Moisture & Loading Capability: how wet paper limits emergency loading
Moisture & Loading Capability is a free Seetalabs® tool for operation and maintenance engineers who must decide an emergency overload or a drying job. From an FDS/PDC result or water in oil, it estimates paper moisture, the bubble inception temperature and the maximum emergency load for the duration you set, with the limit that decides it. It runs in your browser.
How it works
- Moisture. Enter a dielectric response result (FDS or PDC, as paper moisture in %), or an online relative saturation reading with its temperature, or a Karl Fischer water content with the sampling temperature. Choose the liquid (mineral, aged mineral, natural ester, synthetic ester or silicone) and the paper (kraft, thermally upgraded or aramid). A measured DP is optional.
- Load. Enter the rating in MVA, the cooling mode, the top-oil rise, the hot-spot gradient and the ratio R from the heat-run test, the load before the emergency, its duration and the ambient temperature. Without one of the heat-run or duty values the load is shown as "not evaluated" and the missing fields are listed: the tool does not assume 52/26 K or R = 6. Without an ambient value it uses 30 °C and says so.
- Bubbles. The gas pressure at the top of the tank, the oil head and density, and the gas dissolved in the oil (air-saturated, nitrogen blanket, from the DGA, or a value you enter) set the pressure for the bubble calculation. A safety margin is yours to choose; it starts at zero.
- Read the result. Paper moisture with its source and range; relative saturation classes under both editions of IEC 60422; kilograms of water in the paper and in the oil, if you enter the masses; drying targets; the bubble inception temperature; and the maximum load for each constraint, with the one that decides it. A slider shows hot spot, top oil and margins at any load you choose.
You can save your inputs to a JSON file on your own computer, open it again later and print the result.
Method and standards
Paper moisture. A dielectric response measurement takes priority, as CIGRE TB 962 Table 38 prefers it for a quantitative result. Without it, the tool converts water in oil to relative saturation with the saturation curves of IEC 61203:2025 and IEC 62975:2021 Annex A (Table A.1), then reads the oil-paper equilibrium curves of IEC 61203:2025 Annex A: Figure A.2 and Figure A.3 a) for mineral oil, Figure A.3 b) for synthetic ester. The result is a range between the curves, never a single number, and it always carries the warning of IEEE C57.106-2015: equilibrium estimates on operating units can be wrong by up to ±200 %. Use them to screen, and confirm with a dielectric response test.
Relative saturation classes. The two editions of IEC 60422 disagree, and the tool shows both. IEC 60422:2024 Table B.1 applies to an online sensor at operating temperature. IEC 60422:2013 Table A.1 is the older scheme for laboratory samples. The same reading can fall in different classes: 20 % is Fair under the 2024 table and "wet" under the 2013 table.
Drying. No current standard sets a paper-moisture value at which drying becomes necessary. The decision stays with the owner, from the trend and the operating needs. The only criterion the tool applies is that of IEC 60422:2024 Table B.1: consider drying when the online relative saturation exceeds 20 %. Drying targets come from CIGRE TB 962 Table 59 and indicative durations from Tables 57 and 58.
Bubble inception temperature. IEEE C57.91-2011 Annex A, Equation A.2, from paper moisture, pressure and gas content. The pressure is the gas pressure at the top plus the oil head above the winding. The tool computes it at the centre of the moisture range and also at its upper end, and warns when the DP is 670 or lower, following CIGRE TB 741 Figure 1.11.
Thermal limits. Hot spot and top oil come from the difference equations of IEC 60076-7:2018 §8.2.3 with the constants of its Table 4: steady state at the pre-emergency load, then a step to the emergency load for the duration you set. The limits are those of IEC 60076-7:2018 Table 2 for hot spot (140 °C, or 160 °C for a short emergency of 0.5 h or less on medium and large units) and top oil (115 °C), and the current limits of Table 3 for the size of the unit.
Maximum load. The tool searches, by bisection, the highest load that keeps each constraint within its limit: bubbles, IEC hot spot, IEC top oil and current. The lowest of the four is the answer, and the page names it.
