Power Grid & Utility Operations challenge
Transformers, substations, protection and state estimation.
Power & Utilities · Interactive Engineering Model
Transformer Loading Calculator
Losses · Efficiency · Top-oil rise
Move the transformer through its loading range and see where efficiency peaks and why — no-load and load losses, the loading at which the two are equal, and the top-oil rise that follows from the losses rather than from the load.
Base rating of the transformer, at the cooling stage you intend to rely on.
Apparent power actually drawn. Use the peak you care about, not the average.
Core loss from the nameplate or the test report. It is there whenever the transformer is energised, whatever the load.
Copper loss at the nameplate rating, from the test report. It falls with the square of loading.
Start from an example: 1,000 kVA unit at 800 kVA, 30 °C ambient →
Advanced parameters3
- —Efficiency
- —Loading
- —Loading at peak efficiency
- —Total losses
- —No-load loss
- —Load loss at this load
- —Top-oil rise
- —Top-oil temperature
- —Spare capacity
- —Real power delivered
Move a parameter and the transformer answers: what it wastes, what that costs in efficiency, and how hot the oil gets carrying it.
The link carries the figures you entered, in the part of the address that is never sent to a server.
Change one assumption at a time. The model updates from the same calculation used for the reported result.
Try this
Move the transformer through its loading range and watch where efficiency peaks — then compare how fixed and load-dependent losses behave on either side.
How the model works
- The two lossesA transformer wastes power in two ways. The core loss is there whenever it is energised, whatever the load. The winding loss follows the square of the load — double the load and it quadruples.
- The curveBecause one loss is constant and the other grows with the square, efficiency peaks where the two are equal, and falls away either side. That point is usually well below rated load.
- The heatEverything wasted leaves as heat in the oil. The top-oil rise follows the total loss, and the temperature the insulation sees is that rise on top of whatever the ambient is that day.
- The size you choseA larger transformer carries the same load at a lower loading, so its winding loss falls — but its core loss is larger and is paid every hour. That trade is the whole of transformer sizing economics.
Need this based on real transformer operating data?
We can extend this from a screening calculator into a production model connected to your SCADA, asset data and engineering workflow.
Calculation basis 6 steps · view the method →
Loading = load ÷ nameplate rating—Load loss at this load = rated load loss × loading²—Total losses = no-load loss + load loss at this load—Efficiency = output ÷ (output + losses), with output = load × power factor—Peak efficiency occurs where load loss equals no-load loss: loading = √(no-load ÷ load loss)—Top-oil rise, simplified IEEE C57.91-style = rated rise × ((1 + R·K²) ÷ (1 + R))^0.8, with R the loss ratio and K the loading—
Assumptions & limitations Screening estimate for loading review and loss evaluation · 7 assumptions →
This is a screening calculation. It assumes steady load, a constant ambient and a simple top-oil model with the IEEE C57.91 exponent for ONAN cooling — it is not a thermal study, does not produce hot-spot temperature or loss of life, and takes no account of harmonics, unbalance, altitude, solar gain or overload duration. Loading decisions on a real unit need the manufacturer's thermal data and the applicable loading guide.
- Steady load at a constant ambient temperature; no duty cycle or overload excursion is modelled.
- Losses come from the nameplate or test report at reference temperature, with no correction for winding temperature.
- Harmonic content is ignored, though it raises load loss sharply on non-linear loads.
- Top-oil rise uses a simplified IEEE C57.91-style model with an exponent of 0.8 for ONAN cooling. It is not an implementation of the loading guide, produces no hot-spot temperature and no loss of life, and forced cooling behaves differently.
- Hot-spot temperature, insulation ageing and loss of life are not calculated.
- Auxiliary loads such as fans and pumps are excluded from the efficiency figure.
- Efficiency is expressed on real power, using the power factor entered.
Worked examples
- 1,000 kVA unit at 800 kVA, 30 °C ambient
Open with these figures → - The same unit near its peak efficiency, at 350 kVA
Open with these figures → - Overloaded: the same unit at 1,200 kVA
Open with these figures →
What this model reveals
A transformer wastes power in two ways that behave quite differently. Core loss is there whenever it is energised, whatever the load. Winding loss follows the square of the load. Because one is fixed and the other grows, efficiency peaks where the two are equal — usually well below rated load — and everything wasted leaves as heat in the oil. The model keeps the two losses apart so you can see which one a change in rating or load is moving.
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