IEEE
Wind Farm Collector System
Grounding for Personal Safety
Summary of Topics
Content
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1.0
2.0
3.0
4.0
5.0
6.0
7.0
INTRODUCTION
GENERAL REQUIREMENTS
ENVIRONMENT AND GEOTECHNICAL DATA
SOIL RESISTIVITY MODEL
SAFETY DESIGN CRITERIA
GROUND FAULT RETURN CURRENT
MINIMUM GROUND CONDUCTOR CROSS
SECTIONAL AREA
8.0 GROUND ELECTRODE SYSTEM & IMPEDANCE
9.0 GROUND POTENTIALS
10.0 DESIGN OF SUBSTATION GROUND GRID AND
LIGHTNING PROTECTION
1-Introduction
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This document is a summary of
questions/comments on a typical
approach for the minimum design
requirements of a Wind Power Plant
Grounding System.
Aim to Define Topics for Future
Standard or Guide
2 - General requirements
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2.1 Definitions:
– Wind Power Plant (WPP)
A Wind Power Plant is a single or multiple wind
turbine generators electrically interconnected and
where the collection system crosses a single or
multiple private or public properties; and having
one or more points of interconnection to the Utility
electric system.
– WTG
– HV
– LV
– Grounding system
– GPR
– Etc..
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2.2 Reference Standards and Documents
– What reference standards, guides, codes, document to be used ?
– ANSI/IEEE 80 Guide for Safety in AC Substation Grounding
– ANSI/IEEE 81 Guide for Measuring Earth Resistivity, Ground
Impedance, and Earth Surface Potentials of a Ground
System
– ANSI/IEEE 998 Guide for Direct Lightning Stroke Shielding of
Substations
– ANSI/ IEEE 367 Recommended Practice for Determining the Electric
Power Station Ground Potential Rise and Induced
Voltage From a Power Fault
– Other IEEE/ANSI??
– National Electrical Code [NEC]
– National Electrical Safety Code [NESC]
– International Electrotechnical Commission [IEC]
– Underwriters Laboratories [UL]
– Insulated Cable Engineers Association [ICEA]
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2.3 What Consists a WPP
Grounding System
Interpreting NESC/IEEE 80
– Single point,
– Multi point
– 3 wires ? (Ex. Concentric neutral cable only
without additional ground wire
– 4 wires ? (Ex concentric neutral cable +
additional ground wire)
– Ungrounded System?
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2.4
Grounding Design Tools
– Grounding Design analysis software and
calculation tools.
CDEGS ?
 CYME ?
 ETAP?
 EPRI ?
 Other?
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– Hand Calculation
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2.5
Other Design Aspects
– Design Life
– Access to Site and Equipment
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Public, private
– Etc.
3.0Environment and
Geotechnical Data
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3.1
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Climate
Altitude
Normal Ambient Temperature
Extreme Ambient Temperature
Humidity
Max Wind Speed
Precipitation
Seismic Hazard
Lightning Rate
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3.2 Geotechnical data
– What measurement & why? (Electrical
Resistivity; Soil Acidity)
– Measurement method? (Wenner,
Schlumberger…)
– Where?
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Each WTG location?
Each Junction location?
Each switchgear location (pad or pole mounted) ?
Main Substation ?
Interconnection station ?
Met tower location ?
Any other location subjected to electrical hazard?
4.0 Soil Resistivity
Models
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Analyse collected data and determine
an equivalent soil model for each test
location.
– Uniform soil?
– 2 layers soil?
– More than 2 layers
5.0 Safety Design
Criteria
In accordance with IEEE 80: 2000
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Body Resistance
per IEEE 80: 2000
Foot Resistance
per IEEE 80: 2000
Extra Foot Resistance
Applicable or not applicable ?
Maximum HV Ground Fault Disconnection Time: 0.5s or 1s?
Maximum MV Ground Fault Disconnection Time: 0.133s, 0.5s or 1s?
