IEEEIEEEWind Farm Collector Wind Farm Collector System Grounding for System Grounding for Personal SafetyPersonal Safety

Summary of TopicsSummary of Topics

ContentContent

1.01.0 INTRODUCTIONINTRODUCTION 2.02.0 GENERAL REQUIREMENTSGENERAL REQUIREMENTS 3.03.0 ENVIRONMENT AND GEOTECHNICAL DATAENVIRONMENT AND GEOTECHNICAL DATA 4.0 4.0 SOIL RESISTIVITY MODELSOIL RESISTIVITY MODEL 5.0 5.0 SAFETY DESIGN CRITERIASAFETY DESIGN CRITERIA 6.0 6.0 GROUND FAULT RETURN CURRENTGROUND FAULT RETURN CURRENT 7.0 7.0 MINIMUM GROUND CONDUCTOR CROSS MINIMUM GROUND CONDUCTOR CROSS

SECTIONAL AREASECTIONAL AREA 8.08.0 GROUND ELECTRODE SYSTEM & GROUND ELECTRODE SYSTEM &

IMPEDANCEIMPEDANCE 9.09.0 GROUND POTENTIALSGROUND POTENTIALS 10.010.0 DESIGN OF SUBSTATION GROUND GRID DESIGN OF SUBSTATION GROUND GRID

AND AND LIGHTNING PROTECTION LIGHTNING PROTECTION

1-Introduction1-Introduction

This document is a summary of This document is a summary of questions/comments on a typical questions/comments on a typical approach for the minimum design approach for the minimum design requirements of a Wind Power requirements of a Wind Power Plant Grounding System. Plant Grounding System.

Aim to Define Topics for Future Aim to Define Topics for Future Standard or GuideStandard or Guide

2 - General 2 - General requirementsrequirements 2.1 Definitions:2.1 Definitions:

– Wind Power Plant (WPPWind Power Plant (WPP))A Wind Power Plant is a single or multiple A Wind Power Plant is a single or multiple wind turbine generators electrically wind turbine generators electrically interconnected and where the collection interconnected and where the collection system crosses a single or multiple private or system crosses a single or multiple private or public properties; and having one or more public properties; and having one or more points of interconnection to the Utility points of interconnection to the Utility electric system.electric system.

– WTGWTG– HVHV– LVLV– Grounding systemGrounding system– GPRGPR– Etc..Etc..

2.2 2.2 Reference Standards and Reference Standards and DocumentsDocuments

– What reference standards, guides, codes, document to be used ?What reference standards, guides, codes, document to be used ?– ANSI/IEEE 80 Guide for Safety in AC Substation GroundingANSI/IEEE 80 Guide for Safety in AC Substation Grounding– ANSI/IEEE 81 Guide for Measuring Earth Resistivity, Ground ANSI/IEEE 81 Guide for Measuring Earth Resistivity, Ground

Impedance, and Earth Surface Potentials of a Impedance, and Earth Surface Potentials of a Ground Ground System System

– ANSI/IEEE 998 Guide for Direct Lightning Stroke Shielding of ANSI/IEEE 998 Guide for Direct Lightning Stroke Shielding of Substations Substations

– ANSI/ IEEE 367 ANSI/ IEEE 367 Recommended Practice for Determining the Recommended Practice for Determining the Electric Electric Power Station Ground Potential Power Station Ground Potential Rise and Induced Rise and Induced Voltage From a Power Fault Voltage From a Power Fault

– Other IEEE/ANSI??Other IEEE/ANSI??– National Electrical Code [NEC]National Electrical Code [NEC]– National Electrical Safety Code [NESC]National Electrical Safety Code [NESC]– International Electrotechnical Commission [IEC]International Electrotechnical Commission [IEC]– Underwriters Laboratories [UL]Underwriters Laboratories [UL]– Insulated Cable Engineers Association [ICEA]Insulated Cable Engineers Association [ICEA]

2.3 What Consists a WPP 2.3 What Consists a WPP Grounding SystemGrounding System

Interpreting NESC/IEEE 80Interpreting NESC/IEEE 80– Single point,Single point,– Multi pointMulti point– 3 wires ? (Ex. Concentric neutral cable 3 wires ? (Ex. Concentric neutral cable

only without additional ground wireonly without additional ground wire– 4 wires ? (Ex concentric neutral cable + 4 wires ? (Ex concentric neutral cable +

additional ground wire)additional ground wire)– Ungrounded System?Ungrounded System?

2.42.4 Grounding Design Tools Grounding Design Tools – Grounding Design analysis software Grounding Design analysis software

and calculation tools.and calculation tools. CDEGS ? CDEGS ? CYME ?CYME ? ETAP?ETAP? EPRI ?EPRI ? Other?Other?

