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Substation Grounding Tutorial

Substation Grounding Tutorial. Joe Gravelle, P.E. Eduardo Ramirez-Bettoni, P.E. Minnesota Power Systems Conference Thursday, Nov. 9, 2017. Substation Grounding Tutorial. Presenter – Joe Gravelle, P.E.

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Substation Grounding Tutorial

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  1. Substation GroundingTutorial Joe Gravelle, P.E. Eduardo Ramirez-Bettoni,P.E. Minnesota Power Systems Conference Thursday, Nov. 9,2017

  2. Substation GroundingTutorial Presenter – Joe Gravelle,P.E. Joe Gravelle earned BSEEE from NDSU in 1988. After graduation Joe worked in the mining industry in northeastern Minnesota for ten years. Joe is a principal engineer in the Substation Engineering department at Xcel Energy and has held many roles in the department since1998. In addition to substation projects, Joe is active in standards work both at Xcel Energy and in IEEE Substations Committee. Joe is currently the vice chair of the Substations Committee and is active in leadership roles and as an active member in several IEEE workinggroups.

  3. Substation GroundingTutorial Presenter – Eduardo Ramirez-Bettoni,P.E. Eduardo obtained a BSEE with Power System emphasis from University of Costa Rica in 2002. Eduardo has international experience in substation construction, O&M, and design in substations up to 500 kV. He has practiced engineering in the U.S., Canada and Costa Rica. Eduardo has a background in protection & control, substation physical design and power systemgrounding. Eduardo works as principal engineer for Xcel Energy in the Transmission & Substation Standards (TSS) department. TSS creates standard documents for design, specification and installation practices for transmission & substation facilities. Eduardo is an active member in IEEE Substation workinggroups.

  4. XcelEnergy • No. 1 utility wind energy providerin theU.S. • 3.4 millionelectric customers • 2 million naturalgas customers • 12,000employees

  5. Substation GroundingTutorial Agenda • GroundingBasics • Soil Resistivity Testing and SoilModelling • Design & Modelling of SubstationGrid • Break • Grounding DesignVariables • Soil modelvariables • Seasonal modelling ofsoil • Crushed rockresistivity • Fault current designmargin • Fault clearingtime • Summary / Combinedeffect

  6. Substation GroundingTutorial GroundingBasics • Definitions per IEEE 80 &81 • GPR, mesh / step /touch • Currentsplit

  7. Substation GroundingTutorial GroundingBasics • Touch potentials: 3’ hand-to-foot • Step potentials: 3’foot-to-foot • Safe if Calculated Values < Tolerable MaxValues

  8. Substation GroundingTutorial GroundingBasics

  9. Substation GroundingTutorial GroundingBasics

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

  11. Substation GroundingTutorial Grounding Basics -Example

  12. Substation GroundingTutorial GroundingBasics • Body current (IB) is based on let-go current per experimentation • (99.5 % percentile, no ventricularfibrillation) • Body resistance (RB) varies based on exposurevoltage • RB= 500-1000Ω • Source: IEEE80-2015

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

  14. Substation GroundingTutorial Example - BodyVoltage • Tolerable potential across 50 kg body, bodyonly • VB = IB x RB (for TPGcalculations)

  15. Substation GroundingTutorial Example-Touch potential (norock) • Tolerable potential across 50 kg body, step voltage, no surfacerock • Homogeneous soil resistivity: 100Ω-m • Accounts for foot resistance to surface, Vt, 50kg = IB x(RB+1.5) Source: IEEE80-2015

  16. Substation GroundingTutorial Example-Touch potential (withrock) Source: IEEE80-2015

  17. Substation GroundingTutorial Example-Touch potential (withrock) • Tolerable potential across 50 kg body, step voltage, 4” of 2000Ω-m surface rock; homogeneous soil resistivity: 100Ω-m • Vt,50kg = IB x (RB+1.5xCsx);(Cs=0.71)