System X/R Ratio
(default value 20?)
Fibrillation Current Calculation per IEEE 80: 2000 & 50kg Body Weight
Starting Surface Layer Resistivity:
Per applicable Soil Resistivity
Model
Surface Layer Thickness
Sub-Surface Layer Resistivity
Imported Crushed Rock For Starting Surface
– 2000 Ω.m wet Electrical Resistivity. Or more than 2000 Ω.m?
6.0 Ground Fault
Return Current
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1-Phase-to-ground fault current
calculations?
2-Phase-to-ground fault current
calculations?
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6.1 HV Ground fault return current
– Depends on the Main transformer HV winding connection:
Generally Solidly grounded to provide a ground current return
path back to the POI through the transformer HV neutral.
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6.2 MV Ground fault return current
– The 34.5kV system neutral is generally solidly connected to
ground at the main substation, with no intentional neutral to
ground impedance.
– Main substation grounding system connection to WTG?
– The MV cable screening and bare horizontal ground conductor
connection
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6.3 LV Ground fault return current
– The LV system at the WTG with all local metalwork being solidly
bonded to the LV Neutral of the WTG Transformer
7.0 Minimum Ground
Conductor Cross-section Area
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Determine the minimum ground conductors cross
sectional area (Based on IEEE 80: 2000) (Section 11.3.1,
Formula (37) or code tables?)
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Conductor metal
Minimum ground conductor size
Max Ground Ambient temperature
Max Air Ambient temperature
Max bare conductor temperature
Max insulated conductor temperature
System X/R ratio:
HV design total ground fault current (+ Safety margin?)
MV design total ground fault current (+ Safety Margin?)
LV design total ground fault current (+ Safety Margin?)
Fault clearing time ?
8.0Ground Electrode
System & Impedance
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Determine the Ground Electrode System impedance for
the wind farm
Based on soil resistivity models and ground electrode
conductor configurations
– Main S/S & compound
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What ohmic value? Common sense?
– Cable trenches.
– Each wind turbine and it’s associated transformer
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What ohmic value? Common sense?
– Each meteorological mast
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What ohmic value? Common sense?
– Each MV junction box/switchgear
– etc.
9.0Ground Potentials
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9.1 GPR, Transfer, Touch & Step Potential
– Examine Ground potential rise due to HV MV & LV ground
fault currents.
– Compare maximum values to safety criteria for acceptance
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9.2 Zone of Influence (Hot Zone) Contour
– Determine the Zone of Influence of Ground fault potential
(Hot Zone) as per IEEE 367 (and /or local specific code)
– Each site shall then be classified based on a 300V Zone of
Influence contour limit as recommended by IEEE 3671996, section 9.3 as:
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a “Hot Zone of Influence” GPR >/= 300V.
a “Cold Zone of Influence”, GPR is < 300V everywhere across
the Site .
10 Design of Substation
Ground Grid and
Lightning protection
10.1 Base document & references data
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IEEE 80 – IEEE Guide for Safety in AC Substation Grounding
IEEE 81 – IEEE Guide for Measuring Earth Resistivity, Ground
Impedance, and Earth Surface Potentials of a Ground System
IEEE 998 – IEEE Guide for Direct Lightning Stroke Shielding of
Substations
The substation location and prevailing keraunic level & ground
flash density.
Soil resistivity at all grounding points.
Fault Level Schedule.
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10.2 Substation Grounding System
– What design criteria specific to Wind Power Plant?
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Do we specify a Max Ground Grid Impedance, Why?
What acceptable limits for touch & step potential? (in
accordance with IEEE 80).
Fault disconnection time
minimum ground conductor size & material.
Burial depth.
Etc..
10.3 Substation Lightning Protection
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Free standing lightning masts
Lightning rods installed atop substations structures,
Static wire strung between static masts?
Calculation of the Zone of Protection: rolling sphere /
electrogeometric method (EGM)/other methods.
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