Grounding System Design Excellence: Leveraging SES RESAP for Precision Soil Resistivity Modeling

Introduction

In the realm of electrical engineering and safety, grounding systems serve as the invisible armor protecting people, equipment, and infrastructure. At Keentel Engineering, we emphasize not just proper grounding but scientifically verified, site-specific soil modeling for every substation, transmission line, or industrial facility. The backbone of this approach is RESAP, a specialized software module within the SES CDEGS suite, designed for soil resistivity interpretation and modeling.

This article dives deep into RESAP—its principles, methodologies, algorithms, and real-world applications—while showcasing how Keentel uses it to ensure electrical safety, NERC compliance, and project efficiency.

What is RESAP?

RESAP (Resistivity Analysis Program) is a CDEGS module developed by Safe Engineering Services & Technologies Ltd. (SES) that interprets soil resistivity measurement data to produce equivalent multilayer or exponential earth models.

RESAP is critical for:

• Substation grounding design
• Transmission line impedance calculations
• Cathodic protection studies
• Electromagnetic induction (EMI) analysis
  
  

It processes data from industry-standard measurement configurations like Wenner, Schlumberger, and generalized four-point methods.

Why Soil Modeling Matters in Grounding Design

Key Features of RESAP

1. Multiple Earth Structures Supported

• Horizontal multilayer models
• Vertical layer interpretations (up to two layers)
• Exponential resistivity variation with depth

2. Sophisticated Curve Fitting Algorithms

Algorithms used include:

• Steepest Descent
• Levenberg-Marquardt
• Fletcher-Powel
• G-Conjugate Gradients
• Simplex (for specific cases)
  
  

Each algorithm minimizes the difference between measured and modeled resistivity curves using least-square optimization.

3. Flexible Data Input Interfaces

• Windows Toolbox (SWIMS)
• Command-line Interface (SICL)
• ASCII .F05 input files
• Manual input editing

Data Input and Soil Measurement Methods

Measurement Techniques

Three primary configurations are supported:

• Wenner Method: Equally spaced electrodes
• Schlumberger Method: Widely spaced outer electrodes, shorter inner spacing
• General Method: Arbitrary spacings for difficult terrain

Data Requirements

Each test point requires:

• Probe spacing (a)
• Apparent resistance (V/I)
• Electrode depth (for both current and potential rods)

Dealing with Noise

In noisy environments (e.g., substations near live lines), Keentel uses:

• Variable frequency sources (e.g., 70 Hz)
• Selective voltmeters
• Broadband ammeters

This approach ensures accurate resistivity readings, even in interference-rich environments.

Algorithm Spotlight: Levenberg-Marquardt vs Steepest-Descent

Levenberg-Marquardt (LM)

• Fast convergence (up to 10x faster)
• Requires more measurement points
• Sensitive to initial conditions

Steepest-Descent

• More robust in complex/noisy conditions
• Supports fewer data points
• Slower but reliable for multi-layer models

At Keentel, we match algorithm choice to project conditions and client timelines.

RESAP Input File Structure and Execution

A RESAP input file typically contains the following modules:

1. OPTIONS – Units and run ID
2. MEASUREMENTS – Probe data and methodology
3. SOIL-TYPE – Horizontal or vertical layering, number of layers
4. OPTIMIZATION – Algorithm selection and iteration settings
5. COMPUTATIONS – Filters and step sizes for convergence
   
   

Each module is hierarchically structured and allows for automated or manual intervention to fine-tune model accuracy.

Sample Case Studies from RESAP

Two-Layer Soil Model

• RMS Error: ~15.6%
• Layers: 364.6 ohm-m (top), 63.7 ohm-m (bottom)
• Application: Simple substations with clear stratification

Three-Layer Soil Model

• RMS Error: ~5.85%
• Application: Sites with clay overlay on sandy substrate

Five-Layer Complex Model

• RMS Error: ~2.47%
• Layers include high-resistivity rock layer and moist loam
• Ideal for utility-scale solar plants or GIS substations

RESAP in Real-World Projects

Keentel Engineering has used RESAP for:

Utility Substations

• Soil profiling for grid reliability
• NERC PRC-004 and TPL-001 compliance

Renewable Energy Plants

• Utility-scale BESS, wind, and solar
• Line impedance calibration for inverter-based resources

Industrial Facilities

• EMC interference modeling
• Localized grounding for hazardous areas

Integration with CDEGS and SIRPS

RESAP is not a standalone tool. At Keentel, we integrate it with:

• SIRPS: For plotting and reporting
• MALT & MALZ: For grounding grid design
• HIFREQ: For frequency-domain studies
• TRALIN: For transmission line modeling

Custom Reports for Clients

Using RESAP data, Keentel provides:

• Computed vs measured resistivity curves
• Soil model tables with thickness, contrast, and coefficients
• Ground potential rise (GPR) estimation
• Touch/step voltage analysis

All reports are tailored to meet IEEE Std 80, IEEE 81, and utility-specific requirements.

Training and Consulting Services

Keentel offers:

• Hands-on RESAP training for utility engineers
• Soil testing supervision with certified equipment
• Consulting on noise mitigation and probe placement
• Model verification against historic GIS/substation builds

Conclusion

RESAP transforms raw field data into actionable engineering insight. At Keentel Engineering, our expertise in using RESAP extends beyond software — it’s about understanding your site, your utility’s safety standards, and your project’s compliance scope. Whether you’re modeling a complex substation or evaluating grounding for a new solar field, Keentel ensures your system is grounded in science.

20 Technical FAQs on RESAP and Soil Resistivity Modeling