Importance of Earth Grid Design
An earthing grid is used as an electrical component, connected to the earth at zero potential. When a fault condition occurs, the high energy dissipates into the ground through a low-impedance path. The earth grid design system should involve mitigating the hazards arising from step, touch potentials, and bringing down the earth’s resistance.
These problematic issues can be easily eliminated by equipping an earthing grid that should be appropriately designed to minimize the harmful effect.
Problem Statement
To redesign an earth grid for a 765kV substation, as the cost of implementing the proposed earth grid design was exceeding the budget. As a renewable energy project, it created a unique challenge of grid stability and sub-synchronous resonance, at the point of coupling, compared to traditional energy transmission projects.
At the time of fault or lightning, the current interaction capacity (A/m) of Curec+ and Curecon with earth grid conductor is approximately 6 and 1.5 times higher than electrical grade bentonite respectively. This enables to achieve low earth grid resistance and maintains grid potential within permissible limits along with a significant reduction in the buried length of grid conductors. Over and above, these products provide permanency to earthing systems.
| Description | Client Design | Optimized Design |
| Grid spacing (m) | 20/30 | >40 |
| Total length of conductor Lc (m) | 14012 | 11560 |
| Number of earth rods 3m long | 260 | 149 |
The earth grid optimization leads to overall cost savings of 20% along with reduced trench length leading to saving of time and labor costs. This optimization ensured that the dangerous potentials were kept less than the tolerable values. The below table provides the cost comparison of the unoptimized design provided by the client and Manav Energy optimized design.
| Description | Client (In Lakhs) |
Optimized (In Lakhs) |
| 40-mm MS conductor cost | 69 | 57 |
| Earth rods cost | 4 | 2.2 |
| Trenching/Backfilling of conductor | 42 | 35 |
| Earth pit installation cost | 6 | 3 |
| Total cost | 121 | 97.2 |
| Savings in Cost | 23.8 lacs | |
| Saving on Optimized design | 20% | |
Benefits
- Reduced BOQ (conductor length)
- Reduced Project Cost (reduction in trench length, welding joints, labor cost, installation time).
- Achieve the least seasonal variation of grid resistance due to the use of organic earth-enhancing compounds Curec+® and Curecon® conforming to IEEE 80 Clause 14.5 (d) & 14.5 (b) respectively