  18. Substation GroundingTutorial GroundingBasics

  19. Substation GroundingTutorial GroundingBasics Source: IEEE80-2015

  20. Substation GroundingTutorial GroundingBasics

  21. Substation GroundingTutorial SoilAnalysis

  22. Substation GroundingTutorial Soil ResistivityModelling • SoilBasics Homogeneous Multi -Layered Actualsoil

  23. Substation GroundingTutorial SoilResistivity • The soil is part of the faultcircuit • Actual circuit

  24. Substation GroundingTutorial SoilResistivity • The soil is part of the faultcircuit

  25. Substation GroundingTutorial SoilResistivity 7000A 7000A 400Ω-m 400Ω-m 1000Ω-m 100Ω-m 300Ω-m 300Ω-m 100Ω-m 1000Ω-m

  26. Substation GroundingTutorial RockResistivity 400Ω-m 400Ω-m 7000A 7000A 100Ω-m 100Ω-m 300Ω-m 300Ω-m 1000Ω-m 1000Ω-m

  27. Substation GroundingTutorial Preparation Soil ResistivityTesting • Soil boringreport • Substation location & gradingplans • Substation size, sitearea • Propose traverses • Sources ofinterference • Resistivity testschedule • Gradingschedule • Test form • Test equipment checklist

  28. Substation GroundingTutorial Geotechnical Exploration &Review Sand with Silt, fine grained, yellowish brown, moist medium dense (SP-SM) WEATHERED SANDSTONE, yellowish Brown, very dense, [Textural Classification: SAND WITH SILT, fine to medium grained, light brown and yellowish brown, moist, very dense (SP-SM)] Water Table

  29. Substation GroundingTutorial Soil ResistivityTesting • Wenner • Schlumberger Source: IEEE81-2012

  30. Substation GroundingTutorial Soil Resistivity Testing -Wenner

  31. Substation GroundingTutorial Soil Resistivity Testing -Schlumberger • INSERTFORM!

  32. Substation GroundingTutorial Soil ResistivityTesting C1 P1 C2 C1 P1 C2 P2 P2

  33. Substation GroundingTutorial Error Range Based on ProbeSpacing

  34. Substation GroundingTutorial Soil Resistivity TestLayout

  35. Substation GroundingTutorial Grounding DesignVariables

  36. Substation GroundingTutorial Soil Resistivity TestingSummary • How many traverses arerequired? • Are there sources ofinterference? • What is the correct layout oftraverses? • What pin spacing should weuse? • What do I do if there is not adequate space?

  37. Substation GroundingTutorial Soil Resistivity TestingLayout

  38. Substation GroundingTutorial Soil Resistivity Testing Layout

  39. Substation GroundingTutorial Soil Resistivity Testing Layout

  40. Substation GroundingTutorial Plot of Soil ResistivityResults

  41. Substation GroundingTutorial Proposed Soil Resistivity TestTraverses

  42. Substation GroundingTutorial Wenner Data Comparison Pre-&Post-Grading

  43. Substation GroundingTutorial Substation Grading SectionDrawing

  44. Soil ResistivityTesting Wenner vs. Schlumberger SoilModels

  45. Substation GroundingTutorial Wenner vs. Schlumberger SoilModels

  46. Substation GroundingTutorial Soil Resistivity TestDocumentation • Test equipment modelnumber • Test lead resistance(measured) • Ambient Temperature when test wastaken • Date oftest • Standard test resultform • Resistivity & Resistance measurements at various pin spacing • Notes

  47. Substation GroundingTutorial Soil ResistivityModelling • SoilBasics Homogeneous Multi -Layered Actualsoil

  48. Substation GroundingTutorial Ground Grid Design &Modeling

  49. Substation GroundingTutorial Design&Modeling SubstationGround Grid • Ground gridlayout • Fault current system analysis &location • Split factormodeling • Safetyanalysis • Vtouch, Vstep • GPRplots • Safety & SummaryReports

  50. Substation GroundingTutorial General SubstationLayout

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