Transformer Thermal Loading & Ageing (IEC 60076-7)
Top-oil and hot-spot temperatures of an oil-immersed transformer through a load and ambient profile, with the IEC 60076-7 dynamic model, relative ageing and loss of life — and the steady load the transformer can carry at a given ambient.
Transformer
At rated load and 40 °C: top oil 90.0 °C, hot-spot 116.0 °C. Maximum continuous load for a 120 °C hot-spot: 1.04 pu.
Load profile
Result
Open a load profile (or load the sample) to run the dynamic model; the steady-state figures are shown on the left.
What this tool is, how it works and how far to trust it
A thermal model of an oil-immersed transformer that computes top-oil and hot-spot temperatures through a load and ambient profile, with relative ageing and loss of life (IEC 60076-7).
Read this before relying on a result. Every method, formula, assumption and limit is stated here, so you can check the answer yourself instead of accepting it blindly.
What it can do
- Compute the steady top-oil and hot-spot temperatures at rated load and the maximum continuous load for a hot-spot limit.
- Run the IEC 60076-7 difference equations minute by minute through a load and ambient profile, with thermal constants per cooling mode (ONAN, ONAF, OF, OD).
- Compute the relative ageing rate and equivalent ageing for normal or thermally upgraded paper, and check against the normal cyclic limits, 1.5 pu current and 140 °C bubbling.
Why you need it
- A transformer's life is the life of its paper, which ages exponentially with the hot-spot temperature.
- In hot climates and contingencies the question is how hot the winding gets through the day and how much life that costs, not the nameplate MVA.
Understand this first
- Hot-spot = ambient + top-oil rise + hot-spot gradient; both rises grow with the load (with exponents x and y) and lag it with the oil and winding time constants.
- For normal paper the ageing rate doubles for every 6 K above 98 °C; thermally upgraded paper has its reference at 110 °C.
- Normal cyclic loading limits: hot-spot 120 °C, top oil 105 °C, current 1.5 pu.
What you need before you start
- The transformer's cooling mode, top-oil rise, hot-spot gradient and load-loss ratio (from the heat-run test report).
- A load profile CSV (time, load in pu, optionally ambient).
- A current browser (Chrome, Edge, Firefox or Safari). Nothing is installed and your data is never uploaded; it is processed on your own computer.
How to use it
- Enter the transformer data and the ambient; read the steady-state values.
- Open the load profile (or load the sample) and read the assessment and the temperature chart.
- Try the other cooling mode or a reduced peak to see how much margin each option gives.
What you get
- Steady top-oil and hot-spot, maximum continuous load, peak temperatures through the profile, relative ageing, equivalent ageing hours and findings.
How to read the results
- Hot-spot above 120 °C for a few hours with relative ageing below 1: acceptable for life, but outside normal cyclic limits — reduce the peak or start the fans earlier.
- Relative ageing above 1 every day: the transformer is losing life faster than designed — plan load transfer or reinforcement.
Technical basis
- IEC 60076-7:2018 difference equations, thermal constants, ageing rate equations and limits; IEC 60076-2 temperature rises.
Limits: when not to rely on it
- Uses IEC default thermal constants unless better manufacturer values are entered; moisture and oxygen effects on ageing are not modelled.
- The result is only as good as the heat-run data; fibre-optic hot-spot measurements, where fitted, are the reference.
Sample files for this tool
Download, open and compare with what the guide says you should see.
- S13_T1_summer_day_load.csv — Transformer T1 — summer day load and ambient
For preliminary engineering use. Results must be checked by a qualified engineer against the project's standards. How results are checked · Terms & disclaimer · Privacy · Build 7cfcbe2 · 2026-11-14