Grounding Analysis for Utility-Scale Solar Projects with WinIGS: An Integrated Engineering Approach

A Keentel Engineering Technical Publication

Why Solar Plant Grounding Is Not a Bigger Substation Grid and Why the Software Choice Matters

A 100 MW solar plant occupies 500 to 900 acres. A transmission substation occupies five. Yet many grounding studies for utility-scale PV facilities still apply the mental model of the fenced substation yard — design a copper grid, check touch and step voltages inside the fence, stamp it, done. That model fails on solar plants, and it fails in ways that trace directly back to the analysis tool used.

Most grounding software analyzes a grounding system in isolation: the user draws the grid, enters a soil model, assumes a grid current, and the program computes potentials. WinIGS — the Windows-based Integrated Grounding System design program — is built on a fundamentally different premise: the grounding system is modeled together with the power system it belongs to. The network model (sources, transmission lines, transformers, collector circuits) and the geometric grounding model (conductors, rods, piles, fences, foundations) live in one integrated study case, and fault currents, split factors, and ground potential rise emerge from physics rather than from an input field. At Keentel Engineering, this is why WinIGS anchors our grounding practice for utility-scale solar: on a plant this electrically extensive, the assumed-grid-current shortcut is where studies go wrong before the first potential is ever computed.

Three realities of solar plants demand this integrated treatment.

First, the geographic extent changes the physics. A PV plant is not one grounding system; it is thousands of driven steel piles, hundreds of inverter and transformer pads, dozens of miles of medium-voltage collector circuits, a collector substation grid, perimeter fencing, and often a switchyard at the point of interconnection — all electrically interconnected and all embedded in soil that can vary by an order of magnitude across the site. The performance of any one element depends on all the others, and WinIGS's grounding model — built in a full 3-D geometric editor from electrode objects including ground rods, mats, bare and coated horizontal conductors, concrete foundations, and metallic fences — is designed to represent exactly that interconnected reality.

Second, the fault current picture is different. The worst fault for ground potential rise (GPR) at a solar collector substation is very often not a fault at the substation itself but a line-to-ground fault miles away, where the division of fault current between metallic return paths and the earth — the split factor — is least favorable. WinIGS's Maximum GPR analysis automates precisely this: it sweeps fault locations and types across the entire modeled network and identifies the worst fault, reporting its location, type, current, X/R ratio, and the resulting GPR. A study that begins with "assume the grid current is X kA" has already made its most consequential decision by assumption; a WinIGS study makes it by search.



Third, the design objectives multiply: personnel safety per IEEE Std 80 (or IEC 479-1 — both criteria sets are built into WinIGS), a stable reference for protection and communications, lightning protection per IEEE Std 998 via the program's integrated Lightning Shielding Analysis tool, and control of transferred potentials onto fences, pipes, and neighboring infrastructure.

Keentel Engineering's position is that grounding for utility-scale solar must be treated as an integrated, plant-wide power system study — grounding as a first-order design input, not a civil detail closed out at 90% design. WinIGS is the platform on which we execute that philosophy.

The Three-Standards Framework in WinIGS: Design, Verify, Shield

Our methodology organizes the study around three IEEE standards, and each maps onto specific WinIGS capability.

IEEE Std 80 — design criteria, computed natively.

WinIGS computes permissible touch, step, and metal-to-metal voltages directly from the study inputs: electric shock duration (from fault clearing time — we routinely evaluate backup clearing as well as primary), the soil resistivity model, the insulating surface layer thickness and resistivity, and the X/R ratio at the worst-fault location, from which the program derives the decrement factor for DC offset. The surface-layer reduction factor — crushed rock or asphalt derating — is computed to the IEEE Std 80-2000 formulation (the program retains older-edition options only for legacy-study compatibility). Because these inputs flow from the same integrated model that found the worst fault, the criteria and the stresses are always self-consistent.

IEEE Std 81 — measurement in, verification out.