– Hand CalculationHand Calculation

2.5 2.5 Other Design AspectsOther Design Aspects– Design LifeDesign Life– Access to Site and Equipment Access to Site and Equipment

Public, privatePublic, private

– Etc.Etc.

3.03.0 Environment and Environment and Geotechnical Data Geotechnical Data 3.13.1 ClimateClimate

– AltitudeAltitude– Normal Ambient Temperature Normal Ambient Temperature – Extreme Ambient Temperature Extreme Ambient Temperature – Humidity Humidity – Max Wind Speed Max Wind Speed – Precipitation Precipitation – Seismic Hazard Seismic Hazard – Lightning Rate Lightning Rate

3.2 Geotechnical data 3.2 Geotechnical data – What measurement & why? (Electrical What measurement & why? (Electrical

Resistivity; Soil Acidity) Resistivity; Soil Acidity) – Measurement method? (Wenner, Measurement method? (Wenner,

Schlumberger…)Schlumberger…)– Where?Where?

Each WTG location?Each WTG location? Each Junction location?Each Junction location? Each switchgear location (pad or pole mounted) ? Each switchgear location (pad or pole mounted) ? Main Substation ?Main Substation ? Interconnection station ?Interconnection station ? Met tower location ?Met tower location ? Any other location subjected to electrical hazard?Any other location subjected to electrical hazard?

4.0 Soil Resistivity 4.0 Soil Resistivity Models Models Analyse collected data and Analyse collected data and

determine an equivalent soil determine an equivalent soil model for each test location. model for each test location. – Uniform soil?Uniform soil?– 2 layers soil?2 layers soil?– More than 2 layersMore than 2 layers

5.0 Safety Design5.0 Safety Design Criteria Criteria In accordance with IEEE 80: 2000In accordance with IEEE 80: 2000

Body ResistanceBody Resistance per IEEE 80: 2000 per IEEE 80: 2000 Foot ResistanceFoot Resistance per IEEE 80: 2000 per IEEE 80: 2000 Extra Foot ResistanceExtra Foot Resistance Applicable or not applicable ?Applicable or not applicable ? Maximum HV Ground Fault Disconnection Time: 0.5s or 1s?Maximum HV Ground Fault Disconnection Time: 0.5s or 1s? Maximum MV Ground Fault Disconnection Time: 0.133s, 0.5s or 1s?Maximum MV Ground Fault Disconnection Time: 0.133s, 0.5s or 1s? System X/R RatioSystem X/R Ratio (default value 20?) (default value 20?) Fibrillation Current CalculationFibrillation Current Calculation per IEEE 80: 2000 & 50kg Body Weightper IEEE 80: 2000 & 50kg Body Weight Starting Surface Layer Resistivity:Starting Surface Layer Resistivity: Per applicable Soil Resistivity Per applicable Soil Resistivity

ModelModel Surface Layer ThicknessSurface Layer Thickness Sub-Surface Layer ResistivitySub-Surface Layer Resistivity Imported Crushed Rock For Starting SurfaceImported Crushed Rock For Starting Surface

– 2000 Ω.m wet Electrical Resistivity. Or more than 2000 Ω.m?2000 Ω.m wet Electrical Resistivity. Or more than 2000 Ω.m?

6.0 Ground Fault 6.0 Ground Fault Return Current Return Current 1-Phase-to-ground fault current 1-Phase-to-ground fault current

calculations?calculations? 2-Phase-to-ground fault current 2-Phase-to-ground fault current

calculations?calculations?

6.1 HV Ground fault return current6.1 HV Ground fault return current– Depends on the Main transformer HV winding Depends on the Main transformer HV winding

connection: Generally Solidly grounded to provide a connection: Generally Solidly grounded to provide a ground current return path back to the POI through the ground current return path back to the POI through the transformer HV neutral.transformer HV neutral.

6.2 MV Ground fault return current6.2 MV Ground fault return current– The 34.5kV system neutral is generally solidly connected The 34.5kV system neutral is generally solidly connected

to ground at the main substation, with no intentional to ground at the main substation, with no intentional neutral to ground impedance.neutral to ground impedance.

– Main substation grounding system connection to WTG? Main substation grounding system connection to WTG? – The MV cable screening and bare horizontal ground The MV cable screening and bare horizontal ground

conductor connection conductor connection

6.3 LV Ground fault return current6.3 LV Ground fault return current– The LV system at the WTG with all local metalwork being The LV system at the WTG with all local metalwork being

solidly bonded to the LV Neutral of the WTG Transformersolidly bonded to the LV Neutral of the WTG Transformer

