A transformer "burns out" when its internal insulation fails and an arc forms between turns, between windings or to ground. It is almost never an isolated event: the insulation arrives weakened by years of heat, moisture or stress from external short circuits, and an overvoltage, a grid fault or an overload finishes the job. The causes are confirmed with oil gas analysis, electrical tests and protection records.
What “burned out” means
In a plant, a transformer is said to have “burned out” when it trips and never works again, when it smells burnt, or when the medium-voltage fuse blows again and again. Technically, it is almost always an insulation failure: the paper, oil or resin stops insulating. An arc then forms between turns of the same coil, between high-voltage and low-voltage windings, or between a winding and the core or tank.
The energy of the arc breaks the oil down into gases, carbonizes the paper and deforms the conductors. If the protection acts quickly, the damage stays localized; if it is slow, the tank can rupture or a fire can start. That is why part of the answer to “why did it burn out” always lies in the protection.
Causes, by family
| Family | What causes it | Typical early sign |
|---|---|---|
| Thermal aging | Years of overload, harmonics or poor cooling | High temperatures, dark, acidic oil with furans |
| Moisture and contamination | Saturated breather, damaged gaskets, untreated oil | High moisture in oil, low dielectric strength, high insulation power factor |
| Short-circuit stress | Faults on the plant network that the transformer fed | Changes in short-circuit impedance or frequency response |
| Overvoltages | Lightning, switching, ferroresonance | Surge arresters missing, damaged or far from the unit |
| Accessory failures | Bushings, tap changer, internal connections | Arcing or overheating gases, unequal winding resistance |
| Inadequate protection | Fuses or settings that do not clear in time | Late trips, oversized fuses, untested relay |
Thermal aging
Insulating paper ages with heat and does not recover. A transformer that ran hot for years, from overload or harmonics, has mechanically brittle insulation even if it looks fine electrically. We explain this in why a transformer overheats and in overloaded transformer. On online transformer monitoring: transformer monitoring with Qualitrol 509, Hydran and Calisto.
Moisture
Moisture lowers the dielectric strength of the oil and speeds up paper aging. It enters through gaskets, through a saturated silica gel breather or during poorly done maintenance. It is silent: the transformer keeps working until an overvoltage finds the weak point.
External short circuits
Every fault on the plant network drives a very high current through the transformer for as long as the protection takes to clear it. That current produces mechanical forces on the windings. A transformer can withstand the faults it was designed for, but repeated faults, or faults that are slow to clear, loosen and deform the coils, and the next event breaks them. IEEE C57.109 covers the permissible duration of through-fault currents.
Overvoltages
Lightning strikes on the medium-voltage line and switching of breakers and capacitor banks generate overvoltages that the insulation must withstand. A surge arrester only protects if it is well chosen, in good condition and close to the terminals. In medium-voltage networks with cables and unloaded transformers, the opening of a single phase (a blown fuse) can cause ferroresonance. See surge arrester and transient overvoltages in the plant.
Accessories and connections
Bushings with deteriorated insulation, carbonized tap changer contacts or a loose internal connection generate heat or localized discharges that end in failure.
Inadequate protection
An oversized medium-voltage fuse, an overcurrent element with a long time delay or a protection that was never tested lets a small fault grow. The difference between a repair and a lost transformer is often the milliseconds the protection takes. See transformer protection scheme.
Which parts fail in utility transformers
Utilities have kept failure records for decades, and CIGRE gathered them in the survey of Technical Brochure 642. Its conclusions, taken from the brochure’s official page, agree with the list of causes above on three points. Windings are the main cause of major outages. The dielectric failure mode is the most frequent. Bushing failures are the ones that cause the most damage to the surroundings. A study on bushing diagnostics that cites that brochure counts bushing failures in about 17 % of transformer failures, and associates them with explosion, tank rupture and fire.
This experience comes from utility, transmission and generation transformers; it is not a statistic for plant transformers.
What the dissolved gas standards say
IEC 60599 and IEEE C57.104 are the two references for interpreting gas analysis. IEC 60599 (edition 4, 2022) deals with how concentrations of dissolved or free gases are read to assess in-service equipment filled with mineral insulating oil and with solid paper or pressboard insulation, and to suggest the next steps. The standard itself presents its indications as guidance and asks that they be applied with engineering judgment. IEEE C57.104-2019 replaced the 2008 edition and covers gas generation, the purpose and application of the analysis, its quality verification and limitations, interpretation, the definition of fault types and case studies.
Both are interpretation guides and not pass-or-fail lists: the final decision is made by an engineer with the history of the equipment.
How to detect it before the failure
- Dissolved gas analysis (DGA). Each type of fault produces a characteristic mix of gases: overheating of oil or paper, partial discharge or arcing. IEEE C57.104 and IEC 60599 give the criteria. See dielectric oil analysis.
- Periodic electrical tests: power factor and capacitance of the insulation and of the bushings, winding resistance, turns ratio, excitation current and, when deformation is suspected, frequency response analysis (FRA).
- Thermography of bushings, connections and radiators.
- Protection review: check that the settings match the current transformer and that the relay, the CTs and the trip circuit have been tested.
What to do once it has burned out
- Do not re-energize “to see if it works”. A second closing onto an internal fault can turn a repair into a total loss or start a fire.
- Keep the evidence: the relay events and oscillography, the meter record and what was operating in the plant.
- Take an oil sample before moving or draining the unit.
- Run electrical tests to confirm the type and extent of the damage: often this decides whether it is repaired or replaced.
- Find the cause before installing the replacement, and review the protection with the new unit.
How we can help
Our team runs the tests that confirm the damage and the cause of your transformer failure, and preventive diagnostics for the units that remain in service, with a clear report per unit. Field work is done in Colombia, or elsewhere on request; the analysis of the protection events is done remotely, worldwide, through the relay trip analysis, which we quote for you. See power transformer testing and write to us with the nameplate data and what happened.
Related guides
- Why a transformer overheats
- Dielectric oil analysis
- Buchholz or sudden pressure trip
- Frequency response analysis (FRA)
- Transformer Monitoring: Qualitrol 509, Hydran, Calisto
References
- IEEE C57.104, Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers.
- IEC 60599, Mineral oil-filled electrical equipment in service – Guidance on the interpretation of dissolved and free gases analysis.
- IEEE C57.109, Guide for Liquid-Immersed Transformers Through-Fault-Current Duration.
- IEEE C57.152, Guide for Diagnostic Field Testing of Fluid-Filled Power Transformers, Regulators, and Reactors.
- IEC 60076-5, Power transformers – Ability to withstand short circuit.
Frequently asked questions
What is the most common cause of a transformer burning out?
Insulation failure, which is almost always the sum of slow deterioration (heat, moisture, paper aging, stress from earlier short circuits) and an event that finishes it, such as a lightning or switching overvoltage, a short circuit on the plant network or a prolonged overload.
Can it be detected before the transformer burns out?
In many cases, yes. Dissolved gas analysis of the oil detects incipient thermal and electrical faults, and power factor, winding resistance, turns ratio and frequency response tests show changes in the insulation and in the winding geometry before the failure.
What should I do if my plant's transformer burned out?
Do not energize it again, keep the protection and meter records, take an oil sample if it is oil-filled, and run electrical tests to confirm the damage and its cause. Without knowing the cause, the replacement transformer can fail for the same reason.