Worked example: a free-breathing unit with wet paper
Two checks you can repeat from IEEE C57.91-2011 Annex A. With paper at 1.2 % moisture and 926 torr, Equation A.2 gives about 167.3 °C, the worked example of the standard. With paper at 2 % and 760 torr it gives about 142.8 °C, only a few degrees above the 140 °C hot-spot limit of IEC 60076-7:2018 Table 2.
The full example ships with the tool. A 40 MVA ONAN transformer, mineral oil, kraft paper, free-breathing conservator. Heat run: top-oil rise 50 K, hot-spot gradient 23 K, R = 5. Oil sample: 38 mg/kg of water at 55 °C. DP 550. The emergency: from 0.7 p.u. to a higher load for 4 hours at 30 °C ambient, with 1.8 m of oil above the winding.
- Water in oil: relative saturation about 19 %. That is Fair on the IEC 60422:2024 Table B.1 wording (which the standard applies to online sensors) and moderately wet on IEC 60422:2013 Table A.1.
- Paper moisture: about 2.1 %, within a range of 2.1 to 3.3 % between the equilibrium curves, with the warning that equilibrium is not recommended on an operating unit.
- Water inventory: with 4 500 kg of paper and 19 000 kg of oil, about 94 kg of water sits in the paper and less than 1 kg in the oil.
- Bubble inception temperature: about 141.7 °C at the centre of the range. At the upper end of the range, 3.3 %, it falls to about 123 °C. DP 550 triggers the aged-paper warning.
- Maximum load for 4 hours: bubbles 1.39 p.u., IEC hot spot 1.37 p.u., top oil 1.53 p.u., current 1.5 p.u. The IEC hot-spot limit of 140 °C decides: 1.37 p.u. At that load the margin to the bubble temperature is under 2 K.
- If the paper is as wet as the upper end of the range, the bubble limit drops to about 1.21 p.u. and becomes the one that decides. A dielectric response test would tell which case applies.
Limits
The result is an indication for planning, not a loading instruction. Paper moisture estimated from the oil is a screening value; the equilibrium curves are read from the graphs of IEC 61203:2025, so they are approximate. Values reported as below the detection limit ("<x" or "ND") are not accepted. The bubble equation does not apply to aramid paper, and the tool says so. For esters the thermal model uses the constants of mineral oil, with a warning. The effect of a low DP on bubble inception is shown as a warning only, because no primary source gives a formula for it.
Frequently asked questions
Is 20 % relative saturation Fair or wet?
It depends on the edition. IEC 60422:2024 Table B.1, written for online sensors, puts 20 % in Fair; IEC 60422:2013 Table A.1 puts 20 % in "wet". The tool shows both classes next to each other, with the table behind each.
At what paper moisture should I dry the transformer?
No current standard sets a paper-moisture threshold for drying. IEC 60422:2024 Table B.1 advises considering drying when the online relative saturation exceeds 20 %. For the target after drying, CIGRE TB 962 Table 59 gives 1.0 to 1.5 % for a normal unit after repair and 0.5 to 0.7 % for a critical unit after remanufacturing.
Why can bubbles limit the load before the hot spot does?
Wetter paper releases water vapour at a lower temperature. With paper at 2 % and 760 torr, IEEE C57.91-2011 Equation A.2 gives about 142.8 °C, just above the 140 °C hot-spot limit of IEC 60076-7:2018. In the worked example, the upper end of the moisture range lowers the bubble limit from 1.39 to 1.21 p.u.
Related tools
- Insulating Fluid Condition: read the whole oil test, water included, against the table of limits for your fluid and equipment.
- Paper Insulation Remaining Life: see how paper moisture and oxygen change the thermal ageing scenarios of the paper.
Following moisture and gas results across a fleet
The tool answers the question for one transformer, today, and keeps nothing once you close the page. Ronin AI does not set loading limits. It keeps the oil and gas results of every transformer by date and names the missing tests, so you know which units to check here before a high-load season. Tell us how many transformers you follow.
The tool runs entirely in your browser. Its content security policy blocks every network connection (connect-src 'none'), so the values you enter are not sent to Seetalabs® or to anyone else, and nothing is stored in the browser. Your inputs leave the page only if you save them to a file yourself.
Tool version 1.0.3. Seetalabs®.
