A transformer fires are one of the most expensive failures a utility or industrial plant can face. It is not just the cost of the transformer itself. It is the outage, the safety risk to personnel, the damage to nearby equipment, and in many cases, the regulatory scrutiny that follows.
Before we get into the causes, a quick clarification is necessary. CTR does not manufacture transformers. CTR manufactures the protection systems and accessories that keep transformers running safely, including explosion prevention and fire extinguishing systems, oil surge relays, breathers, bushings, and isolation valves. This blog looks at why transformer fires happen, and where the right protection system makes the difference between a contained fault and a catastrophic failure.
Why Transformer Fires Are So Destructive?
Most power and distribution transformers use oil filling. The oil acts as both an insulator and a coolant. This same oil is also combustible. When an internal fault occurs, whether from a short circuit, an arc, or a winding failure, the fault energy breaks down the oil and generates gases at very high speed. Pressure inside the tank builds up rapidly. If one does not relieve it in time, the tank ruptures, the hot gases meet the surrounding air, and the oil ignites almost instantly.
This is why transformer fires escalate so fast compared to other electrical fires. By the time a conventional fire detector or sprinkler system reacts, the explosion has often already happened.
Top 10 Reasons Transformer Fires Happen
1. Moisture Ingress into the Insulation
Moisture is one of the most common, and most preventable, causes of long-term insulation failure. Every time a transformer heats up and cools down, it breathes air in and out through its conservator. If technicians do not properly dry that air, moisture builds up in the oil and paper insulation over time, lowering its dielectric strength. Eventually, this weakened insulation cannot withstand normal electrical stress and breaks down.
2. Overloading and sustained overheating
Engineers design transformers to operate within a defined thermal limit. Running a unit beyond its rated capacity for extended periods, often to meet unplanned demand, accelerates insulation ageing far faster than normal operation. Once thermal degradation degrades insulation, it takes far less to trigger a fault.
3. Faults in the On Load Tap Changer
The tapchangers do not operate constantly; they operate frequently depending on application. However, high voltage constantly connects to them. As a result, engineers design tap changers to withstand long-term electrical stress while delivering reliable and precise voltage control.
4. Bushing Failure and External Flashover
Bushings carry current from outside the tank into the winding, and they sit at one of the highest stress points on the entire transformer. Contamination, cracking, or partial discharge inside a bushing can lead to flashover, which in turn can ignite the oil at the point where the bushing meets the tank.
5. Loose or Deteriorated Electrical Connections
Over years of thermal cycling and vibration, connections inside and around a transformer can loosen. A loose connection creates a high resistance point, which generates localised heating. Left unnoticed, this heating point can eventually become hot enough to start a fire, particularly at bushing terminations and cable box connections.
6. Oil Contamination and Reduced Dielectric Strength
Transformer oil degrades naturally over time, but the process accelerates with moisture, oxidation, and particulate contamination. Degraded oil loses much of its ability to insulate and cool effectively, which raises the risk of internal flashover under normal load conditions.
7. Lightning Strikes and Switching Surges
Lightning strikes and switching operations elsewhere in the network expose transformers connected to overhead lines to transient over voltages. These surges can puncture insulation that would otherwise have held up for years, triggering a sudden internal fault with very little warning.
8. Short Circuits and Winding Faults
Winding faults, whether from insulation breakdown, manufacturing defects, or external short circuits reflected back into the transformer, generate intense localised heat and arcing. This is one of the most direct causes of the explosive gas generation that leads to tank rupture and fire.
9. Ageing Transformers without Adequate Protection Upgrades
A large share of the transformer fleet across Indian utilities and industries has been in service for decades. As transformers age, their insulation margin shrinks, and internal faults can escalate into fires more easily, especially if operators have not upgraded the transformer’s protection systems accordingly.
10. Delayed Fault Detection and Slow Fire Response
This last cause is less about why a fault starts and more about why it turns into a full-scale fire. Conventional Buchholz relays and pressure relief devices detect a fault, but the time between detection and actual fire suppression matters enormously. If there is no fast-acting suppression system in place, even a well-detected fault can still result in an explosion and fire within seconds.
