Why do we spread gravel in the transformer yard?

 

There are many reasons why gravel is used to cover the topsoil in a transformer yard but before we delve into the reasons behind this practice, we will explore the important concepts of step and touch voltage:

1. Insulation: Gravel primarily acts as an insulator, reducing both step and touch voltage. This is vital to ensure safety in the transformer yard. To understand this better, let's explain the concepts of step and touch voltage:


   - Step Voltage: Step voltage refers to the voltage that develops between the two feet of a person standing on the ground when fault current flows through it. For instance, if an energized 11KV cable falls to the ground (topsoil), the fault current dissipates into the ground. As you walk towards the cable, there's a voltage differential between the points where your feet meet the topsoil. If this step voltage is sufficiently high, it can potentially push current sufficient enough to cause injury or death through your body. 

In essence, if an energized 11KV cable falls to the ground, the fault current would be dissipated into the ground consequently voltage at the point where the cable is in contact with the ground would not be the same as the voltage 10 meters away.


   - Touch Voltage: Touch voltage, on the other hand, is the voltage that develops between an earthed equipment body and the ground during a fault current event.


Gravel, with its high resistivity, restricts the flow of current on the upper surface of the ground. This, in turn, reduces the potential difference between two points, minimizing the risk of dangerous step and touch voltages.


2. Vegetation Control: Gravel's nutrient-deficient nature makes it an effective means of controlling vegetation. Unwanted plants like weeds and climbers can grow unpredictably into parts of the transformer, potentially causing equipment malfunctions or damage.


3. Stability: Gravel contributes to the stability of the underlying soil. By preventing puddle formation and rainwater accumulation, it maintains a consistent ground condition. Rainwater efficiently passes through the gravel into the soil, lowering the resistivity of the lower layers—a desirable characteristic for power system protection. Additionally, it prevents rapid evaporation of rainwater.


4. Fire Protection: In the event of an oil spillage from the transformer, gravel plays a crucial role. It allows the spilled oil to seep through, preventing it from remaining on the surface. This, in turn, prevents the upper area from catching fire and helps contain the spread of fires in the switchyard, aligning with the guidelines outlined in IEEE 980 Substation Oil Containment.

 


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