We often see large quantities of cobblestones placed beneath transformers. What exactly is their purpose? Is it merely for aesthetics?
[Image]
Common transformers fall into two categories: dry-type transformers and oil-immersed transformers.
Oil-immersed transformers use oil as the primary insulation medium and rely on it for cooling-utilizing methods such as oil-immersed self-cooling, forced-air cooling, water cooling, or forced oil circulation. Key components include the iron core, windings, oil tank, oil conservator, breather, explosion-proof vent (or pressure relief valve), radiator, insulating bushings, tap changer, gas relay, thermometer, and oil purifier.
Compared to dry-type transformers, oil-immersed transformers offer advantages such as lower costs and easier maintenance, while effectively handling high-capacity heat dissipation and high-voltage insulation requirements. However, because the cooling oil used is flammable, these transformers possess an inherent drawback: the risk of fire or explosion.
[Image]
This is where components like cobblestones come into play. The area beneath the transformer is typically referred to as an oil discharge pit or sump, which connects to an emergency oil collection pit or basin.
In the event of an accident-such as an oil spray or explosion-transformer oil drains into the discharge pit and flows toward the emergency collection basin.
These pits may or may not be fitted with a grating. If a grating is used, the cobblestones are placed on top of it; if not, they are placed directly inside the pit. The decision to use a grating depends on the transformer's type, capacity, and voltage rating, all of which are subject to specific regulations. Image
Placing cobblestones beneath oil-immersed transformers takes the following seven factors into account:
1. After prolonged use, transformer components may suffer from aging or leaks; a bed of cobblestones absorbs leaked oil, allowing it to flow smoothly into the emergency oil pit and thereby reducing the risk of accidents.
2. In the event of an accident, the cobblestones prevent the oil inside the transformer from spraying outward, helping to avert an explosion.
3. In the event of an explosion or fire, the cobblestones act as a barrier, preventing the fire from spreading to the ground and facilitating firefighting efforts.
4. They provide a slight cooling effect; if the transformer overheats, the cobblestones help dissipate heat.
5. Cobblestones are electrically insulating, making it safer and easier for maintenance and operations personnel to perform inspections.
6. Cobblestones provide shock absorption. They function similarly to the ballast stones on railway tracks, adding a layer of cushioning.
7. They inhibit weed growth.
Image
# Fire safety regulations explicitly require the following:
1. Outdoor oil-immersed transformers and other oil-immersed electrical equipment containing more than 1,000 kg of oil must be equipped with an oil storage pit and oil drainage facilities.
2. The volume of the oil storage pit must be determined based on either 100% or 20% of the equipment's total oil capacity. If the pit is sized for 20% of the oil capacity, the bottom must feature a drainage pipe to channel leaked oil into an emergency oil reservoir. The drainage pipe must have an inner diameter of at least 100 mm, allow for rapid oil discharge during an emergency, and be fitted with an iron grate or mesh screen at the inlet.
3. The oil storage pit must contain a grid with a clear spacing of no more than 40 mm; a layer of cobblestones at least 250 mm thick, with particle sizes ranging from 50 to 80 mm, must be laid over this grid.
When a centralized emergency oil pit is installed, its volume must be determined based on the total oil capacity of the largest single piece of oil-filled electrical equipment. If a fixed water-spray fire suppression system is installed, the volume of the centralized emergency oil pit must also account for the volume of the spray water, ensuring an adequate safety margin.
Why is water required in the emergency oil pit? So, what exactly is an accident oil pit?
Let me first briefly introduce the background.
[Image]
Currently, among the major electrical equipment in substations,
oil-immersed power transformers
are widely used.
In the event of a transformer accident,
a large amount of mineral oil
can spray out from the transformer
and land in the surrounding area within a short time.
Without specific protective measures,
this would not only pollute the substation and the surrounding environment,
but the sprayed oil could easily trigger a massive fire;
the large volume of leaked oil would undoubtedly escalate the severity of the accident.
Therefore,
whether considering environmental protection
or fire safety,
it is essential to safely drain this oil
into a dedicated facility,
isolating it from flammable materials in the vicinity,
storing it at a reduced temperature,
and keeping it for future separation, recovery,
processing, and reuse.
