This simulator models the oil circuit and cooling banks of a 10 MVA power transformer — 100 kW winding loss at rated current and 75°C, 15 kW energized core loss — and lets you switch between ONAN, ONAF and OFAF cooling arrangements to see how forced air and forced oil circulation change how quickly heat moves from the windings to ambient air.
• Thermal plant tab: a real-time 3D oil circuit and cooling-bank model with a tank toggle (cutaway view), home view reset, auto-rotate camera and an expand control, showing the orange supply pipe, blue return pipe and two fan-bank rotors. • A live heat-transfer breakdown and operating-status readout alongside the 3D view. • Control desk: run-time playback, advance 10 min / advance 1 hour time-step buttons, isolate-primary and energize-primary controls, a playback-speed selector (real time, 60x, 300x), a cooling-arrangement selector (ONAN, ONAF, OFAF), an auxiliary-control mode (automatic or manual), a manual fan-bank count selector (0/2, 1/2, 2/2), a manual oil-pump request checkbox, a teaching thermal-trip enable toggle, and a trip-reset button. • Trends & heat balance tab: a temperature history chart (winding, bulk oil, radiator and ambient reference traces), an energy-accounting panel with governing equations, fan-control and thermal-parameter settings (thermal capacitances, natural and pump/fan-assisted conductances), and an 8-hour controlled cooling comparison across ONAN, ONAF and OFAF. • Experiments & diagnostics tab: a fault-fixture selector (fan bank unavailable, pump failed, cooling auxiliary supply lost, severe radiator air blockage, oil radiator valve restriction, sensor bias, sensor lost) with an adjustable sensor-bias slider, guided experiments, a 20-check verification bench, and an exportable event log. • Learn & assess tab: lessons on transformer thermal behavior, model-scope notes, external references, and a knowledge-check quiz.
A transformer generates heat from two sources: winding (copper) loss, which rises with load current squared, and core (iron) loss, which is roughly constant whenever the transformer is energized. That heat has to move from the windings into the surrounding oil, then from the oil to the radiators, and finally from the radiators to the surrounding air — three steps modeled here as three coupled thermal nodes (winding, bulk oil, radiator) connected by thermal conductances.
ONAN (Oil Natural, Air Natural) relies entirely on natural convection at every step — hot oil rises through the windings and radiators while ambient air circulates past the radiator fins on its own. ONAF (Oil Natural, Air Forced) adds fan banks that force air across the radiators, increasing the radiator-to-air conductance without touching how oil moves. OFAF (Oil Forced, Air Forced) goes further and adds an oil pump, which increases both the winding-to-oil and oil-to-radiator conductances, giving the fastest heat rejection of the three arrangements and the highest sustainable load for a given temperature rise.
The temperature history chart plots four lumped values over time: the winding node (orange), bulk oil (cyan), radiator node (violet) and the current ambient reference (gray). Because this is a lumped three-node model rather than a full hot-spot or CFD simulation, the winding trace should be read as a representative average rather than a calibrated hot-spot prediction — the model explicitly does not resolve oil stratification or a winding hot spot.
The heat-transfer panel shows instantaneous power flow at each of the three conductance steps in real time, letting you see, for instance, that switching on a second fan bank raises the radiator-to-air conductance immediately while the oil and winding temperatures only catch up gradually because of their much larger thermal capacitances (20 MJ/K for oil vs. 2 MJ/K for the winding). The two-stage fan and pump logic uses a threshold-plus-hysteresis pattern — stage 2 starts 15°C above stage 1's threshold, and each stage only stops once temperature drops below its own threshold minus the hysteresis margin — which the trend chart makes visible as clearly separated on/off temperature bands rather than a single toggle point.
These are IEC/IEEE cooling-class codes describing oil and air circulation. ONAN is Oil Natural, Air Natural — oil and air both circulate by natural convection. ONAF is Oil Natural, Air Forced — fans force air across the radiators while oil still circulates naturally. OFAF is Oil Forced, Air Forced — a pump forces oil circulation in addition to forced-air cooling, giving the highest cooling capacity of the three.
The model uses three coupled thermal nodes: the winding, the bulk oil, and the radiator. Heat generated by winding copper loss and core loss flows from the winding node into the oil node, then from the oil node into the radiator node, and finally from the radiator to ambient air — each step modeled as a thermal conductance that can be increased by pump or fan operation.
The teaching thresholds are an alarm at 120°C winding or 95°C oil, and a delayed trip at 140°C winding or 110°C oil sustained for 10 seconds. These are educational settings built into the model for demonstration purposes, not real operating limits or a manufacturer's protection recommendation.
Stage 1 forced cooling starts once temperature reaches its threshold; stage 2 starts once temperature reaches a second threshold 15°C higher. Each stage stops only when temperature falls below its own threshold minus a hysteresis margin, which prevents the fans or pump from rapidly cycling on and off right at the threshold.