On the front end, WinIGS ingests field soil data in all the standard forms — Wenner four-pin, Schlumberger, and driven-rod — and automatically estimates a two-layer soil model with statistical rigor: parameter tolerances at a user-set confidence level (90% default), a validity depth telling you how deep the survey can actually see, and graphical bad-data screening where outlier measurements are marked and excluded, visibly tightening the model tolerances. On the back end, WinIGS supports the verification workflow: its Point-to-Point Impedance analysis (Model B or D) predicts what continuity testing of the as-built system should measure, and its Smart Ground Meter integration allows field measurement locations to be imported and interface points generated automatically — closing the loop between model and measurement that IEEE 81 exists to enforce.

IEEE Std 998 — shielding in the same model.



The WinIGS Lightning Shielding Analysis tool implements all three IEEE 998 methods — the full electrogeometric model, the rolling sphere method, and the shielding angle method — operating directly on the same 3-D geometric model used for grounding: buildings, buswork, outdoor equipment, shield wires, and masts are modeled once and analyzed for both fault safety and stroke interception.

Design to 80, verify to 81, shield to 998 — one model, one tool, no re-entry of data between siloed programs.

Step 1: Soil Characterization Across a Heterogeneous Site

Everything rests on the soil model, and on a multi-hundred-acre site the biggest error we encounter in third-party studies is a single soil model extrapolated across the whole plant.

Our practice is to specify Wenner traverses at a density matched to site geology — several distributed across the array area plus dedicated sets at the collector substation and switchyard — with probe spacings pushed as far as logistics allow, because WinIGS's reported validity depth makes the limitation explicit: a survey with spacings to 100 feet supports a model to roughly 150 feet of depth and no further, and for an electrode the size of a solar plant, deep-layer resistivity governs.

Each traverse's data — either measured resistance or apparent resistivity; WinIGS accepts both and cross-computes the other, along with probe length and meter frequency — is processed through the program's soil interpretation engine, which fits the two-layer model and superimposes the fitted curve over the measured points. That plot is where field data quality becomes visible: measurements contaminated by buried metal or instrument error sit conspicuously off the curve, are marked as bad data with the Mark/Unmark function, and the re-run fit reports dramatically tighter parameter tolerances. A defensible soil model is one whose uncertainty is quantified, and this workflow quantifies it.



On sites with genuinely distinct soil regions — river terrace against upland clay, landfill cell against native ground — we model multiple conductor groups and soil regions rather than averaging site geology into a fiction that exists nowhere.

Step 2: The Integrated System Model in WinIGS

A WinIGS study case has two tightly coupled halves, edited in the program's two specialized environments.

The Network Editor

holds the power system as a single-line diagram: equivalent sources, transmission lines with their physical construction (conductor types and positions, tower geometry from the program's tower library, span length, structure footing resistance, shield wires), transformers including autotransformers with tertiaries, collector circuits, cable systems built from the cable library, and the grounding system elements that tie it all to earth. Our standard scope for a utility-scale PV model: the collector substation grid in full detail; the interconnecting transmission line modeled physically for the first several miles; equivalent sources at interface buses beyond; the 34.5 kV collector system with actual cable construction; representative inverter/transformer pads; the pile and racking network; and the perimeter fence.

Two modeling disciplines deserve emphasis. Equivalents must be verified, not assumed. The external network is reduced to equivalent sources and inter-bus equivalent circuits at the interface buses, with parameters taken from the utility's short-circuit model (ASPEN, CAPE, or similar). The acceptance test is non-negotiable: the reduced WinIGS model must reproduce the utility's fault duties at the POI before any grounding conclusion is drawn. Exact match is not expected — WinIGS's physically based three-phase models capture asymmetries that classical short-circuit programs ignore — but the discrepancy must be quantified and small. Everything needs a path to remote earth, and everything conductive is part of the answer. WinIGS enforces the first as a modeling rule (a floating delta tertiary will stop the solver, for good reason); the second is engineering judgment the tool supports: reinforced-concrete foundations are modeled as steel mat electrodes on a dedicated Foundations layer — capturing their Ufer-effect contribution while WinIGS's layer system keeps them out of the auto-generated Bill of Materials, since they're structure, not purchased copper.

