Substation Earthing – A Simplified and Complete Guide

A power substation requires the earthing system for three main purposes.

The neutral point earthing for each network which has to be earthed at the power station or substation. In other words, it is mainly the earthing of neutral point of power transformers.
Earthing of non-current carrying metalwork of different apparatus. For example, transformer tanks, power cable sheaths etc.
Earthing the other metalwork not associated with the power systems. For example boundary fences, metal gates etc.

The purpose of earthing is to keep any voltage on metal frameworks below dangerous levels during normal or fault conditions. In directly earthed neutral system, it is not always possible to keep metal parts exactly at true earth potential when fault current flows. Therefore, the main goal is to provide low-impedance path to the earth of all the parts that a person may touch at the same time. So, the large fault currents do not flow through the body of the person.

Designing Earthing System for a Substation

For designing the earthing system of a substation, we follow the guidelines given in IS 3043 and IEEE 80 (1986 or 2000 edition). First, we select the portion of land on which we will construct the switchyard. During the selection of the switchyard area, we also include the area for the forecasted future expansion.

Measuring Soil Resistivity

Now we use the Wenner four electrode method to measure the soil resistivity. Because soil resistivity is the main factor that governs the entire earth mat design. For this, we first select suitable locations for measurement. We may select multiple measurement locations to bring the entire area under measurement. We have a dedicated article on the Wenner four electrode method. You can check it if required.

Calculating Tolerable Touch and Step Potentials

Tolerable Touch Potential

Suppose, when a person touches a metal structure, a fault occurs. The fault current flows through the metal structure to the ground. This current creates a potential gradient in the structure and ground. As a result, a potential difference appears between his or her hand and feet. Because of this potential difference, current flows through the human body.

Tolerable touch voltage is the maximum voltage for which the current flowing through from hand to foot remains within a safe limit for a permissible duration. The expression of tolerable touch voltage $E_{touch}$ given by

$E_{\text{touch}}=\left(1000+1.5\rho_s\right)\times\frac{0.157}{\sqrt{t_s}}$

Here, $\rho_s$ is the surface layer resistivity (Ω·m). $t_s$ is the fault clearing time in second.

Tolerable Step Potential

When a person stands or walks on the ground and an electrical fault occurs, fault current flows through the ground. Because the ground has resistance, a voltage difference develops between the person’s two feet. This voltage difference causes current to flow through the body.

Tolerable step voltage is the maximum voltage limit at which a tolerable current can flow through his or her body for a permissible duration without causing harmful electric shock. The expression of tolerable step voltage $E_{step}$ given by

$E_{\text{step}}=\left(1000+6\rho_s\right)\times\frac{0.157}{\sqrt{t_s}}$

Designing the Grid

Now, we consider some basic data. These include soil resistivity, grid size (length and breadth), grid conductor cross-section, mesh size (grid spacing), and the depth of the buried earth mat. We also consider fault current, fault duration, and other relevant factors. In addition, we include factors such as the current diversion factor $(S_f)$ and the decrement factor $(D_f)$ .

Grid Current

First, we calculate the grid current. This shows how the system fault current distributes into the earthing grid during a fault.

$I_g = D_f \times S_f \times I_f$

Grid Resistance

Next, we calculate the grid resistance. For this, we use the standard formula,

$R_g = \frac{\rho}{4\sqrt{A}}$

Where A is the total area of the switchyard for which the grid is designed, and $R_g$ is the grid resistance. Here, $\rho$ is measured soil resistivity.

Grid Potential Rise (GPR)

Now we come to Grid Potential Rise (GPR). This is a very important parameter. Grid potential rise is simply the product of grid current and grid resistance.

$GPR = I_g \times R_g$

Actual Mesh Voltage

After that, we calculate the actual mesh voltage that appears during a fault for the designed grid (for example, with 10-meter spacing). The equation uses soil resistivity $(\rho)$ , mesh factor $(K_m)$ , irregularity factor $(K_t)$ , grid current $(I_g)$ , and the total conductor length $(L_T)$ .

$E_m = \rho \, K_m \, K_i \frac{I_g}{L_T}$

Here, $L_T$ represents the total length of the grid conductors.

