Wind Turbine Foundation Grounding Considerations

IEEE Wind and Solar Plant Collector System Design Working Group (WandSPCSDwg)

Task Force on Wind and Solar Plant Grounding for Personal Safety http://grouper.ieee.org/groups/td/wind/; http://grouper.ieee.org/groups/td/safety/

Abdou Sana, Eng., Ph.D.

Secretary of the Task Force on Wind and Solar Plant Grounding for Personal Safety

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I Introduction

• WPP Grounding System consists of many Small grounding stations distributed in a wide area with various soil types and characteristics.

• The Small grounding Stations are interconnected to form a wide network spread across the WPP.

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3

• Typical Wind Power Plant Feeder Ground grid

Main Purpose of the Grounding system:

• Personal Safety

• Equipment Safety

For any accidental Circuit (Short-circuit to ground at or lightning strike on a Tower), the questions are:

• Is a person working at walking by the tower, safe?

• How do we plan, design and build the WTG grounding system to be safe and .. . Affordable?

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Factors to be considered when designing a WTG grounding system:

• Codes and Regulations

• Ground conductor (Material, Size, connections)

• Ground Grid Layout and Resistance

• Soil Electrical Resistivity

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2 - Reference standard • 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

• National Electrical Code [NEC] • National Electrical Safety Code [NESC] • International Electro-technical Commission [IEC]

– IEC 62305-3, Protection against lightning – Part 3: Physical damage to structures and life hazard

– IEC 61400-24, Wind turbine generator systems – Part 24: Lightning protection

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3 - Ground Conductor Sizing Conductor size as a function of conductor current can be

obtained from IEEE Std-80

Assumptions:

• All heat will be retained in the conductor (adiabatic process).

• The product of specific heat (SH) and specific weight (SW), TCAP, is approximately constant because SH increases and SW decreases at about the same rate.

• Fault duration within a few seconds

Minimum conductor size to curry a given current when the conductor material constants are available:

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I rms current in kA

Amm2 minimum Conductor cross section in mm2

Akcmil minimum Conductor cross section in kcmil

Tm Maximum allowable temperature in °C

Ta Ambient temperature in °C

Tr Reference temperature for material constants in °C

αo Thermal coefficient of resistivity at 0 °C in 1/°C

αr Thermal coefficient of resistivity at reference temperature Tr in 1/°C

ρr Resistivity of the ground conductor at reference temperature Tr in ìΩ-cm

Ko 1/αo or (1/αr) – Tr in °C

tc duration of current in s

TCAP: Thermal capacity per unit volume of material in J/(cm3·°C) (see table)

αr and ρr to be evaluated at the same reference temperature of Tr °C .

(Tables provides data for αr and ρr at 20 °C).

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Amm2 I

1

TCAP104

tc r r

lnK0 Tm

K0 Ta

A.kcmil I

197.4

TCAP

t.c .r .r

lnK.0 T.m

K.0 T.a

Minimum Conductor Cross-section Area

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a From ASTM standards. b Copper-clad steel rods based on 0.254 mm (0.010 in) copper thickness. c Stainless-clad steel rod based on 0.508 mm (0.020 in) No. 304 stainless steel thickness over No. 1020

steel core.

Description Material conductivity

(%)

αr factor at 20 °C (1/°C)

Ko at 0 °C (0 °C)

Fusinga

temperature Tm (°C)

ρr 20 °C (μΩ·cm)

TCAP thermal capacity

[J/(cm3·°C)]

Copper, annealed soft-drawn

100.0 0.003 93 234 1083 1.72 3.42

Copper, commercial hard-drawn

97.0 0.003 81 242 1084 1.78 3.42

Copper-clad steel wire 40.0 0.003 78 245 1084 4.40 3.85

Copper-clad steel wire 30.0 0.003 78 245 1084 5.86 3.85

Copper-clad steel rodb 20.0 0.003 78 245 1084 8.62 3.85

Aluminum, EC grade 61.0 0.004 03 228 657 2.86 2.56

Aluminum, 5005 alloy 53.5 0.003 53 263 652 3.22 2.60

Aluminum, 6201 alloy 52.5 0.003 47 268 654 3.28 2.60

Aluminum-clad steel wire

20.3 0.003 60 258 657 8.48 3.58

Steel, 1020 10.8 0.001 60 605 1510 15.90 3.28

Stainless-clad steel rodc 9.8 0.001 60 605 1400 17.50 4.44

Zinc-coated steel rod 8.6 0.003 20 293 419 20.10 3.93

Stainless steel, 304 2.4 0.001 30 749 1400 72.00 4.03

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4- WTG Grounding Layout and Resistance

Typical WTG Ground Grid

4.1 Standalone /Local Ground Grid Resistance

WTG ground grid resistance measured/calculated when the WTG ground grid is disconnected from the rest of the WPP grounding systems.