The Ground Editor



holds the physical grounding model in true 3-D — top, side, perspective, and rendered views — with the site's foundation drawing imported as a scaled background image (DXF import/export is supported for CAD interchange), so grounding conductors are placed against real equipment locations, not idealized rectangles. Analysis model fidelity is selectable: Model A for rapid equipotential-conductor screening through Models B, C, and D, which represent voltage drop along conductor lengths — with the enhanced Model C in current releases capturing ground-conductor self-inductance at near-Model-D accuracy for a fraction of the runtime, a meaningful advantage on plant-scale models with thousands of electrode segments.

Step 3: Worst-Fault Search and Split Factor the Maximum GPR Analysis

With the model built, WinIGS's Maximum GPR analysis runs a large automated series of fault simulations — every fault type, swept along every line and bus within a user-set radius (or the whole system) — and identifies the fault producing maximum GPR at the monitored node. The report is the design basis in one screen: worst fault type and location, fault current magnitude and phase, X/R at the fault, and the GPR.

Three phenomena make this search essential on solar plants. The split factor — reported directly in WinIGS's Ground Resistance report as the ratio of earth current to fault current — varies strongly with fault location, and the worst case is routinely a fault out on the line, not at the station. Transformer winding effects are captured physically: WinIGS's Internal I/O reporting shows the circulating current in a delta tertiary during ground faults — a current that can exceed 10 kA while the winding's terminal currents read zero, and that raises GPR for remote faults while lowering it for local ones. And on the collector side, the grounding-source configuration for the 34.5 kV system shapes touch voltage at every inverter pad in the plant.



Typical well-designed collector substations on effectively grounded transmission systems see split factors in the 20–40% range. An isolated-grid study assuming 100% of fault current enters the earth over-designs by a factor of three to five; one that guesses low gambles with compliance. WinIGS replaces the guess with a computation.

Step 4: Safety Assessment — Touch, Step, and Transferred Potentials

Against the worst-fault condition, WinIGS computes earth-surface potentials and evaluates touch and step voltages over user-defined assessment regions rectangular or polygonal plot frames drawn directly on the grounding model. Region definition is engineering judgment the tool makes explicit: touch-voltage frames cover everywhere a person can stand and reach grounded metal — substation interior plus a band extending three feet outside the fence, every inverter pad and enclosure row; step-voltage frames emphasize the areas just outside perimeter fences where gradients peak; and each frame is assigned its own permissible basis crushed-rock derating inside the station, native-soil criteria in the arrays and beyond the fence.

The results read as color-coded equipotential contour plots with the maximum location flagged and the permissible value printed alongside compliance or violation at a glance plus a rendered 3-D touch-voltage surface where violations rise as literal red peaks above the plant. Those peaks are a design instruction: they show precisely where conductor must be added, which is a far more economical process than uniformly densifying the grid. Two referencing details matter for correctness at plant scale, and WinIGS handles both: touch voltage referenced to the nearest grounding point (the recommended setting for Models B/C/D, where conductor voltage drop is represented) rather than a single remote reference; and step voltage computed across the standard 3-foot stride distance.

Transferred potential closes the assessment. WinIGS's user-defined contour feature draws the zone-of-influence boundary commonly the 300 V contour directly on the site plot, making visible which pipelines, fences, communication routes, and neighboring facilities fall inside it during the worst fault and therefore require isolation, gradient control, or dedicated mitigation.

