer oil, assess the aging and deterioration degree of transformer oil, monitor the condition of insulating medium in power transformers, and provide data support for transformer maintenance and oil replacement.

Test Samples

Oil samples collected from field‑operated transformers

Test Instruments

1. SH259B Automatic Water‑Soluble Acid Tester
2. Multiple 250 mL conical flasks
3. Boiled and cooled distilled water
4. Bromocresol purple indicator, bromocresol green indicator
5. Auxiliary cleaning consumables for tests

Operating Procedures

1. Sample preparation: Pipette 50 mL of transformer oil sample into a 250 mL conical flask, add 50 mL of freshly boiled carbon‑dioxide‑free distilled water, and tighten the flask stopper. Mount the conical flask into the corresponding sample slot of the instrument, connect pipelines properly and close the instrument chamber cover.
2. Reagent installation: Place bromocresol purple and bromocresol green indicators at the reagent compartment on the right side of the instrument, connect the corresponding hoses and fit the peristaltic pump hose pressure block. Ensure reliable earthing of the instrument housing, then switch on the power supply.
3. Parameter setting: Navigate to the system‑setting interface. Set the heating temperature to 75 °C, shaking time to 5 min and standing time to 5 min in compliance with national standard requirements. Return to the test‑start page; mark cup positions for samples to be tested as “Yes”, and set unused cup positions as “No”.
4. Initiate test: Select the standard mode test. The instrument automatically completes the full workflow including heating, shaking‑extraction, static stratification, aqueous‑phase extraction and colorimetric measurement. Test data will be saved automatically.

Note: Bromocresol purple is used as the primary indicator. In case of test failure, the instrument triggers an audible alarm. Replace with an empty beaker as prompted by the instrument and confirm; the instrument will automatically switch to bromocresol green indicator and resume measurement.

5. Post‑test operation: View, export or print test results from the historical‑data interface after measurement. When the instrument will remain idle for a long period, perform pipeline cleaning and release the pressure blocks of peristaltic pump hoses to protect pipelines and guarantee testing accuracy for subsequent tests.

Data Analysis and Result Evaluation

According to GB/T7598‑2008, the SH259B water‑soluble acid tester features a pH measuring range of 3.6‑7.0 with measuring error ≤ ±0.1 pH, which meets national‑standard requirements. The extracted pH value of new transformer oil is mostly within 5.4‑6.2. For normally operating transformer oil, pH generally stays between 4.5‑5.4. A pH drop to 4.2‑4.5 indicates onset of oil oxidation and requires increased testing frequency. If pH ≤4.2, severe oil aging occurs; organic acids will accelerate degradation of insulating paper and corrosion of metal parts. Oil regeneration or replacement should be implemented promptly. The instrument supports simultaneous testing of 1‑3 samples with favorable repeatability. The pH deviation of repeated measurements for the identical sample can be controlled within 0.1. With functions for data storage and printing, it is suitable for batch screening of oil samples in electric power laboratories.