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Substation Grounding TutorialJoe Gravelle, P.E.Eduardo Ramirez-Bettoni, P.E.Minnesota Power Systems ConferenceThursday, Nov. 9, 2017

Substation Grounding TutorialPresenter – Joe Gravelle, P.E.Joe Gravelle earned BSEEE from NDSU in 1988. Aftergraduation Joe worked in the mining industry innortheastern Minnesota for ten years. Joe is a principalengineer in the Substation Engineering department atXcel Energy and has held many roles in the departmentsince 1998.In addition to substation projects, Joe is active instandards work both at Xcel Energy and in IEEESubstations Committee. Joe is currently the vice chairof the Substations Committee and is active inleadership roles and as an active member in severalIEEE working groups.

Substation Grounding TutorialPresenter – Eduardo Ramirez-Bettoni, P.E.Eduardo obtained a BSEE with Power Systememphasis from University of Costa Rica in 2002.Eduardo has international experience in substationconstruction, O&M, and design in substations up to 500kV. He has practiced engineering in the U.S., Canadaand Costa Rica. Eduardo has a background inprotection & control, substation physical design andpower system grounding.Eduardo works as principal engineer for Xcel Energy inthe Transmission & Substation Standards (TSS)department. TSS creates standard documents fordesign, specification and installation practices fortransmission & substation facilities. Eduardo is an activemember in IEEE Substation working groups.

Xcel Energy No. 1 utility windenergy provider inthe U.S. 3.4 million electriccustomers 2 million natural gascustomers 12,000 employees

Substation Grounding TutorialAgenda1.2.3.4.5.Grounding BasicsSoil Resistivity Testing and Soil ModellingDesign & Modelling of Substation GridBreakGrounding Design Variables––––––Soil model variablesSeasonal modelling of soilCrushed rock resistivityFault current design marginFault clearing timeSummary / Combined effect

Substation Grounding TutorialGrounding Basics Definitions per IEEE 80 & 81 GPR, mesh / step / touch Current split

Substation Grounding TutorialGrounding Basics Touch potentials: 3’ hand-to-foot Step potentials: 3’ foot-to-foot Safe if Calculated Values Tolerable Max Values

Substation Grounding TutorialGrounding Basics

Substation Grounding TutorialGrounding Basics

Substation Grounding TutorialGrounding Basics - Example Future fault current: 7000 A Earth current: 4200 A, (split factor Sf 60%) Grid resistance: 1.07 Ω GPR nom 1.07Ω x 4200 A 4500 V Mesh voltage 210 V Body current IB (500 ms, 50 kg) 0.116/SQRT(0.5) 164mA

Substation Grounding TutorialGrounding Basics - Example

Substation Grounding TutorialGrounding Basics Body current (IB) is basedon let-go current perexperimentation(99.5 % percentile, noventricular fibrillation) Body resistance (RB) variesbased on exposure voltage RB 500-1000 ΩSource: IEEE 80-2015

Substation Grounding TutorialGrounding Basics - Example Future fault current: 7000 A Earth current: 4200 A, (split factor Sf 60%) Grid resistance: 1.07 Ω GPR nom 1.07 Ω x 4200 A 4500 V Mesh voltage 210 V Body current IB (500 ms, 50 kg) 0.116 / SQRT(0.5) 164 mA

Substation Grounding TutorialExample - Body Voltage Tolerable potential across 50 kg body, body onlyPercentileIB (A)RB (Ω)VB (V)Body Configuration95%0.1641000164Hand-hand; Foot-foot95%0.164750123Hand-both feet95%0.16450082Both hands-both feet95%0.164700115Hand-trunk VB IB x RB (for TPG calculations)

Substation Grounding TutorialExample-Touch potential (no rock) Tolerable potential across 50 kg body, step voltage, no surface rock Homogeneous soil resistivity: 100 Ω-m Accounts for foot resistance to surface, Vt, 50kg IB x (RB 1.5ρ)PercentileIB (A)RB (Ω)Vtouch (V)Body Configuration95%0.1641000201Hand-hand; Foot-foot95%0.164750148Hand-both feet95%0.164500107Both hands-both feet95%0.164700140Hand-trunkSource: IEEE 80-2015

Substation Grounding TutorialExample-Touch potential (with rock)Source: IEEE 80-2015