Typical Requirements (IEC 61400-24 clause 9):

Rg ≤ 10.0 Ohm

• Local soil resistivity is required for calculating Rg

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4.2 Lightning Protection Requirement IEC 62305-3, Clause 5.4.2.2 - Type B - Level I and IEC 61400-24, Clause 9 & Annex I – Type B Class I

Combination of Horizontal and Vertical Electrode:

Lr= L1-re

Lv=(L1-re)/2

Lr: Horizontal ground electrode length

Lv: Vertical ground electrode length

re= Equivalent radius of the ring electrode

L1: IEC 62305-3 2010 - fig. 3

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4.3 Case Study : • Case 1 : - 2x Ground Rings & 4x Vertical Electrodes

• Case 2: Rebar only

• Case 3: Rebar with 2x Ground Rings & 4x Vertical Electrodes

• Case 4: Case 3 and 1x 50m horizontal ground electrode

• Case 5: Case 3 and 2x 50m horizontal ground electrode

Concrete slurry was not included & Only uniform soil electrical resistivity was considered

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Case 1

Case 2

Case 3

Cases 4 &5

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0.1

1

10

100

10 100 1000 10000

Gro

un

d G

rid

Re

sist

ance

(O

hm

)

Equivalent Soil Resistivity (Ohm.m)

WTG Ground Grid Resistance (Uniform Soil Resistivity)

2x Ring & 4& V.Rods

Rebar Only

Rebar & 2 Rings& 4xV.Rods

Rebar & 2 rings&4Rods&1x50mHorizontal cond

Rebar & 2 rings&4Rods&2x50mHorizontal cond0

2

4

6

8

10

0 200 400 600 800

• The 2 rings and 4 rods case exhibit the highest ground impedance and is conservative.

• Adding the rebar to the 2 rings and 4 rods improves the ground resistance.

• There is not a significant difference between Rebar Alone and Rebar with 2 rings external to the foundation.

• Adding 1x or 2 x 50m horizontal conductor to the rebar & 2 rings further improves ground grid impedance.

• For soils with resistivity below 500Ohm.m a ground grid resistance <10Ohm can be achieved easily.

• For soil resistivity above 500Ohm.m, achieving a ground grid resistance of <10Ohm is challenging. Additional cost (Equipment & work) work/ equipment such as ground enhancing materials or chemical rods will be required.

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4.4 WTG Interconnected Ground Resistance

Resistance measured / Calculated when the WTG ground grid is interconnected to the wind power plant grounding system (functional Ground grid Resistance)

Typical Requirement:

Rg ≤2.0 Ohm

Easily Achievable thanks to the interconnection of many small grounding stations

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Soil electrical resistivity is critical as it directly impacts:

• The safety criteria (safe voltages) computations

• The ground grid impedance which in turn, affects actual ground potentials

At which locations, soil resistivity should be tested (how many

tests should be carried out?)

What soil equivalent resistivity should be used in the calculations for safety assessment? .

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5- Soil Resistivity for WPP Grounding Design

5-1 Soil Resistivity and Safety Criteria

Safety criteria (safe touch and Step voltages) are determined based on local conditions (soil conditions).

• For body weight of 50kg (IEEE std. 80)

– Estep step voltage in V

– Etouch touch voltage in V

– Cs De-rating factor (protective surface layer) ~

– ρs resistivity of the surface material in Ω·m

– ts duration of shock current in seconds

– If no protective surface layer is used, then Cs =1 and ρs = ρ.

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Etouch50 1000 1.5Cs s

0.116

ts

Estep50 1000 6Cs s

0.116

ts

(IEEE Std 80) For b = 0.08m Cs versus hs curves for different parameter K values

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5.2 Actual Step & Touch voltages

• Actual Step and Touch Potentials are a fraction of the Ground potential rise which is directly impacted by the interconnected grounding systems impedances

GPR=Rg*Ig = Rg*(3I0) *SF

– Rg= ground grid resistance

– I0 is the Zero Sequence current (“ground” current)

– SF is the split factor

– Rg=is a function of

• Soil electrical resistivity

• Ground grid geometry & material (surface, depth, shape, copper, coppercald steel etc..)

