Problem Statement

MEPL had to redesign an earth grid for 765/400/220 kV substation that belonged to 765/400 kV intra-state transmission project, 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.

Solution

The grid was re-designed and optimized using CDEG software andby enveloping the ground conductors with our trademarked ground enhancement materials Curec+® and Curecon® which perform as per IEEE 80 Clause 14.5 d & 14.5 b respectively.

At the time of fault or lightning, the current interaction capacity (A/m2) 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 significant reduction in buried length of grid conductors. Over and above, these products provide permanency to earthing system.

Design Details

Soil resistivity -40 Ohm-m, Fault current -50 kA , Area of the Grid -685 x 500 m , Initial spacing of the grid – 30 m

Design Result

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 lead to overall cost savings of 20% along with reduced trench length leading to saving of time and labor cost. This optimization ensured that the dangerous potentials were kept less than the tolerable values. Below table provides the cost comparison of unoptimized design provided by client and MEPL 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%

Design Benefits

  1. Reduced BOQ (conductor length)
  2. Reduced Project Cost (reduction in trench length, welding joints, labor cost, installation time).
  3. Achieve least seasonal variation of grid resistance due to use of organic earth enhancing compounds Curec+® and Curecon® conforming to IEEE 80 Clause 14.5 (d) & 14.5 (b) respectively.
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