This is what a typical substation accident oil pit looks like:
[Image]
The "200 cubic meters" shown on the nameplate in the photo
refers to the total volume of the pit,
not its maximum oil storage capacity.
The inlet of the accident oil pit
connects to the main transformer's foundation oil pit-
the area beneath the transformer filled with cobblestones-
allowing oil from the main transformer
to flow into the accident oil pit via a drainage pipe.
At this point,
many of you might be wondering:
We've all seen the cobblestones beneath transformers,
but why are they there?
Could we use gold, silver, or jewels instead?
Well, let me explain.
First of all, we simply don't have that much money...
[Image]
But back to the point:
The cobblestones actually serve as an isolation layer;
in the event of a transformer fire,
they help suppress the flames.
Secondly,
the high-temperature transformer oil
is cooled as it passes through the cobblestones,
which also helps reduce the fire's intensity
and aids in extinguishing it.
So, what is the operating principle of an accident oil pit?
Let's take a look at a cross-sectional view.
[Image]
Simply put,
an accident oil pit functions as a communicating vessel. When no spill oil is present,
if the spill containment pit contains water,
the water levels in both compartments (A and B) are equal.
Oil and water have different densities,
are immiscible, and separate naturally.
Since oil is less dense than water,
it floats on the surface
(as anyone who cooks would know).
Once spill oil enters the containment pit,
it exerts pressure on the water surface in the main compartment (Compartment A),
forcing the water through the overflow opening into the other side (Compartment B);
as the volume of spill oil increases,
the water is displaced into the wastewater sump.
If you still don't quite grasp this,
don't worry-
there is no need to question your intelligence,
as the concept is indeed a bit complex to explain.
Let's use some diagrams
to describe the process
clearly and concisely.
[Image]
Initially, the spill containment pit holds water.
If the main transformer or high-voltage reactor catches fire,
the water spray system activates,
and a large volume of insulating oil and oil-water mixture
flows into Compartment A.
After settling and separating in Compartment A,
the oil floats at the top, while the water sinks to the bottom.
Driven by the pressure of the oil,
the fluid passes through the overflow opening into Compartment B,
and is then discharged through the outlet.
Eventually, it reaches a stable, balanced state:
[Image]
This allows the oil to be retained in Compartment A,
facilitating subsequent analysis and recovery.
If the pit is initially empty of water,
and a massive oil leak occurs first-
filling the pit with insulating oil-
followed by a fire in the transformer or reactor
that triggers the water spray system,
a large mixture of oil and water enters Compartment A.
[Image]
After settling and separating in Compartment A,
the water sinks to the bottom, and the oil floats at the top.
However, a small amount of oil in the upper part of Compartment B
(calculated at a maximum of approximately 1.7 m³ for the Dieling Station pit)
will eventually be discharged into the surrounding environment through the outlet.
[Image]
Once the oil in Compartment B has been discharged,
the system reaches that same stable, balanced state
(shown below),
meeting design requirements.
[Image]
Under normal conditions, the pit should be kept filled with water. According to the provisions of GB 50229-2006,
"Code for Design of Fire Protection for Fossil Fuel Power Plants and Substations":
When a central emergency oil storage pit
equipped with oil-water separation measures is provided,
its capacity should be determined based on 60% of the oil volume of a single oil tank.
This means the emergency oil pit
must be capable of holding
60% of the oil from one transformer.
Let's run the numbers:
For example:
At the DL Substation, the oil volume for a single phase of the #2 and #3 main transformers is 65 t,
while that for the #4 main transformer is 60.5 t,
and the oil volume for the high-voltage shunt reactor is even lower, at 13 t.
Therefore, the calculation is based on a single phase of the #2 or #3 main transformer.
The density of transformer oil
is found to be 0.895 kg/m³,
and based on the formula ρ=m/V, 65 t of oil corresponds to 72.6 m³.
60% of the oil tank's capacity is 43.56 m³.
The maximum oil storage volume of the DL Substation's emergency oil pit
was calculated to be
47.55 m³ (which is greater than 43.56 m³),
so it complies with the standard.
By now, I believe everyone understands
the structure and operating principle of the emergency oil pit~