Step 5: Design Enhancement — Where the Money Is

The study is complete when the as-built system is measured and reconciled. Our commissioning scope: fall-of-potential impedance measurement with leads long enough to escape the zone of influence WinIGS plotted; plant-wide continuity verification, with WinIGS's Point-to-Point Impedance report generating the predicted value for every test-point pair — the complete pairwise report exports to file for direct comparison against field data; and investigation of every divergence before energization. A measured point-to-point impedance that disagrees with the model is not noise — it is a missing bond, a broken conductor, or a soil surprise, found while it is still cheap to fix.



WinIGS is a proprietary software product of its respective developer and owner. Keentel Engineering is an independent consulting engineering firm and is not affiliated with, endorsed by, or sponsored by the developers of WinIGS or any other software vendor or standards organization referenced herein. IEEE Std 80, IEEE Std 81, and IEEE Std 998 are publications of the Institute of Electrical and Electronics Engineers. All product names and trademarks are the property of their respective owners.

Case Study

Case Study 1: 120 MW PV Plant — Three Bonding Strategies Settled by One WinIGS Model

Situation

A utility-scale PV facility in the southeastern U.S. — roughly 120 MW across approximately 700 acres, 34.5 kV collection, 115 kV interconnection — engaged Keentel to resolve a design dispute between the EPC and the owner's independent engineer over array bonding philosophy: (A) isolated array grounding, each inverter block grounded locally and deliberately not bonded to the collector substation; (B) fully integrated bonding tying racking, pads, and station into one continuous system; (C) a hybrid integrating the collector grounding while insulating fence sections remote from the substation.

Approach

Rather than argue philosophy, we built one WinIGS integrated study case and ran all three schemes as variants of it. The Network Editor model carried the 115 kV interconnection with shield wires and structure grounds, equivalent sources tuned until the reduced model reproduced the utility's fault duties at the POI, the full collector system with actual cable construction, and the BESS-ready station arrangement; the Ground Editor model carried the station grid, pads, pile network, and fencing over the imported site drawing. Fourteen Wenner traverses were processed through the soil interpretation engine — site resistivity ranged from about 90 Ω·m over 30 Ω·m in the bottomland to over 400 Ω·m in the upland third, so two soil regions were modeled with separate conductor groups rather than averaged. The Maximum GPR analysis was run for each variant against both transmission and collector systems.

Findings

The isolated scheme (A) — defended as preventing station GPR from "exporting" into the arrays — performed worst. The model showed the isolation to be largely fictional (cable neutrals and incidental paths defeat it), while depriving inverter pads of the gradient control the integrated network provides: during the worst collector-side fault, the touch-voltage plot frames at three pads showed exceedances over 40% above the native-soil permissible. The integrated scheme (B) cleared the pad violations and cut station impedance by roughly a third — the pile network's contribution, now visible in the Ground Resistance report — but the transmission-side worst fault transferred elevated potential onto remote fence runs, and the fence-line frames lit up nearly half a mile from the station. The hybrid (C) met criteria everywhere: integrated bonding delivered the pad safety and impedance benefit, insulated fence breaks with local gradient loops handled the remote transfer.

Outcome

Scheme C became the grounding design basis. The quantified pile contribution supported a reduction of the station grid from the EPC's preliminary layout, offsetting most of the fence-mitigation cost. At commissioning, point-to-point continuity results matched WinIGS predictions across the test-point set, and the fall-of-potential measurement of the completed system landed within 12% of the model's computed impedance.

Case Study 2: High-Resistivity Mountain Site — WinIGS Split-Factor Analysis Beats a Copper Crisis

Situation

A 150 MW solar project in mountainous western-U.S. terrain hit a grounding crisis at 60% design: shallow soil over rock, survey models showing 600–1,100 Ω·m upper layers, and an EPC study — an isolated-grid analysis with an assumed grid current — concluding that compliance required massive grid densification plus deep wells at a cost the project could not absorb.

