Wind Turbine Foundation Grounding Considerations

Transcription

1 1 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 Abdou Sana, Eng., Ph.D. Secretary of the Task Force on Wind and Solar Plant Grounding for Personal Safety

2 2 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.

3 Typical Wind Power Plant Feeder Ground grid 3

4 4 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?

5 5 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

6 6 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 , Protection against lightning Part 3: Physical damage to structures and life hazard IEC , Wind turbine generator systems Part 24: Lightning protection

7 7 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:

8 8 Minimum Conductor Cross-section Area 1 A mm2 I A.kcmil I TCAP10 4 K 0 T m ln t c r r K 0 T a TCAP t.c.r.r ln K.0 T.m K.0 T.a I rms current in ka A mm2 minimum Conductor cross section in mm 2 A kcmil minimum Conductor cross section in kcmil T m Maximum allowable temperature in C T a Ambient temperature in C T r Reference temperature for material constants in C α o α r Thermal coefficient of resistivity at 0 C in 1/ C Thermal coefficient of resistivity at reference temperature T r in 1/ C ρ r Resistivity of the ground conductor at reference temperature T r in ìω-cm K o 1/α o or (1/α r ) T r in C t c duration of current in s TCAP: Thermal capacity per unit volume of material in J/(cm 3 C) (see table) α r and ρ r to be evaluated at the same reference temperature of T r C. (Tables provides data for α r and ρ r at 20 C).

9 9 Description Material conductivity (%) α r factor at 20 C (1/ C) K o at 0 C (0 C) Fusing a temperature T m ( C) ρ r 20 C (μω cm) TCAP thermal capacity [J/(cm 3 C)] Copper, annealed softdrawn Copper, commercial hard-drawn Copper-clad steel wire Copper-clad steel wire Copper-clad steel rod b Aluminum, EC grade Aluminum, 5005 alloy Aluminum, 6201 alloy Aluminum-clad steel wire Steel, Stainless-clad steel rod c Zinc-coated steel rod Stainless steel, a From ASTM standards. b Copper-clad steel rods based on mm (0.010 in) copper thickness. c Stainless-clad steel rod based on mm (0.020 in) No. 304 stainless steel thickness over No steel core.

10 10 4- WTG Grounding Layout and Resistance Typical WTG Ground Grid

11 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 clause 9): R g 10.0 Ohm Local soil resistivity is required for calculating R g

12 Lightning Protection Requirement IEC , Clause Type B - Level I and IEC , Clause 9 & Annex I Type B Class I Combination of Horizontal and Vertical Electrode: L r = L 1 -r e L v =(L 1 -r e )/2 L r : Horizontal ground electrode length L v : Vertical ground electrode length r e = Equivalent radius of the ring electrode L 1 : IEC fig. 3

13 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 Case 1 Case 2 Case 3 Cases 4 &5

14 Ground Grid Resistance (Ohm) 14 WTG Ground Grid Resistance (Uniform Soil Resistivity) 100 2x Ring & 4& V. Rods 10 Rebar Only 10 8 Rebar & 2 Rings & 4xV.Rods Equivalent Soil Resistivity (Ohm.m) Rebar & 2 rings &4Rods&1x50m Horizontal cond Rebar & 2 rings &4Rods&2x50m Horizontal cond

15 15 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.

16 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: R g 2.0 Ohm Easily Achievable thanks to the interconnection of many small grounding stations

17 17 5- Soil Resistivity for WPP Grounding Design 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?.

18 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) E touch C s s E step C s s t s E step step voltage in V E touch touch voltage in V C s De-rating factor (protective surface layer) ~ ρ s resistivity of the surface material in Ω m t s duration of shock current in seconds If no protective surface layer is used, then Cs =1 and ρ s = ρ t s

19 (IEEE Std 80) For b = 0.08m C s versus h s curves for different parameter K values 19

20 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=R g *I g = R g *(3I 0 ) *S F R g = ground grid resistance I 0 is the Zero Sequence current ( ground current) S F is the split factor R g =is a function of Soil electrical resistivity Ground grid geometry & material (surface, depth, shape, copper, coppercald steel etc..)

21 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)

22 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.

23 Earth Apparent Resistance (Ohm) Earth Apparent Resistance (Ohm) Case Study Overall Max Overall Average Overall Median Overall Min WPP 1 - Earth Apparent Resistance Profiles for a WPP on high resistivity soils (East Pennsylvania, USA) Average Electrode Spacing (m) Average Electrode Spacing (m) Overall Max Overall Average Overall Median Overall Min WPP 2 - Earth Apparent Resistance Profiles for a WPP on medium resistivity soils (Central Texas, USA) WPP 3 - Earth Apparent Resistance Profiles for a WPP on a low resistivity soils (Plains East Colorado)

24 24 Electrode length for lightning protection* using median soil profile (m) Electrode length for lightning protection* using Max soil profile (m) WPP 1 WPP 2 WPP ** 40 5 *Minimum length of Earth electrode (vertical and horizontal conductors) per IEC , Clause 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 Computed maximum local grid impedance using Max soil profile (Ω) Computed maximum local grid impedance using Median soil profile (Ω) Maximum measured local grid impedance in the field after construction Computed maximum interconnected grid impedance across WPP using Median soil profile (Ω) Computed maximum interconnected grid impedance using Max soil profile (Ω)

25 25 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

26 26 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

27 27 6 Conclusion Various WTG Manufacturer have various requirements for WTG ground grid Layout and resistance. Most WTG manufacturer require R g 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

28 28 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