Transformer overheating or tripping in summer: understand it and act

Why summer is the hardest time for a transformer

The power stamped on a transformer's nameplate is not an absolute figure: it is guaranteed for a reference ambient temperature. Product standards (IEC / EN 60076) typically assume an average ambient of 20 °C and a maximum around 40 °C. Below that, the unit sheds its heat easily. Above it, the margin melts away.

A loaded transformer constantly produces heat (iron losses + copper losses). With natural cooling (ONAN for oil-immersed, AN for dry-type), that heat flows into the surrounding air. When that air is already at 35 or 40 °C, the exchange works poorly: the hot-spot temperature of the windings climbs, and it is that temperature which decides the health of the insulation.

What the symptoms mean

The warning signs are not random, they tell a story:

• Trip on thermal protection (DGPT2 on an oil unit, PT100 sensor on a dry one): the internal temperature exceeded its threshold. The protection is doing its job, but it signals an insufficient margin.
• Abnormally hot oil or windings: cooling is overwhelmed, or the load is too high for the day's conditions.
• Louder hum, cut-outs at end of day: load (air-conditioning, processes) peaks exactly when ambient is highest, a cumulative effect.
• Trips getting more frequent year on year: often the sign of a unit undersized for a load that has grown, or of ageing insulation.

The real causes, beyond the weather

The heatwave is the revealer, rarely the sole cause. Behind a transformer that suffers every summer, you almost always find one of these three factors:

• Undersizing: the load has grown (extension, new equipment, vehicle charging) without the transformer keeping pace.
• Poorly ventilated room: a substation whose hot air cannot escape turns any warm day into a furnace. See our guide on transformer room ventilation.
• Ageing: insulation that has already taken years of thermal cycles loses its margin; what passed ten years ago no longer does.

What to check straight away

Without replacing your operator or maintener, a few reflexes limit the damage in the short term: reduce or spread the load during the hottest hours, check that the room's air inlets and outlets are clear (grilles, fans, obstacles), and log the temperatures to make the problem objective rather than just endured. These measures buy time; they do not fix a structural cause.

Repair or replace: the honest trade-off

If the transformer is simply poorly ventilated, the action is on the room. But if it is undersized for today's load or at end of life, each repair merely postpones the next trip. The honest calculation weighs the cost of outages (stopped production, data, reputation) against the cost of a new, correctly sized, low-loss unit.

A well-run replacement resolves the cause: power headroom that accounts for future summers and load growth, Tier 2 efficiency that reduces heat produced and the bill, and the right cooling mode for the site. That is exactly the purpose of our replacement / retrofit offer.

This article gives general operating guidance and does not replace a diagnosis by a qualified professional on your installation. Exact thresholds and conditions depend on the unit, the applicable standard and your site.