Moisture in transformer oil calculator: ppm, relative saturation, IEC 60422:2024 and IEEE C57.106-2015

What this calculator does

It converts the water content of a mineral oil between milligrams per kilogram (what a Karl Fischer test gives) and relative saturation in per cent (what an online capacitive probe gives), at whatever temperature you specify. It calculates two things that almost never appear in the report: the solubility of the oil at that temperature and the saturation temperature, below which that same water begins to appear as free water. And it assesses the oil with the standard of your choice, in two tabs: IEC 60422:2024, which classifies it as good, fair or poor according to the equipment category, for water and for five other routine tests; and IEEE C57.106-2015, which gives the limits for water and the other tests by voltage class for in-service, new or reconditioned oil and for load tap changers, and uses its annex B to estimate how much breakdown voltage drops when the oil cools down.

Why ppm is misleading

Water dissolved in oil does not matter for its absolute amount, but for how close it is to saturating the oil. And solubility depends very strongly on temperature. For new mineral oil:

log₁₀(Ws) = 7.0895 − 1567 / T

with T in kelvin and Ws in mg/kg. It is formula 1 of IEEE C57.106-2002 (from Bruce, Christie and Griffen), known as the Oommen curve, and one of the literature curves collected in figure 1 of IEC 60422:2024 (A = 7.09; B = 1567). That figure shows how much they differ: their average is A = 7.11 and B = 1585, and the standard does not fix any of them. It gives about 55 mg/kg at 20 °C, 243 at 60 °C and 449 at 80 °C.

The practical consequence: an analysis of 25 mg/kg taken with the transformer hot looks reassuring —10 % saturation at 60 °C—, but that same water, when the unit cools down to 20 °C, is at 45 % saturation. That is why a sample without the oil temperature recorded is of little use, and why online probes report relative saturation rather than ppm.

The formula is for new mineral oil. An aged or oxidised oil dissolves more water, and IEC 60422:2024 warns that in heavily oxidised oils the difference can be large and that there is no universal formula. Starting from the mg/kg of a Karl Fischer test, the actual saturation would be somewhat lower than calculated: the result errs on the conservative side. Starting from a probe saturation, the opposite happens: the real mg/kg would be somewhat higher and any mg/kg-based assessment errs on the optimistic side.

The saturation temperature

This is the most useful figure in this calculator and the one that is rarely calculated. Given the measured water content, it indicates the temperature at which that oil would reach 100 % saturation. Below it, free water may appear, which is what really collapses the dielectric strength and causes failures on cold starts or after a prolonged shutdown in winter.

A transformer with 40 mg/kg saturates at around 12 °C. If it is installed in a room that drops below that temperature, the risk is not theoretical.

Classification according to IEC 60422:2024

IEC 60422:2024 is the guide for the supervision and maintenance of mineral insulating oils in electrical equipment. The calculator applies edition 5.0 of 2024, which gives good, fair and poor limits by equipment category, according to the highest voltage for equipment (Um) and with one table for each type of equipment:

• Transformers and reactors (table 5): A, above 170 kV or of any voltage where continuity of supply is vital; B, above 72.5 kV up to 170 kV; C, up to 72.5 kV (distribution) and traction. Water good below 15, 20 and 30 mg/kg and poor above 20, 30 and 40; breakdown voltage good above 60, 50 and 40 kV and poor below 50, 40 and 30; plus acidity, dissipation factor and resistivity at 90 °C, and interfacial tension, which differs for inhibited and uninhibited oil.
• Instrument and protection transformers (table 7): D, above 170 kV, and E, up to 170 kV. Only water (cut-offs at 20 and 30 mg/kg, and at 30 and 40) and dissipation factor (0.01 and 0.10, and 0.10 and 0.30).
• On-load tap-changer diverters (table 6), category F: water good below 15 mg/kg and poor above 40 for star point or 30 for delta or line end; breakdown voltage is only good (at least 30 or 40 kV) or poor.
• Circuit breakers and switchgear (table 8): G, above 72.5 kV, and H, up to 72.5 kV. Water, breakdown voltage and acidity.
• Bushings (table 9): K, above 170 kV, and L, up to 170 kV. Water, breakdown voltage and dissipation factor.

Tables 6 to 9 do not cover every test: for the rest they refer to the appropriate category of table 5, and the calculator marks them “See table 5”. Compared with the 2013 edition, category O is merged into A, the dissipation factor of instrument transformers above 170 kV becomes 0.01 and 0.10, and tap changers, circuit breakers and bushings come in with their own tables.

What each state means: good, sampling and testing at the normal frequency are sufficient; fair, deterioration is already detectable and more frequent testing is advisable; poor, there is abnormal deterioration and corrective action should be considered, such as drying, reconditioning or regeneration of the oil. The calculator classifies only the tests that are filled in and summarises with the worst of them. A value exactly at the boundary of an interval counts as fair.

Four caveats. Water is given at operating temperature, it is always read together with breakdown voltage, and the standard asks not to act on a single result but to resample. Below 35 °C water diffuses too slowly to approach equilibrium with the paper, and the calculator flags it; with very hot oil in highly loaded transformers, the quick moisture exchange can give higher values. To trend the same unit, annex C, informative and presented as historical practice, normalises water to 20 °C with f = 2.24·e^(−0.04·ts): 10 mg/kg measured at 40 °C are equivalent to 4.5. It only holds with the sample at 35 °C or above and is not compared with the tables. And estimating paper moisture is outside the scope of the standard.

For relative saturation, table B.1 gives a guideline for online measurement at operating temperature: below 10 %, dry transformer; between 10 % and 20 %, moisture elevated compared with a new unit, with a risk of lower breakdown voltage on a cold start or when cooling after high loading; and above 20 %, moist, with reduced breakdown voltage, and drying of the solid insulation should be considered. The standard gives it for follow-up of at least one month; the calculator applies it to saturation at the oil temperature, and from a single sample it is only indicative.