Actual Step Voltage

The step voltage can also be calculated using its standard step-factor formula.

$E_s = \rho \, K_s \, K_i \frac{I_g}{L_T}$

Here, $K_s$ represents the step factor.

Compare with Tolerable Limits

Finally, we compare the actual step and touch voltages with the tolerable step and touch voltages.

$E_m < E_{\text{tolerable touch}}\\E_s < E_{\text{tolerable step}}$

If the actual mesh voltage is lower than the tolerable touch voltage, and the actual step voltage is lower than the tolerable step voltage, the design is safe.

Redesigning the Earthing Grid

Otherwise, we must improve the design. We can also reduce grid spacing by adding more grid conductors. We can also install more vertical electrodes (typically 3-meter rods) to lower the grid resistance. Also, we can increase the cross section of the grid rod. We can also use the tool for designing earth mat.

Connections with the Earth Grid

Transformer Neutral and Metallic Parts

We connect all power transformer neutrals to the main earth mat. Obviously, we connect them through earth conductors and earth pits. We also connect all metallic structures and metallic bodies to the main earth mat. We use risers for these connections. For that we use rods below the ground. Additionally, we use flats above the ground. We use rods underground because corrosion damage is acceptable. Similarly, we avoid flats underground because corrosion makes them thin.

Ground Surface Preparation

Lastly, we dress the ground surface. Then we provide a 75 mm thick PCC layer. We use jhama bricks in PCC. Because Jhama allows water to soak and keep the soil wet. We do not use crushed stone in PCC. After that, we lay a 100 mm thick crushed stone or gravel layer. The average size of crushed stone is about 40 mm. Crushed stones increase ground resistivity. Therefore, this improves tolerable step and touch potential and increases safety.

Earthing of Structures

We connect each structure to the earth mat using at least two risers. We connect one riser to the nearest grid rod along the X-axis. Then, we connect another riser to the nearest grid rod along the Y-axis. We join all connections by welding.

Ring Earth Mesh around Buildings

We keep the ring at least 1 meter away from the building. Obviously, we place it about 600 mm below ground level. Then, we connect this ring to the main earth mat at several points using earth rods.

Earthing of Equipment and Metallic Structures

We connect all station equipment to the main earth mat using at least two risers. We place these risers in diagonal opposite directions. For examples, we connect steel columns, metallic stairs, rail linings, and other metallic structures to the earth mat. For the railway tracks on the rail-cum-road, we connect the rails to the main earth mat at every 30 meters. We also connect lightning poles, junction boxes, BMK, CT, VT, CBT, cable boxes, and glands to the earth mat using risers.

Crossing Concrete Foundations

If an earth conductor crosses a concrete foundation, we pass it at least 300 mm below the foundation.

Earth Pits

We connect lightning arresters, reactors, transformer neutrals, CVT, and PT to the earth mat through earth pits. For these, we use treated earth pits for these connections. We use a treated earth pit for transformer neutrals. We connect every earth pit to the main earth grid.

Welding and Corrosion Protection

When we connect equipment with risers, we provide five welded joints. We do not allow nut-bolt joints in between. We treat all welding surfaces with barium chromate. Then we apply bitumen compound or coal tar to prevent corrosion.

Local Mesh below Isolator Handles

We install a local earth mesh below each isolator handle. We place it 300 mm below finished ground level (FGL). The mesh size is 1.5 m × 1.5 m. The mesh spacing is 500 mm × 500 mm. We connect this mesh to the main earth mat at two diagonal points. We also install similar meshes at the corners of the main earth grid.

Outer Periphery Earthing

We place two parallel earth conductors along the outer boundary of the main earth grid. These conductors reduce sudden voltage gradient changes. They improve safety.

Quick Easy Summary

Connect all equipment and metallic parts to the main earth mat.
Use rods underground and flats above ground.
Prepare the ground using PCC and crushed stone layers.
Use two risers for safe earthing connections.
Provide ring earthing around buildings.
Use proper welding and corrosion protection.
Install local meshes at isolators and grid corners.
Use parallel conductors at the outer boundary for better safety.