Why Fast Response Matters More Than Prevention Alone?
No utility can eliminate every one of the causes above. Ageing infrastructure, lightning exposure, and load growth are realities that every transformer fleet has to live with. This is why fire protection strategy has to include fast response, not just prevention.
CTR’s Transformer Explosion Prevention and Fire Extinguishing System is built around this principle. The system immediately responds to a serious internal fault by draining oil from the top of the tank and injecting nitrogen gas into the newly created space, cutting off the oxygen a fire needs to sustain itself. The system extinguishes external fires at the bushing or radiator within 30 seconds. The system also includes a transformer Conservator isolation valve, which blocks the passage of oil from the conservator, preventing the fault from escalating further. It can be retrofitted on to existing units with minimal outage. It can be tested on an energised transformer. One can also operate it manually if the power supply fails, which matters during an actual fault condition.
This kind of system does not prevent the underlying causes listed above. What it does is buy the critical seconds needed to stop a fault from turning into a fire that destroys the transformer, damages neighbouring equipment, and puts personnel at risk.
How CTR's Protection Systems Address These Risks
Each of the causes above has a corresponding protection point, and CTR builds accessories for most of them. Moisture ingress is addressed through maintenance free breather systems, which reduce the manual upkeep that conventional silica gel breathers demand and keep insulation dry over the long term. The Oil Surge Relay picks up faults inside the on load tap changer, detects abnormal oil flow toward the conservator, and disconnects the tap changer and transformer before the fault spreads. Properly rated epoxy bushings built for the transformer’s actual load and voltage class reduce bushing-related risks. And where a fault does escalate despite these measures, the explosion prevention and fire extinguishing system is the last line of defence, cutting off oxygen before the oil can ignite.
CTR builds its full range of transformer accessories around this layered approach, catching different failure points at different stages, rather than relying on a single device to cover everything.
What Utilities Should Look for in a Transformer Fire Protection System?
If you are evaluating a fire protection system for your transformer fleet, a few questions are worth asking before you finalise a vendor. Can we test the system on an energised transformer without taking the unit out of service? Can it retrofitted onto existing transformers, not just new installations? Does it have a manual backup option in case the power supply to the control cubicle fails during a fault. And does the vendor have a track record of approvals from utilities, railways, and transmission companies for similar installations.
Utilities, railways, and transmission companies across India approve CTR’s system, and they have tested and installed it on transformers and reactors up to 765kV, covering power, furnace, rectifier, and generator transformers.
Final Word
Single factor rarely causes transformer fires. A fault, whether from moisture, ageing insulation, a loose connection, or an OLTC issue, that escalates without a fast enough response usually causes them. For utilities, railways, and industrial plants running critical transformers, the practical question is not whether a fault will ever occur. It is whether the protection system around that transformer can catch the fault before it turns into an explosion.
Talk to CTR’s engineering team to assess your transformer fleet’s current fire protection and find out where the gaps are.
FAQ
Insulation failure from moisture ingress and long-term oil degradation is among the most common underlying causes, though the fault that actually triggers the fire is often a short circuit, winding failure, or OLTC arcing that the weakened insulation could no longer withstand.
Not entirely. Ageing infrastructure, lightning exposure, and load fluctuations are ongoing risks. The more realistic goal is reducing the likelihood of a fault through good maintenance, and pairing that with a fast-acting suppression system that stops a fault from becoming a fire.
Very fast. Once internal arcing generates enough gas pressure to rupture the tank, the resulting oil fire can escalate within seconds, which is why response time matters as much as detection.
No. CTR manufactures transformer accessories and protection systems, including explosion prevention and fire extinguishing systems, oil surge relays, bushings, breathers, and isolation valves, used on transformers manufactured by OEMs across utilities, railways, and industrial plants.
CTR’s explosion prevention and fire extinguishing system can be retrofitted onto existing transformers and reactors with minimal outage, in addition to being fitted on new installations.