Assessment according to IEEE C57.106-2015

IEEE C57.106-2015 is the IEEE guide for acceptance and maintenance of insulating mineral oil, the usual reference in North America. The calculator applies the 2015 edition, which revised the 2006 one and changed the water criterion: there is no longer a maximum based on the average oil temperature, as in table 5 of the 2002 edition, but a fixed maximum in mg/kg per voltage class, and the estimate of paper moisture is gone.

The tab lets you choose the table according to the condition of the oil:

• Table 3, in-service oil: maximum water of 35, 25 and 20 mg/kg for up to 69 kV, 69 to 230 kV and 230 kV and above; ASTM D1816 breakdown of at least 23, 28 and 30 kV with 1 mm or 40, 47 and 50 kV with 2 mm; maximum power factor of 0.5 % at 25 °C and 5 % at 100 °C; minimum interfacial tension of 25, 30 and 32 mN/m; and maximum acidity of 0.20, 0.15 and 0.10 mg KOH/g. ASTM D877 breakdown (25 kV) applies only to reactors and regulators, not to transformers.
• Table 4, reconditioned or reclaimed oil, after filling and before energizing: water of 35, 20 and 15 mg/kg (below 10 desirable at 230 kV and above), breakdown of 25, 30 and 35 kV with 1 mm or 45, 55 and 60 kV with 2 mm, interfacial tension of 35 mN/m and acidity of 0.05 mg KOH/g.
• Table 2, new oil in new equipment or after filling, before energizing: water of 20, 10 and 10 mg/kg, power factor of 0.05 % at 25 °C, interfacial tension of 38 mN/m and acidity of 0.03 mg KOH/g.
• Tables 6 and 7, load tap changers: new oil before energizing and in-service oil, with their own limits for the neutral end and for the line end up to 69 kV and above 69 kV (in service, water of 40, 30 and 25 mg/kg).

For in-service oil, the calculator applies the classification of subclause 4.2.1: class I if everything is met; class II if only breakdown voltage or water fail, in which case the guide recommends reconditioning by filtration or vacuum dehydration; and class III if anything else fails, in which case it recommends reclaiming with Fuller’s earth or replacing the oil, and restoring the inhibitor. ASTM D1816 breakdown is not comparable with IEC 60156:2025 at 2.5 mm, so a value is not carried from one tab to the other.

The standard warns in every table that these mg/kg may be excessive at low temperatures, because saturation rises as the oil cools. For that, its annex B (informative, for new oil) relates saturation to ASTM D1816 2 mm breakdown voltage: it computes saturation with Ws = 10^(7.42 − 1670/(273.16 + t)) and relative breakdown with 0.95·e^(−(%RS/63.15)²) + 0.05. In the standard’s example, 15 mg/kg is 18 % saturation at 30 °C and 74 % at 0 °C, and a 50 kV breakdown measured at 30 °C would drop to about 15 kV when energizing at 0 °C. The IEEE tab does that calculation with the second temperature; for cold energization the standard refers to IEEE C57.93-2019.

Worked example

This is the in-service case published by CIGRE (table 3 of the same paper): a 15-year-old transformer, a sample of 21.5 mg/kg taken with the oil at 57 °C and a breakdown voltage of 57 kV. These are the values the calculator loads when opened.

• Solubility at 57 °C: 220 mg/kg → saturation 9.8 %: dry according to table B.1 of IEC 60422:2024 (below 10 %), although at the limit.
• Solubility at 20 °C: 55.5 mg/kg → saturation 38.8 %: if the unit cools down, well above the 20 % of table B.1.
• Saturation temperature: −1.0 °C.

CIGRE gives 38.7 % and 9.7 %; the difference is only due to rounding. Its conclusion is what the tool shows: measured in the laboratory at 20 °C, the oil looks much wetter than it actually is in the unit.

With the IEC 60422:2024 classification: in a category A transformer, the 21.5 mg/kg is poor (the cut-off is 20) and the 57 kV is fair; in category B, water becomes fair and breakdown voltage good; in C, both are good. The same oil changes state depending on the unit it is in. With IEEE C57.106-2015 and in-service oil (table 3), the maximum for 230 kV and above is 20 mg/kg: the water does not meet it and, if that is the only failure, the oil is class II and must be reconditioned. Between 69 and 230 kV (25 mg/kg) and up to 69 kV (35 mg/kg) it does. According to annex B, saturation goes from 9.3 % at 57 °C to 41 % at 20 °C, and breakdown voltage would fall to about 69 % of the measured value if the unit cools down.

What this calculator does not do

It does not tell you how much water is in the paper, which is where most of it is and what has to be dried; IEC 60422:2024 leaves that estimate outside its scope. Equilibrium between paper and oil takes days or weeks to establish and depends on temperature and load history, so estimating paper moisture from a single oil sample is unreliable. The temperature correction of breakdown voltage in the IEEE tab is that of annex B of IEEE C57.106-2015, informative and obtained with new oil: it shows the trend and does not replace a test at that temperature. It does not cover esters or silicone oils, whose solubility and limits are different, and it does not replace the laboratory report.

How it is measured

Water content is determined by Karl Fischer titration in the laboratory, and relative saturation with capacitive probes installed in the oil circuit, which makes it possible to see the trend rather than a snapshot. The dielectric oil analyser and the other oil measurement equipment cover the testing side; drying and degassing, the treatment side. If the oil is wet, it is also worth looking at the dissolved gas analysis: paper degradation leaves a trace in CO and CO₂.