Transformer Cooling System Thermal Inspection: Radiators, Fans and Oil Temperature

Why Transformer Cooling Fails

Power transformers are designed to run hot — but only within a narrow window. The insulating paper in a transformer winding ages according to the Arrhenius law: for roughly every 6–8 °C of sustained temperature increase, the insulation lifetime is cut in half. A transformer that runs its top oil 10 °C above design temperature can lose decades of its useful life in a single hot season. In tropical and high-altitude Latin American locations, ambient temperatures of 35 – 45 °C are normal, which means the cooling system has far less margin than the same transformer would have in a temperate climate.

The cooling system has three jobs: move heat out of the windings, move heat out of the oil, and keep the top-oil temperature inside the design envelope. Radiators and coolers transfer heat to the air; fans and oil pumps force the airflow and oil flow that make the transfer efficient. When any link in that chain degrades — a clogged radiator, a stuck fan, a blocked oil path — the transformer compensates by running hotter, and the hotter it runs, the faster every other component ages.

Most cooling failures develop gradually and invisibly. A radiator fin corrodes and stops transferring heat; a fan bearing seizes and the thermal relay never notices because the oil temperature stays just inside limits; a conservator runs low on oil and the top of the tank starts to overheat. By the time the temperature alarm sounds, the damage to the insulation is already done. That is why a handheld thermal camera is the most practical early-warning tool in a transformer yard: it sees the cooling problem months before the winding temperature rises.

"Our 30 MVA unit kept tripping its winding temperature alarm every summer. The thermal scan showed two of the four radiator banks were blocked with salt deposits and one fan was dead. Cleaning and a new fan motor cost us two days — a rewinding would have cost six months and the whole feeder." — Substation Maintenance Engineer, Ecuador

Thermal Signatures of Cooling Problems

Learn to read these patterns when you scan a transformer:

• Cold radiator bank among warm ones: The clearest cooling failure signature. On an ONAF/ONAF transformer, a radiator bank that is significantly cooler than its neighbors is not transferring heat — its fins are clogged, the oil path is blocked, or its fan is dead. The oil inside that bank is stagnating while the rest of the transformer carries the load.
• Hot top tank with cool radiators: Oil is circulating poorly or the level is low. The tank top runs hot while the radiators stay cool, meaning the oil is not reaching the cooling surfaces.
• Uneven radiator temperature top-to-bottom: A healthy radiator is warm at the top (hot oil enters) and progressively cooler toward the bottom. A radiator that is uniformly cold, or hot at the bottom, points to blocked oil flow or a stuck thermosiphon.
• Localized hot spot on the tank wall: Points to a core or winding hot spot conducting through the tank, or an internal problem such as a failing tap changer compartment. Cross-check with a partial discharge scan before scheduling invasive work.
• Hot conservator or breather area: Low oil level or excessive moisture in the oil. Compare the conservator surface temperature against the tank top under the same load.
• Fan motors running hot: A fan motor that is much hotter than its neighbors is overloaded, has failing bearings, or is about to trip. A dead fan shows up as a cold, motionless patch in the air stream — check with the fan running and again after it cycles.

Bare aluminium radiators have moderate emissivity, but painted surfaces and oily films make emissivity vary from panel to panel. Use a consistent emissivity setting and compare radiators against each other rather than trusting absolute temperatures. The EasIR Series with adjustable emissivity and a 15 mK NETD resolves the 1–2 K differences that reveal a partially clogged radiator.

Delta-T Criteria for Transformer Cooling

These thresholds follow standard thermography practice for oil-filled transformers (ISO 18434-based programs and utility maintenance guides). Always compare like for like: same load, same ambient, same time of day. The comparison between two radiator banks on the same transformer is the most reliable indicator because both banks see identical load and ambient conditions.

"We now scan every transformer bank at peak load twice a year. The radiator-to-radiator comparison is our gold standard — it caught a blocked cooler on a spare transformer that was sitting in 'standby' but was actually being called on every hot afternoon." — Asset Manager, Peru

ONAN, ONAF and OFAF: Reading the Cooling Configuration

Know your transformer's nameplate configuration before you scan. On OFAF units, the oil pump is the critical component: if it fails, the radiators go cold while the tank heats rapidly. Compare the radiator gradient against the expected behavior for the configuration, and always note which fans and pumps are running at scan time.

Safe Transformer Thermal Inspection Procedure

1. Scan at or near peak load. Cooling problems are most visible when the transformer is working hardest — typically midday to early afternoon, or during peak summer loading.
2. Record load, ambient temperature, and weather. Sun, wind and rain change surface temperatures dramatically. Note them for every scan so trends stay comparable.
3. Scan the tank top first, then each radiator bank, then accessories. Work in a fixed order: conservator, tank top, bushing turrets, radiator banks (top to bottom), fans, pump motors, cable boxes.
4. Keep a safe distance and angle. Respect approach distances for the voltage class. Scan at a consistent angle — reflections from the sun or the sky can create false hot or cold spots on oil surfaces.
5. Use consistent camera settings. Set emissivity, distance and span identically across all banks. Save a reference image of each bank at the same settings for month-to-month comparison.
6. Cross-check hot findings. A hot tank wall can be a cooling problem or an electrical problem. Pair the thermal scan with an SC Series PD detector and an acoustic camera to distinguish thermal, partial discharge, and mechanical sources before deciding on corrective action.

Do not rely on the transformer's own temperature gauges alone — they measure one or two points. Thermography gives you the full surface picture: a tank that is uniformly hot on one side tells you something the top-oil gauge never will.

Choosing the Right Camera for Transformer Work

For most substations the EasIR is the workhorse: light enough to carry across the yard and sensitive enough (15 mK) to see the 1–2 K difference between a healthy and a partially clogged radiator. Teams that inspect large EHV transformers, where radiators are high off the ground, prefer the Hammer II for its dual-FOV optics that zoom into a distant fan or oil gauge without losing the wide view of the whole tank.

Building a Transformer Cooling Inspection Program

Transformer thermography pays off when it is scheduled and trended:

• Scan all critical transformers twice a year at peak load — before summer and after the rainy season, when cooling deposits and corrosion are worst.
• Scan after any cooling system work (fan replacement, radiator cleaning, oil top-up) to verify the fix restored the expected thermal profile.
• Trend every bank and every fan. A radiator moving 2 K colder over two scans is worth more than a single alarming snapshot.
• Pair thermal with oil analysis. Hot radiators, high moisture, and rising dissolved gas all reinforce each other. Review them together.
• Add PD screening. Overheating accelerates insulation aging, and insulation aging produces partial discharge. An SC Series PD detector on the same visit catches the electrical side of the same problem.

In hot climates the margin is thin and the season is long. A radiator cleaning and fan repair that takes one day prevents a failure that would take a transformer out of service for months — and that is what a scheduled thermal program is really buying.