Substation Grounding TutorialExample-Touch potential (with rock) Tolerable potential across 50 kg body, step voltage, 4” of 2000 Ω-msurface rock; homogeneous soil resistivity: 100 Ω-mPercentileIB (A)RB (Ω)Vtouch (V)Body 3Hand-both feet95%0.164500432Both hands-both feet95%0.164700464Hand-trunk Vt,50kg IB x (RB 1.5xCsxρ); (Cs 0.71)

Substation Grounding TutorialGrounding Basics

Substation Grounding TutorialGrounding BasicsSource: IEEE 80-2015

Substation Grounding TutorialGrounding Basics

Substation Grounding TutorialSoil Analysis

Substation Grounding TutorialSoil Resistivity Modelling Soil BasicsHomogeneousMulti - LayeredActual soil

Substation Grounding TutorialSoil Resistivity The soil is part of the fault circuit Actual circuit

Substation Grounding TutorialSoil Resistivity The soil is part of the fault circuit

Substation Grounding TutorialSoil Resistivity7000 A7000 A400 Ω-m400 Ω-m1000 Ω-m100 Ω-m300 Ω-m100 Ω-m300 Ω-m1000 Ω-m

Substation Grounding TutorialRock Resistivity400 Ω-m7000 A400 Ω-m100 Ω-m100 Ω-m300 Ω-m300 Ω-m1000 Ω-m1000 Ω-m7000 A

Substation Grounding TutorialPreparation Soil Resistivity Testing Soil boring report Substation location & grading plans Substation size, site area Propose traverses Sources of interference Resistivity test schedule Grading schedule Test form Test equipment check list

Substation Grounding TutorialGeotechnical Exploration & ReviewSand with Silt, fine grained, yellowishbrown, moist medium dense (SP-SM)WEATHERED SANDSTONE, yellowishBrown, very dense, [Textural Classification:SAND WITH SILT, fine to medium grained,light brown and yellowish brown, moist,very dense (SP-SM)]Water Table

Substation Grounding TutorialSoil Resistivity Testing Wenner SchlumbergerSource: IEEE 81-2012

Substation Grounding TutorialSoil Resistivity Testing - Wenner

Substation Grounding TutorialSoil Resistivity Testing - Schlumberger INSERT FORM!

Substation Grounding TutorialSoil Resistivity TestingC1P1P2C2 C1P1P2C2

Substation Grounding TutorialError Range Based on Probe Spacing

Substation Grounding TutorialSoil Resistivity Test Layout

Substation Grounding TutorialGrounding Design Variables

Substation Grounding TutorialSoil Resistivity Testing Summary How many traverses are required? Are there sources of interference? What is the correct layout of traverses? What pin spacing should we use? What do I do if there is not adequate space?

Substation Grounding TutorialSoil Resistivity Testing Layout

Substation Grounding TutorialSoilResistivityTestingLayout

Substation Grounding TutorialSoilResistivityTestingLayout

Substation Grounding TutorialPlot of Soil Resistivity Results

Substation Grounding TutorialProposed Soil Resistivity Test Traverses

Substation Grounding TutorialWenner Data Comparison Pre-& Post-Grading

Substation Grounding TutorialSubstation Grading Section Drawing

Soil Resistivity TestingWenner vs. Schlumberger Soil Models

Substation Grounding TutorialWenner vs. Schlumberger Soil Models

Substation Grounding TutorialSoil Resistivity Test Documentation Test equipment model number Test lead resistance (measured) Ambient Temperature when test was taken Date of test Standard test result form Resistivity & Resistance measurements at variouspin spacing Notes

Substation Grounding TutorialSoil Resistivity Modelling Soil BasicsHomogeneousMulti - LayeredActual soil

Substation Grounding TutorialGround Grid Design & Modeling

Substation Grounding TutorialDesign & Modeling Substation GroundGrid Ground grid layoutFault current system analysis & locationSplit factor modelingSafety analysis Vtouch, Vstep GPR plots Safety & Summary Reports

Substation Grounding TutorialGeneral Substation Layout

Substation Grounding TutorialGeneral Ground Grid Layout

Substation Grounding TutorialGeneral Ground Grid Layout1.2.3.4.5.6.7.Grid area – Loop 3’ outside fenceMain components – adjacent grid linesComplete grid spacingGate(s) grading ringGrading runs - UG facilitiesRodsBond all structures & equipment

