The oil in a power transformer does two jobs: it insulates and it cools. As it ages it oxidises, picks up water and collects dissolved and suspended contaminants, and every one of those changes weakens it as an insulator and attacks the paper insulation it surrounds. No single test captures all of that. Oil condition programmes, such as those described in IEC 60422, combine several measurements, and four of the most common are dielectric dissipation factor (tan delta), interfacial tension, water content and acid number.
Dielectric Dissipation Factor (Tan Delta) and Resistivity
When an AC voltage is applied across an ideal insulator, the current leads the voltage by exactly 90°. Real oil lets a small in-phase (resistive) current flow as well, and the tangent of the angle by which the current falls short of 90° is the dissipation factor, tan δ. Clean, dry new oil has a very low value. Polar contaminants, oxidation products, dissolved metal compounds and moisture all raise it, so tan delta is a sensitive general indicator that something has got into the oil.
The test is made in a three-electrode test cell, with the guard electrode removing stray capacitance and surface leakage from the measurement. Because tan delta rises with temperature, the oil is heated to a set temperature before measuring — 90°C is widely used for mineral oil — so that results can be compared over time. Most instruments also apply a DC voltage across the same cell to measure DC volume resistivity, which responds to the same family of contaminants.
MEWOI's insulating oil dielectric loss testers integrate the cell, induction heater, bridge, SF6-filled standard capacitor and DC supply. The supplier specifies a 2mm electrode gap, temperature setting up to 120–125°C, an AC test voltage adjustable up to about 2200V, tan δ from 0.00001 to 100 and DC resistivity from 2.5 MΩ·m to 20 TΩ·m on the resistivity-capable models.
Interfacial Tension (IFT)
Interfacial tension is the force, in mN/m, needed to pull a platinum ring up through the boundary between oil and water. Pure oil and water meet at a sharp interface with a high tension. As oil oxidises it forms polar, soap-like compounds that gather at that boundary and lower the tension. IFT therefore falls early in the ageing process, often before the acid number has risen noticeably, which makes it a useful early warning that sludge-forming products are building up.
The ring method is standardised in ASTM D971. The MEWOI301 automates it with an electromagnetic force-balance sensor; the supplier gives a range up to 200 mN/m with 0.1 mN/m resolution.
Water Content (Karl Fischer)
Water reduces the breakdown strength of oil and accelerates the ageing of cellulose paper. In service, water moves between the paper and the oil as temperature changes, so a result is only meaningful with the oil temperature at sampling recorded alongside it.
At the low levels found in transformer oil, water is measured by coulometric Karl Fischer titration (IEC 60814, ASTM D1533). The instrument generates iodine electrically, the iodine reacts with the water in the sample, and the charge needed to reach the end point gives the mass of water directly, typically reported in mg/kg (ppm). The MEWOI401 trace moisture tester specifies 0.1 µg sensitivity and ±3 µg accuracy between 10 µg and 1000 µg of water.
Acid Number
Oxidation produces organic acids. They attack paper insulation and metals and lead to sludge, which blocks cooling ducts. The acid (neutralisation) number is the mass of potassium hydroxide, in mg KOH per gram of oil, needed to neutralise the acids in a sample. A rising trend shows that oxidation is well under way.
The MEWOI703 automates the titration — dosing, mixing and end-point detection — and stores the extraction solvent in glass and the alkaline titrant in polyethylene with a CO₂ and moisture trap. The supplier specifies a linear range of 0.001 to 0.900 mg KOH/g, 0.001 mg KOH/g resolution and an analysis cycle of about two minutes.
How the Four Results Fit Together
| Test | Mainly responds to | Typical role |
|---|---|---|
| Tan delta / resistivity | Polar contaminants, ageing products, moisture | General contamination indicator |
| Interfacial tension | Polar oxidation products at the oil–water boundary | Early sign of ageing and sludge risk |
| Water content | Dissolved water | Dielectric strength and paper ageing risk |
| Acid number | Organic acids from oxidation | Confirms advanced oxidation |
A high tan delta with normal IFT and acidity points towards contamination rather than ageing. Falling IFT followed by rising acidity is the classic oxidation pattern. Water content is read together with breakdown voltage and the sampling temperature. Limits differ with equipment voltage class and type, so assess results against the categories in IEC 60422 or the equipment owner's own criteria, and above all against previous results from the same unit.
Frequently Asked Questions
Do these tests replace dissolved gas analysis? No. Dissolved gas analysis looks for faults inside the transformer, such as overheating, partial discharge and arcing. These four tests describe the condition of the oil itself. Most maintenance programmes use both.
Why heat the oil for tan delta? Tan delta changes strongly with temperature. Measuring at a fixed, elevated temperature makes results repeatable and comparable from year to year.
Is acid number the same as neutralisation number? Yes. Both terms mean the mg KOH per gram of oil needed to neutralise its acidic constituents.