Why Are Gravel Stones Laid Under Oil-Immersed Transformers?

Oil-immersed transformers are widely used in substations for their cost-effectiveness, superior heat dissipation, and insulation properties. However, they carry a significant risk: the cooling oil is highly flammable. To mitigate fire and environmental hazards, a thick layer of gravel is strategically placed beneath these units.

According to electrical safety codes, the specifications are strict: stones must have a diameter of 50–80 mm​ and a thickness of at least 250 mm. Far from being decorative, this layer serves seven critical functions:

1.Containment: It absorbs leaked oil and directs it into the drainage system.

2.Fire Safety: It prevents oil spray during explosions and acts as a thermal barrier to stop fire spread.

3.Cooling: It reduces the temperature of overheating equipment or hot oil.

4.Safety: It provides insulation for maintenance personnel and cushions the transformer's weight, much like railway ballast.

5.Maintenance: It suppresses weed growth and keeps the pit clean.

Beneath the Surface: The Accident Oil Pool

Underneath the gravel lies the oil discharge pit, which connects to the accident oil pool. This underground reservoir is sized to hold the oil volume of the largest transformer in the station. If a water spray fire suppression system is present, extra capacity is added to handle the runoff.

The Science: Automatic Oil-Water Separation

The true genius of the system lies in how the accident oil pool handles spills. It operates as a communicating vessel utilizing the basic physics of density differences. Transformer oil (density ~890 kg/m³) is lighter than water (1000 kg/m³), meaning it naturally floats.

Normal Conditions:

The pool is kept partially filled with water, with equal levels in both chambers (A and B).

During an Incident:

1Intake: Leaked oil or a water-oil mixture from fire sprinklers enters Chamber A.

2.Separation: Due to density differences, the oil floats to the top while water remains at the bottom.

3.Displacement: As oil accumulates, it exerts hydrostatic pressure on the water below. This forces the water at the bottom to move through an opening in the central partition into Chamber B.

4.Discharge: The displaced water exits the pool through an outlet pipe into the sewage system.

5.Retention: The lighter oil is trapped in Chamber A, isolated from the environment.

This passive system ensures that even during a major fire, the bulk of the pollutant is contained on-site for later recycling, while clean water is safely drained away. Without this water-presence protocol, oil could escape into the environment before separation occurs.

Conclusion

From the gravel bed to the underground reservoir, every element is designed for safety and environmental compliance. The next time you see a transformer pad, you’ll recognize it not just as a pile of rocks, but as a sophisticated, life-saving engineering solution.