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IEEEWind Farm Collector System Grounding for Personal Safety Summary of Topics
Content • 1.0INTRODUCTION • 2.0GENERAL REQUIREMENTS • 3.0ENVIRONMENT AND GEOTECHNICAL DATA • 4.0 SOIL RESISTIVITY MODEL • 5.0 SAFETY DESIGN CRITERIA • 6.0 GROUND FAULT RETURN CURRENT • 7.0 MINIMUM GROUND CONDUCTOR CROSS SECTIONAL AREA • 8.0GROUND ELECTRODE SYSTEM & IMPEDANCE • 9.0GROUND POTENTIALS • 10.0DESIGN OF SUBSTATION GROUND GRID AND LIGHTNING PROTECTION
1-Introduction • 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 • 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..
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]
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?
2.4 Grounding Design Tools • Grounding Design analysis software and calculation tools. • CDEGS ? • CYME ? • ETAP? • EPRI ? • Other? • Hand Calculation
2.5 Other Design Aspects • Design Life • Access to Site and Equipment • Public, private • Etc.
3.0 Environment and Geotechnical Data • 3.1 Climate • Altitude • Normal Ambient Temperature • Extreme Ambient Temperature • Humidity • Max Wind Speed • Precipitation • Seismic Hazard • Lightning Rate
3.2 Geotechnical data • What measurement & why? (Electrical Resistivity; Soil Acidity) • Measurement method? (Wenner, Schlumberger…) • Where? • 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 • 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 CriteriaIn accordance with IEEE 80: 2000 • 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 • 1-Phase-to-ground fault current calculations? • 2-Phase-to-ground fault current calculations?
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. • 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 • 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 • Determine the minimum ground conductors cross sectional area (Based on IEEE 80: 2000) (Section 11.3.1, Formula (37) or code tables?) • 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 ?
8.0 Ground Electrode System & Impedance • Determine the Ground Electrode System impedance for the wind farm • Based on soil resistivity models and ground electrode conductor configurations • Main S/S & compound • What ohmic value? Common sense? • Cable trenches. • Each wind turbine and it’s associated transformer • What ohmic value? Common sense? • Each meteorological mast • What ohmic value? Common sense? • Each MV junction box/switchgear • etc.
9.0 Ground Potentials • 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 • 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 367-1996, section 9.3 as: • 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 • 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.
10.2 Substation Grounding System • What design criteria specific to Wind Power Plant? • 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 • 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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