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5.3 Soil Resistivity Tested at a limited number of locations

If Soil Resistivity is tested only at few locations on site, then consider using the most conservative data from the test for all grounding processes:

• Use Maximum soil profile for ground grid impedance and ground potential calculations

• Use Minimum soil profile for Safety Criteria computation

• The max and min soil profiles are not necessary true maximum and true minimum and Design Risks needs to be assessed and controlled (Overdesign, Safety concerns)

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5.4 Soil Resistivity tested at all grounding station locations

If Soil Resistivity is tested at all locations subject to safety hazard on Site, then consider the following:

- For stand alone ground grid impedance calculations, use local soil resistivity models

- For Interconnected ground grid impedance and ground potential calculation, use an averaged soil resistivity profile to apply across site.

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0.01

0.10

1.00

10.00

100.00

1000.00

10000.00

0.10 1.00 10.00 100.00

Ear

th A

pp

aren

t R

esis

tan

ce (

Oh

m)

Average Electrode Spacing (m)

Overall

Max

Overall

Average

Overall

Median

Overall

Min

WPP 1 - Earth Apparent Resistance Profiles for a WPP on high resistivity soils (East Pennsylvania, USA)

WPP 2 - Earth Apparent Resistance Profiles for a WPP on medium resistivity soils (Central Texas, USA)

0.10

1.00

10.00

100.00

1000.00

10000.00

0.10 1.00 10.00 100.00

Ear

th A

ppar

ent R

esis

tance

(O

hm

)

Average Electrode Spacing (m)

Overall

Max

Overall

Average

Overall

Median

Overall

Min

WPP 3 - Earth Apparent Resistance Profiles for a WPP on a low resistivity soils (Plains East Colorado)

5.5 Case Study

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WPP 1 WPP 2 WPP 3

Electrode length for lightning protection*

using median soil profile (m) 55 13 5

Electrode length for lightning protection*

using Max soil profile (m) 80** 40 5

*Minimum length of Earth electrode (vertical and horizontal conductors) per IEC 62305-3,

Clause 5.4.2.2 - Type B - Level, and a ground grid effective radius of 9.0 m

** Value limited to 80m corresponding to max 3000Ohm.m resistivity in IEC 62305-3

Computed maximum local grid impedance using

Max soil profile (Ω) 24.99 37.93 2.68

Computed maximum local grid impedance using

Median soil profile (Ω) 6.03 15.24 0.75

Maximum measured local grid impedance in the

field after construction 13.43 9.02 0.750

Computed maximum interconnected grid

impedance across WPP using Median soil

profile (Ω)

1.13 1.02 0.20

Computed maximum interconnected grid

impedance using Max soil profile (Ω) 1.99 2.12 0.36

For low to medium resistivity soil ( ρ ≤ 500Ohm.m)

• Using the max soil resistivity profile or the median soil resistivity profile does not make a big difference

• Zg <10.0Ω can be achieved easily at each local WTG without additional grounding conductors or soil enhancing materials.

• Safety criteria calculations and safety assessment should be based on the measured local soil resistivity, or the minimum soil profile across site

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For medium to high soil resistivity soils ( ρ >500Ohm.m)

• Max resistivity profile may result in extended additional ground grid conductors and/or ground enhancing material to comply with WTGs requirement for local ground grid impedance.

• This suggests that the soil resistivity testing be conducted for each WTG location and the local ground grid be designed accordingly.

• For the interconnected or functional ground grid impedance only, there is not a significant difference between the Max soil profile and the median soil profile. 2.0 Ohms can be achieved easily. Safety criteria calculations and safety assessment have to be based on the measured local soil resistivity or the minimum profile soil resistivity across site

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• Various WTG Manufacturer have various requirements for WTG ground grid Layout and resistance.

• Most WTG manufacturer require Rg≤10.0Ω

• Local soil resistivity is critical to WTG standalone ground resistance.

• If soil electrical resistivity measurement is completed at all grounding points, then averaged soil resistivity can be used for WTG interconnected ground grid resistance and ground potentials calculation.

• If soil electrical resistivity data is available only at a few locations across the WPP, then “Maximum” profile data should be used for ground resistance calculations and “Minimum” soil resistivity profiles data should be used for safety criteria (safe step and touch potential) calculations

• For soil electrical Resistivity ρ>500Ω.m there is a significant cost increase for achieving a standalone WTG ground grid resistance Rg≤10.0Ω

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6 Conclusion

Work On Progress • Effect of Multi-layered soil structures

• 3-D soil models,

• Effect of concrete slurry

• Effect of Ground enhancing material (such as bentonite).

• Hand calculations of WTG ground grid impedance

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