Adventures in Wind System Grounding
Tales From A-field (literally)
SWT Grounding System Evaluation Project
• Commissioned by *NYSERDA to evaluate the effectiveness of
grounding systems designed or recommended by turbine
manufacturers
• Owners of NYSERDA funded wind turbines were asked if they had
experienced lightning damage
• 15% of the respondents indicated that their systems may have been
damaged by lightning
• Perhaps a “one size fits all” approach to small wind turbine tower
grounding is not sufficient?
* New York State Energy Research & Development Authority
SWT Grounding System Evaluation Project
• 18 sites were selected for testing
– wide variation in terrain and soil types
– testing region covered 9 counties and an area of 80 x 260 miles
• Turbines from 5 different manufacturers
– Bergey, Endurance, Gaia, WTI (Jacobs) and Xzeres
– 8.9 kW to 55 kW in rated capacity
• Four different tower types
– 9 guyed lattice, 7 free standing lattice, 1 fixed monopole and one
guyed tilting pipe
– Installation ages spanned 7 years, from April 2006 to May 2013
• This diverse group was chosen to help identify commonalities between
the various grounding methods and tower types
Project Goals
• Measure & record grounding electrode resistance values
• Observe the effect that turnbuckles and other hardware
have on electrical conductivity between guy wires and the
grounding electrode
• Measure and record the resistance between the tower
structure and the grounding system
• Evaluate the overall integrity of the grounding system
• Observe the effect that workmanship and variations in
installation techniques have on grounding system integrity
Tower Grounding
• For small wind turbines there are two types of grounding
– Safety grounding to prevent electrical shock hazard
– Grounding metal structures for lightning protection
• The primary focus of this project was on tower grounding
• So the question is- what constitutes a good ground?
• Let’s start with a grounding electrode resistance value
• And where do we find this elusive value?
Grounding Electrode Resistance?
• Wind turbine installation manual grounding
recommendations vary widely between manufacturers
• Most give fairly specific guidelines for installing the
grounding system but few give grounding electrode
resistance recommendations
• One of the exceptions found during the field testing was
Endurance Wind Power. Their specifications call for a
grounding electrode system having a resistance to
ground of 10 ohms or less
Grounding Electrode Resistance?
• 2011 NEC Article 250.53 (2), the Exception mentions a resistance of
25 ohms or less for grounding electrodes, but this isn’t for towers
• Article 694.40 covers small wind tower grounding in some detail but
never mentions a minimum grounding electrode resistance value
• NFPA 780-2011- Standard for the Installation of Lightning Protection
Systems- chapter 9 covers Protection for Wind Turbines, but is this
primarily for utility scale turbines
– But nowhere in the document does it provide any electrode resistance values!
• How about best practices from other industries?
– Lightning protection industry recommends 10 ohms or less
– The Telecomm industry is now recommending 5 ohms or less but
this is more for protection against RF than for lightning
The Testing Process
• A 3 lead Extech Earth Resistance Tester was used
• One lead attaches to the grounding electrode or other test point
• The other two leads attach to two spikes driven into the ground, one
at 33 feet, the other at 66 feet
• The ground tester injects voltage into the ground and measures the
voltage drop between the three test points
• This allows it to calculate the resistance between these points
The Testing Process
• The guyed lattice towers all had 4 ground rods (electrodes), one at
the tower base and one at each of the 3 guy anchor points
• Free standing lattice towers had each leg grounded.
– Some installations connected each leg to its own grounding electrode.
– Others connected each leg to a grounding system consisting of multiple
electrodes bonded together to make a grounding ring
• The free standing monopole tower was connected to a grounding
electrode system consisting of multiple electrodes bonded together
to make a grounding ring
• The guyed tilting tower had an electrode at all 4 guy anchor points,
and one at the tower base.
The Testing Process
• All electrodes were measured first
• Then a comparative reading was made between the electrodes and
each guy wire, tower leg, turbine disconnect ground buss bar and a
spot 7 feet up on the tower
• When accessible, the tower ground to
premises ground bonding conductor was tested
• A visual inspection of the installation was
also conducted to assess code compliance
and quality of workmanship
Findings- Guyed Lattice Towers
• Grounding electrode readings ranged from 2.9 to 15.3 ohms
– some sites had almost a 10 ohm variance between electrodes
• Guy wire to ground readings ranged from 3.1 to 8.2 ohms
– Most guy wire to ground resistance values were less than the
grounding electrode resistance at that anchor point, due to the
multiple paths to ground via the tower and other guy wires
– Conductivity through the guy hardware and foundation anchor
rod was much better than anticipated. Guy wire tension and
vibration appear to be maintaining healthy electrical connections
• Variance between guys and electrodes ranged from 0 to 9.8 ohms
– The only tower with no variance had all the guy wires at each
anchor bonded together with a continuous conductor and
connected directly to the grounding electrode
Findings- Free Standing Lattice Towers
• The Endurance installations had the lowest resistance values of all
the towers tested, with 1.5 ohms at one site and 1.7 at the other
– This value was consistent for the entire structure to ground
– no variance between any of the test points at either site
• With only one exception, the electrodes at other lattice tower sites
varied from 1.5 to 6.5 ohms
– The exception was an installation on bedrock. An engineered
grounding system was installed consisting of 8 electrodes,
bonded to each other and to the tower with exothermic welds.
Additional top soil was placed over the system
– Tower to soil resistance values ranged from 9.6 to 10.3 ohms
Findings- Guyed Tilting Tower
• Grounding electrode readings ranged from 7.7 to 9.1 ohms
• Variance between guys and their corresponding electrode ranged
from 0 to 0.2 ohms
– The guy wires at every anchor were bonded together with a
continuous conductor and connected directly to the grounding
electrode
• As with the guyed lattice towers, the conductivity through the guy
wire thimbles, turnbuckles and ground anchor rod was much better
than anticipated
• Guy wire tension and vibration appear to be helpful in maintaining
healthy electrical connections
Findings- Monopole Tower
• Although the grounding electrodes were below ground level and
inaccessible for testing, the grounding electrode conductor and one
point on the tower both measured 6.1 ohms
Findings- Mechanical
• The most common issue found was improper installation of the
grounding electrode conductors
– Conductors installed with no slack, broken due to turbine
vibration and/ or thermal cycling
– Poor conductor routing methods, leading to damage by abrasion,
foot traffic and livestock
– Poor or lost electrical connections due to inadequate fasteners
– Fastening techniques that allowed contact between dissimilar
metals which caused corrosion due to electrolysis
• Missing and/ or improperly installed tower ground to premises
ground bonding conductor
Conclusions
• The manufacturers of all the wind turbines that were tested did a
reasonably good job specifying grounding systems for their turbines
and no significant deficiencies were observed with their designs
• Most of the issues that were found were caused by the effects of
vibration on grounding connectors and conductors
• The majority of these were workmanship related, all of which could
be easily rectified by the installers more closely following industry
best practices
• No matter how robust a wind turbine is or how well it is installed,
annual inspections are absolutely critical to maintaining electrical,
structural and aesthetic integrity
• And that it’s damned difficult to fit all this into 15 minutes or less!