Approach

Keentel rebuilt the problem in WinIGS as an integrated study. The 230 kV interconnection was modeled physically for the first several miles — both shield wires, tower geometry from the tower library, measured footing resistances — with the external network reduced to equivalent sources and inter-bus equivalents verified against the utility's short-circuit duties at the POI. The Maximum GPR analysis, searching all fault locations, placed the governing fault on the 230 kV line roughly a mile and a half from the plant — and the Ground Resistance report put the split factor near 25%: only about a quarter of the fault current actually entered the earth at the station. The EPC study's assumed grid current had been more than triple physical reality.

Findings

With the computed earth current, the crisis shrank but did not vanish: touch-voltage frames along one switchyard edge still showed exceedances around 20%, and the station GPR kept a fiber route and a pipeline crossing inside the 300 V zone-of-influence contour WinIGS plotted on the site plan. Two mitigations were engineered in the same model. Grid enhancement needed on the order of 1,300 additional feet of 4/0 in rock trench — and the re-run showed GPR essentially untouched, leaving the transferred-potential exposures alive. The counterpoise alternative used the Model Conversion tool to convert the first two spans to the mutually coupled multiphase line model, adding a buried counterpoise conductor bonded to structure grounds and the station grid through a corridor of deeper soil. Result: station GPR down approximately 25%, every touch frame under permissibles with better than 10% margin, the 300 V contour pulled inside the pipeline crossing — at under two-thirds the conductor length of the copper-only option, in trenchable soil.

Outcome

The counterpoise design was built. Total grounding cost came in below half the EPC's original mitigation estimate, and the study package — worst-fault basis, split-factor derivation, soil model with confidence tolerances, IEEE 81 verification plan — cleared utility review on first submittal. On hard-soil sites, the cheapest copper is the copper the split-factor computation proves you don't need.

Case Study 3: Solar-Plus-Storage Collector Station — the Worst Fault Wasn't Where Anyone Looked

Situation

A solar-plus-storage facility — approximately 200 MW PV with a 100 MW / 400 MWh BESS sharing the collector substation — engaged Keentel after the owner's engineer flagged that the preliminary grounding study had evaluated only station bus faults. The station was electrically busy: two 34.5 kV collector buses, a BESS medium-voltage system with its own grounding transformer, a main transformer with a buried delta tertiary, and a 138 kV interconnection.

Approach. The WinIGS model carried the full station, the 138 kV line with shield wires, and both feeder systems with their distinct grounding sources; the delta tertiary was modeled explicitly (with its required path to remote earth). The Maximum GPR analysis was configured with the search radius at zero — every circuit, transmission and medium-voltage alike — and the tertiary's behavior was examined via Internal I/O reports under both remote and local fault cases.

Findings

The governing fault was a line-to-ground fault on a 34.5 kV BESS feeder a short distance from the station — not the 138 kV bus fault the preliminary study assumed conservative. Two effects an isolated study cannot see drove it. The BESS grounding transformer fed ground-fault current returning through station earth for close-in feeder faults, over a cable system whose metallic return was weaker than the transmission side's shield wires — an unfavorable split, visible directly in the split-factor output. And the delta tertiary, inert at its terminals, circulated over 10 kA in the Internal I/O report during the feeder fault, adding to the earth current for that case while reducing GPR for the bus-fault case — the exact inversion of the preliminary study's assumption. Touch-voltage plot frames around the BESS enclosure rows, evaluated against the feeder fault with the decrement factor from that location's X/R, showed exceedances at eleven enclosure positions.

Outcome

Mitigation was surgical: gradient-control loops around the affected rows, a revised neutral-grounding-resistor specification for the BESS grounding transformer that trimmed fault contribution without degrading protection sensitivity, and one relocated feeder ground bond  roughly 900 feet of added conductor per the final Bill of Materials, versus the several-thousand-foot blanket densification a bus-fault-only reading would have driven. At commissioning, WinIGS's exported point-to-point impedance predictions exposed one unbonded BESS enclosure row construction had missed — the measured value disagreed with the model, and the model was right. The station has since cleared two real collector-system ground faults without incident.

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