How do I balance economics and safety in grounding design?

Introduction

Challenging grounding designs are no rarity in the world of grounding analysis: poor soil conditions, small site dimensions, and lack of data are among the many contributors impeding a balance of cost and safety in the design process. These challenges are often hedged by overly-conservative assumptions, which can push a design to costly and seemingly unreasonable design recommendations.
Our client was presented with a design calling for multiple ground wells for a substation in a mountainous area. The implementation cost would be enormous, so SR3 was consulted for a second opinion.

Approach

The following multi-stage approach was utilized: 
  1. The existing model and design basis was evaluated to identify areas where design margin or other aspects of the model might be considered overly conservative. A summary of findings and recommendations was produced for client consideration.
  2. A new design would be developed that would incorporate the recommendations approved by the client.

Evaluation

SR3’s evaluation determined that no split factor was utilized in the original design (meaning all fault current was considered earth current) which resulted in a very large ground potential rise. SR3 gathered the necessary data to compute a split factor, including the effects of the autotransformer at the site, which resulted in a more realistic fault current distribution and orders of magnitude reduction in earth current. Additional scenarios were run to evaluate future system growth and the eventual removal of the autotransformer.

Results

The re-computed current distribution resulted in a ground grid design that did not require any ground wells. Furthermore, the results indicated adequate margin for future system growth and substation reconfiguration. The client estimated that the revised design saved over $1,000,000 in construction costs.

Figures

The results indicated that the new 115 kV line did have an impact on the pipeline, but it was marginal in comparison to the effects of the existing transmission lines; furthermore, the marginal effects did not cause any thresholds to be exceeded when all lines were in operation.
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