I have been conducting transformer oil quality testing for many years, and I have found that many novice laboratory technicians know to measure interfacial tension, but after receiving the values, they are not sure how to correspond them to the actual oil state evaluation. Clarify this set of judgment logic, only then can the data truly be useful.

What is oil-water interfacial tension

The interfacial tension between transformer insulation oil and water is measured as the intermolecular tension at the interface between the two phases of oil and water. During testing, the lower layer is distilled water, the upper layer is the tested insulating oil, and the platinum ring measures the interfacial tension between the two phases at the oil-water interface. The higher this value, the fewer polar impurities in the oil and the purer the oil.

Qualification criteria

According to the national power industry standards, the basic judgment reference for the interfacial tension of transformer insulation oil is as follows:

New oil: The interfacial tension is usually ≥ 40mN/m, and high-quality new oil can reach 45mN/m or more

Oil in operation: ≥ 35mN/m is generally considered to be in good condition

Attention should be paid: When<30mN/m, it indicates that the oil has significantly deteriorated, and the detection frequency should be increased and combined with other indicators for comprehensive analysis

Need to deal with: When<25mN/m, it is usually judged that the oil quality has severely deteriorated, and oil replacement or regeneration treatment should be arranged

GB/T 7595 stipulates that the oil-water interfacial tension of new oil should be ≥ 40mN/m (25 ℃), and the interfacial tension should be ≥ 35mN/m during pre operation acceptance.

Why is interfacial tension more sensitive than breakdown voltage

The aging of transformer oil is a gradual process, and the early aging products are mainly polar organic acids and ketones, which preferentially accumulate at the oil-water interface, causing a decrease in interfacial tension. At this stage, the breakdown voltage and dielectric loss may still be within the qualified range, and the insulation performance may still be "normal" on the surface.

The decrease in interfacial tension often occurs earlier than changes in breakdown voltage and dielectric loss, and is the most sensitive indicator for detecting early deterioration of oil products. That's why interface tension is listed as a mandatory test item in transformer preventive testing, rather than simply using breakdown voltage as a substitute.

The Importance of Comparing Historical Data

The single measurement result is only a reference, and more importantly, longitudinal tracking - the interfacial tension values of the same transformer in previous tests. If the values continue to decline in recent tests, the trend itself is an important warning signal, even if the current absolute value has not yet reached the alarm line. The 255 historical records (including time stamps) function of HZZL-3 provides a data foundation for this trend analysis.