7.0 Minimum Ground 7.0 Minimum Ground Conductor Cross-section Conductor Cross-section AreaArea

Determine the minimum ground conductors cross Determine the minimum ground conductors cross sectional area (Based on IEEE 80: 2000) (Section sectional area (Based on IEEE 80: 2000) (Section 11.3.1, Formula (37) or code tables?)11.3.1, Formula (37) or code tables?)– Conductor metalConductor metal– Minimum ground conductor sizeMinimum ground conductor size– Max Ground Ambient temperatureMax Ground Ambient temperature– Max Air Ambient temperatureMax Air Ambient temperature– Max bare conductor temperatureMax bare conductor temperature– Max insulated conductor temperatureMax insulated conductor temperature– System X/R ratio:System X/R ratio:– HV design total ground fault current (+ Safety margin?)HV design total ground fault current (+ Safety margin?) – MV design total ground fault current (+ Safety Margin?)MV design total ground fault current (+ Safety Margin?)– LV design total ground fault current (+ Safety Margin?)LV design total ground fault current (+ Safety Margin?)– Fault clearing time ? Fault clearing time ?

8.08.0 Ground Electrode Ground Electrode System & ImpedanceSystem & Impedance

Determine the Ground Electrode System Determine the Ground Electrode System impedance for the wind farmimpedance for the wind farm

Based on soil resistivity models and ground Based on soil resistivity models and ground electrode conductor configurations electrode conductor configurations – Main S/S & compoundMain S/S & compound

What ohmic value? Common sense? What ohmic value? Common sense? – Cable trenches.Cable trenches.– Each wind turbine and it’s associated transformerEach wind turbine and it’s associated transformer

What ohmic value? Common sense? What ohmic value? Common sense? – Each meteorological mastEach meteorological mast

What ohmic value? Common sense? What ohmic value? Common sense? – Each MV junction box/switchgearEach MV junction box/switchgear– etc.etc.

9.09.0 Ground Ground PotentialsPotentials 9.19.1 GPR, Transfer, Touch & Step PotentialGPR, Transfer, Touch & Step Potential

– Examine Ground potential rise due to HV MV & LV Examine Ground potential rise due to HV MV & LV ground fault currents.ground fault currents.

– Compare maximum values to safety criteria for Compare maximum values to safety criteria for acceptanceacceptance

9.29.2 Zone of Influence (Hot Zone) ContourZone of Influence (Hot Zone) Contour– Determine the Zone of Influence of Ground fault Determine the Zone of Influence of Ground fault

potential (Hot Zone) as per IEEE 367 (and /or local potential (Hot Zone) as per IEEE 367 (and /or local specific code)specific code)

– Each site shall then be classified based on a 300V Zone Each site shall then be classified based on a 300V Zone of Influence contour limit as recommended by IEEE 367-of Influence contour limit as recommended by IEEE 367-1996, section 9.3 as:1996, section 9.3 as:

a “Hot Zone of Influence” GPR >/= 300V.a “Hot Zone of Influence” GPR >/= 300V. a “Cold Zone of Influence”, GPR is < 300V everywhere a “Cold Zone of Influence”, GPR is < 300V everywhere

across the Site . across the Site .

10 Design of Substation 10 Design of Substation Ground Grid and Ground Grid and Lightning protection Lightning protection

10.1 Base document & references data10.1 Base document & references data

– IEEE 80 – IEEE Guide for Safety in AC Substation IEEE 80 – IEEE Guide for Safety in AC Substation GroundingGrounding

– IEEE 81 – IEEE Guide for Measuring Earth Resistivity, IEEE 81 – IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground Impedance, and Earth Surface Potentials of a Ground SystemGround System

– IEEE 998 – IEEE Guide for Direct Lightning Stroke IEEE 998 – IEEE Guide for Direct Lightning Stroke Shielding of SubstationsShielding of Substations

– The substation location and prevailing keraunic level & The substation location and prevailing keraunic level & ground flash density. ground flash density.

– Soil resistivity at all grounding points.Soil resistivity at all grounding points.– Fault Level Schedule.Fault Level Schedule.

10.2 Substation Grounding System10.2 Substation Grounding System– What design criteria specific to Wind Power Plant?What design criteria specific to Wind Power Plant?

Do we specify a Max Ground Grid Impedance, Why? Do we specify a Max Ground Grid Impedance, Why? What acceptable limits for touch & step potential? (in What acceptable limits for touch & step potential? (in

accordance with IEEE 80).accordance with IEEE 80). Fault disconnection timeFault disconnection time minimum ground conductor size & material.minimum ground conductor size & material. Burial depth.Burial depth. Etc..Etc..

10.3 Substation Lightning Protection10.3 Substation Lightning Protection– Free standing lightning mastsFree standing lightning masts– Lightning rods installed atop substations structures, Lightning rods installed atop substations structures, – Static wire strung between static masts? Static wire strung between static masts? – Calculation of the Zone of Protection: rolling sphere Calculation of the Zone of Protection: rolling sphere

/ electrogeometric method (EGM)/other methods. / electrogeometric method (EGM)/other methods.

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