Substation Grounding TutorialGeneral Ground Grid Layout

Substation Grounding TutorialGeneral Ground Grid Layout

Substation Grounding TutorialGround Rod Application Guideline UL listed Length, diameter Spacing Perimeter Equipment Open spaces

Substation Grounding TutorialCoffee Break

Substation Grounding TutorialFault Current Analysis1. Short circuit study1. LLG, SLG, fault duration, X/R2. Design margin2. Identify ground sources1.2.3.4.Transformer connections# of T-lines & feedersGrounding transformersGenerator Step-Up TR connections

Substation Grounding TutorialFault Current Analysis

Substation Grounding TutorialSplit Factor Analysis Sf Ig / (3I0) TL shield wire & feeder neutral currents Grid vs. OHSW /neutral currents “Hand” (IEEE 80 Annex C) vs. Software calculation

Substation Grounding Tutorial115 kV Triple Circuit with Two Shield Wires

Substation Grounding Tutorial115 kV Double Circuit Line with Two Shields

Substation Grounding TutorialSplit Factor Analysis

Substation Grounding TutorialSplit Network One-LineInjected current (3I0):Current in shields:Earth current:Split factor:5687.8 A, 21.6 779.3 A, -153.9 4911 A, 20.9 Sf 4911/5687.8 86% into remote earth

Substation Grounding TutorialExample 1 - Fault Analysis

Substation Grounding TutorialExample 1 - Symmetrical Comp. Analysis

Substation Grounding TutorialExample 2-Fault Current Analysis

Substation Grounding TutorialExample 3-Fault Current AnalysisF2

Substation Grounding TutorialSafety Analysis - Touch

Substation Grounding TutorialSafety Analysis - Step

Substation Grounding TutorialParametric Safety Analysis

Substation Grounding TutorialSafety Analysis - Mitigation

Substation Grounding TutorialSafety Analysis - Mitigation

Substation Grounding TutorialMitigation Options Reduce clearing time Add conductor Add rods / extend rod length Ground wells Increase area Surface rock Satellite grid Soil enhancement Bond adjacent grids

Substation Grounding TutorialLeakage CurrentL1L2𝜌𝜌 50𝜌𝜌 500

Substation Grounding TutorialTesting Fall of Potential, IEEE 81-2012Source: IEEE 81-2012

Substation Grounding TutorialTesting – 3 Pin Method (Grid Resistance)

Substation Grounding TutorialTest LifecycleDesignMaintenance

Substation Grounding TutorialGrounding Study Report

Substation Grounding TutorialGrounding Study Report

Substation Grounding TutorialGrounding Study Report

Substation Grounding TutorialSafety Analysis Report Data Soil boring report Soil resistivity test report Fault study report, current magnitude, duration, safetymargin, X/R Rock, soil resistivity models Body weight – IEEE 80 formulas Simulations different conditions Split Factor – calculation method, assumptions Photos Drawings Protection data

Substation Grounding TutorialGround Grid Report Design variables Soil model data Safety analysis(touch-step potentials) Simulation files Simulation log

Substation Grounding TutorialGrounding Design Variables– Soil model variables (variance test results)Pre- & Post-grade Approximation method used of raw data,error, discard data Impact of fill material Limited test data

Substation Grounding TutorialGrounding Design Variables

Substation Grounding TutorialGrounding Design Variables

Substation Grounding TutorialSubstation Grading Section DrawingRough GradeOriginal Surface

Substation Grounding TutorialGrounding Design Variables– Seasonal modelling of soils Drought, flood, winter, summer, early spring-late fall Map frost depth Multiplication factor discussion Table with various soil resistivities Mitigation options

Substation Grounding TutorialSpring Conditions

Substation Grounding Tutorial

Substation Grounding Tutorial

Substation Grounding TutorialFall Conditions

Substation Grounding TutorialWinter Conditions

Substation Grounding TutorialMap Frost Depths

Substation Grounding TutorialGrounding Design Variables– Crushed rock resistivity Last developments - IEEE 81 Task Force Testing Examples of values Maintenance Foreign materials (wind erosion, contamination,snow, salt spray, vehicle mud & traffic, etc.)

Substation Grounding Tutorial

Substation Grounding Tutorial