A dry-type transformer has no oil to sample, so its condition is tracked through temperature, cleanliness, insulation and partial discharge. Its life depends on keeping the winding below the limit of its thermal class (F, 155 °C; H, 180 °C per IEC 60085), which requires free ventilation, working fans and probes, and loading within design. Maintenance combines inspection and cleaning with electrical tests trended over time.
What changes compared with an oil-filled transformer
In an oil-filled transformer, oil analysis tells you what is happening inside. A dry-type transformer offers no such window: its windings are cooled by air and, in cast-resin units, encapsulated in epoxy resin. Maintenance therefore relies on three signals: temperature, the condition of the surfaces (dust, moisture, discharge tracks) and electrical tests. The general differences between the two types are in dry-type vs. oil-filled transformers, and oil-filled maintenance is covered in transformer maintenance.
Temperature: the number that rules
Insulation life depends on the temperature of the hottest spot in the winding. The nameplate shows the thermal class of the insulation system under IEC 60085, normally F (155 °C) or H (180 °C). That class limits the temperature the windings can withstand; the loading guide for dry-type transformers, IEC 60076-12, lets you estimate how much life is used up by running above rating.
Do not confuse it with other nameplate classes: IEC 60076-11 also classifies dry-type transformers by environment (E), climate (C) and fire behavior (F0, F1). A transformer can be both «class F» and «F1», but the two mean different things.
How it is monitored
Medium-voltage dry-type transformers usually come with temperature probes (for example, PT100 RTDs or thermistors) in the low-voltage windings, wired to a temperature relay or controller with three thresholds: fan start, alarm and trip. The manufacturer sets the values for each unit. See transformer temperature indicators.
Why it overheats
| Cause | Typical sign |
|---|---|
| Overload | Temperature follows the load and exceeds the usual level |
| Harmonics from drives and rectifiers | Higher temperature than the measured current would suggest |
| Insufficient room ventilation | High ambient temperature in the room; the whole transformer is hot |
| Cooling ducts clogged with dust | One zone or coil hotter than the others |
| Failed or badly controlled fans | Temperature climbs fast when cooling should have started |
| Load unbalance | One phase hotter |
| Internal fault | Localized heating that the load does not explain |
The causes are developed in why a transformer overheats. A thermographic scan under load shows whether the heat is general or localized, and helps separate a ventilation problem from an internal one.
Routine maintenance
- Cleaning of coils, insulators and cooling ducts with the transformer de-energized and grounded, using the methods the manufacturer specifies. Dust, especially if conductive or combined with moisture, promotes surface discharge and reduces cooling.
- Medium- and low-voltage connections: condition, signs of heating and torque per the manufacturer; thermal cycling loosens them.
- Temperature system: check that the probes read correctly, that the relay starts the fans, raises the alarm and trips at its thresholds, and that the trip actually reaches the breaker.
- Fans: operation, noise, bearings.
- Surfaces: cracks in the resin, surface-discharge tracks (tracking) on coils and supports, discoloration.
- No-load tap changer: links in the correct tap and properly tightened.
- Room: temperature, ventilation, water ingress, no stored objects.
Moisture and time out of service
A dry-type transformer that has been de-energized for a while in a humid environment, or that has suffered condensation or water ingress, may have a damp insulation surface. Before re-energizing it, measure the insulation and follow the manufacturer’s instructions for drying if needed. Energizing damp equipment is a known cause of failures from surface discharge.
Electrical tests
| Test | What it detects |
|---|---|
| Insulation resistance | Moisture, contamination and insulation deterioration |
| Turns ratio | Shorted turns, wrong tap |
| Winding resistance | Loose connections, tap-changer contacts, open turns |
| Partial discharge | Defects in the resin insulation (voids, cracks) |
| Probe and temperature relay check | That the thermal protection works |
Partial discharge testing is especially useful on cast-resin units: IEC 60076-11 includes it among the type and routine tests of dry-type transformers, and in service it can reveal cracks or voids before breakdown. See partial discharge testing. Results are compared with the factory values and with earlier tests: the trend matters more than a single value.
Common mistakes
- Raising the temperature thresholds «so it stops tripping». If the transformer alarms, the problem is the load, the ventilation or the equipment itself, not the relay.
- Fans left disconnected because of noise or failure and never repaired.
- Using the room as storage, blocking air circulation.
- Cleaning with the wrong method, for example with products that leave conductive residue or with humid air.
- Not testing the temperature trip, which in many dry-type transformers is the only protection against sustained overload.
How often
There is no universal interval. A dry-type unit in a clean, dry, well-ventilated room can go long between cleanings; one in a plant with dust, fibers or a humid atmosphere needs frequent cleaning. The substation maintenance plan should set the interval for each piece of equipment according to the type of substation and its environment, and follow the local code and the utility’s requirements. See how often to maintain a substation.
How we help
At Tiza Group our team performs diagnostic and commissioning tests on dry-type and oil-filled transformers in industrial plants and medium-, high- and extra-high-voltage substations, in Colombia, or elsewhere on request. The tests cover insulation, ratio, windings, partial discharge and verification of the temperature system and its trip. We deliver the results compared with the nameplate and with earlier tests.
Learn about our transformer testing and contact us with the rating and make of your dry-type transformer.
Related guides
- Dry-type or oil-filled transformer: which to choose
- Trihal, GEAFOL, RESIBLOC and WEG dry-type transformers
- Transformer maintenance: dry-type and oil-filled
- Transformer temperature indicators
- Partial discharge testing
- MV/LV distribution transformer
- Faults in a plant transformer
References
- IEC 60076-11, Power transformers – Part 11: Dry-type transformers.
- IEC 60076-12:2008, Power transformers – Part 12: Loading guide for dry-type power transformers.
- IEC 60085, Electrical insulation – Thermal evaluation and designation.
Frequently asked questions
What temperature is normal for a dry-type transformer?
It depends on the thermal class, the design, the load and the ambient temperature; the manufacturer sets the alarm and trip values for each unit. As a reference, the insulation thermal class (IEC 60085) limits the hot-spot temperature: 155 °C for class F and 180 °C for class H. A temperature that rises while the load does not points to a ventilation problem, a fan problem or an internal fault.
What does class F or class H mean on a dry-type transformer?
It is the thermal class of the insulation system under IEC 60085: F corresponds to 155 °C and H to 180 °C. It should not be confused with class F1 of IEC 60076-11, which describes fire behavior. Both appear on the nameplate.
What tests are performed on a dry-type transformer?
Insulation resistance, turns ratio, winding resistance and, for cast-resin units, partial discharge when the scope justifies it. The temperature probes, the temperature relay with its alarm, trip and fan thresholds, and fan operation are also verified.