What happens when a power transformer fails?

When a power transformer fails, the situation can be very serious, with consequences ranging from damage to the equipment itself to the paralysis of the entire power grid, and even safety incidents such as fire or explosion. Exactly what happens depends on the type of fault, its severity, the design of the transformer, and how quickly the protective devices can operate.

Here are some possible scenarios:

Abnormal phenomena (observable signs):

Overheating: A large amount of heat is generated locally at the fault point, causing the oil temperature or winding temperature to rise sharply. The thermometer or thermal imager will alarm.

Abnormal sound: Strong "buzzing", "crackling", "bursting" or even "roaring" sounds are heard inside. This is caused by strong electromagnetic vibrations caused by arc discharge, insulation material rupture, loose core or severe overcurrent.

Abnormal oil level change: Gas generated by internal faults or large amounts of gas generated by high-temperature decomposition of insulating oil by arcs may cause abnormal oil level increase (increased pressure) or decrease (leakage).

Oil spray or oil leakage: A sharp increase in internal pressure may cause the pressure relief valve to spray oil, or oil tanks, pipes, radiators and other parts may rupture and leak oil due to overheating, pressure or mechanical stress.

Smoke and fire: High temperature and arcs may ignite insulating oil or solid insulating materials, causing the transformer to smoke or even catch fire.

Gas generation: Insulating oil decomposes under high temperature and arcing to produce gases such as hydrogen, methane, ethane, ethylene, acetylene, carbon monoxide, carbon dioxide, etc. (Dissolved gas analysis/DGA is an important fault diagnosis method). Large amounts of gas accumulation may cause a sudden increase in pressure.

Shell deformation or rupture: In extreme cases, huge internal pressure or arc energy may cause the transformer tank to swell, deform or even burst.

Internal damage:

Winding failure:

Turn-to-turn short circuit: The insulation between adjacent turns in the same winding is damaged, forming a short-circuit loop and causing local overheating.

Interlayer short circuit: The insulation between winding layers is damaged.

Phase-to-phase short circuit: The insulation between different phase windings is broken.

Winding short circuit to ground: The insulation between the winding and the core or tank (ground) is broken.

Winding open circuit: The wire is broken or the connection point is unsoldered.

Winding deformation/displacement: The huge short-circuit electromotive force causes the winding to mechanically deform, loosen or even collapse.

Core failure:

Core multi-point grounding: The core should be designed to have only one reliable grounding point. If there is an additional grounding point, a circulating current will be formed, causing local overheating or even melting of the core.

Short circuit between core pieces: Damage to the insulating paint leads to short circuit between pieces, resulting in increased eddy current loss and overheating.

Insulation system failure:

Aging, moisture, and breakdown of solid insulation (cardboard, stays, etc.).

Aging, moisture, contamination, carbonization, and decreased breakdown strength of insulating oil.

Tap switch failure: Poor contact, contact erosion, insulation breakdown, mechanical jamming, or drive mechanism failure.

Bushing failure: Flashover, dirty discharge, internal moisture or cracking leading to breakdown, or seal failure and oil leakage.

Cooling system failure: Radiator blockage, fan/oil pump stoppage, cooling pipeline leakage, resulting in poor heat dissipation, temperature increase, accelerated insulation aging or failure.

Impact on electrical system:

Relay protection action: Transformers are equipped with multiple protections (differential protection, gas protection, overcurrent protection, pressure release protection, temperature protection, etc.). When a fault occurs, the relevant protection devices will quickly detect the abnormality (current imbalance, gas generation, pressure increase, excessive temperature) and act:

Trip: Disconnect the circuit breaker connected to the transformer and isolate the faulty transformer from the power grid. This is the most critical link, aimed at preventing the accident from expanding.

Alarm: Send out sound and light signals or remote alarm information.

Voltage fluctuation or drop: The fault itself or the protection tripping will cause the bus voltage connected to the transformer to drop or fluctuate instantly, affecting the power supply quality of downstream users.

Power supply interruption: If the faulty transformer is a key node in the power supply chain, its tripping will cause a large-scale power outage in the area it supplies power.

System stability issues: The tripping of a large main transformer fault may disrupt the power balance and stability of the power grid, and in severe cases may cause a larger-scale power outage or even system collapse (cascading failure).

Short-circuit current shock: A short-circuit fault inside the transformer will generate a huge short-circuit current, which will not only cause devastating damage to the transformer itself, but also cause huge electromotive force and thermal stress shock to the busbars, switchgear, lines, etc. connected to it.

Safety risks:

Fire and explosion: The sprayed high-temperature flammable insulating oil is very likely to cause a fire when it encounters air or electric arc. In a confined space, the oil-gas mixture may explode. This is the most dangerous situation.

Toxic substance release: Burning insulating oil and insulating materials will release toxic smoke and gas.

Equipment damage splash: Explosion or oil tank rupture may cause high-temperature oil, debris, and parts to splash, causing harm to personnel and nearby equipment.

Environmental pollution: Large amounts of insulating oil leakage will pollute soil and water sources.