ASK THE EXPERTS: Wind Turbine Tower Grounding

Intermediate

A wind turbine installer I recently spoke to claims that the connection between the equipment and tower grounds is unnecessary and dangerous. I disagree, but don’t have the code or research to back up my opinion.

Mark S. Mayhew • NYSERDA

The NEC is clear—you have to follow code, or convince your inspector otherwise (which is unlikely).There is a safety reason for bonding the tower ground. A case in point: Years ago, a Skystream was wired improperly at the nacelle and the four connections (two line, neutral, and ground) were all off by one position. There was no separate bond at the tower base to the equipment grounding system. The result was that the tower was “live” when the turbine was first energized. An installer happening to put a large wrench on a tower bolt would have been at risk for a deadly shock.

Uninspectable ground connections at the tower top, and the possibility of a live connection to tower ground via loose wire or perhaps alternator failure pose safety hazards. For this reason, requirements were written into National Electrical Code’s (NEC) Article 694 that specify an inspectable ground to the tower. This requirement echoes the general requirements on grounding in the NEC: all exposed metal must be grounded. While there are some exceptions—rotors, tails, and guy wire, for example—the relevant  2017 NEC sections are 690.40 (A), (2), (3) and (4):

694.40 Equipment Grounding and Bonding.

(A) General. Exposed non–current-carrying metal parts of towers, turbine nacelles, other equipment, and conductor enclosures shall be grounded and bonded to the premises grounding and bonding system. Attached metal parts, such as turbine blades and tails that are not likely to become energized, shall not be required to be grounded or bonded.

(2) Bonding Conductor. Equipment grounding conductors or supply-side bonding jumpers, as applicable, shall be required between turbines, towers, and the premises grounding system.

(3) Tower Connections. Equipment grounding, bonding, and grounding electrode conductors, where used, shall be connected to metallic towers using listed means. All mechanical elements used to terminate these conductors shall be accessible.

(4) Guy Wires. Guy wires used to support turbine towers shall not be required to be connected to an equipment grounding conductor or to comply with the requirements of 250.110.

However, there is controversy about tower-to-premises bonding and, similarly, array-to-premises bonding for PV systems. The counterargument is that a lightning strike at the tower or array will cause a ground bounce at the premises through the connected equipment-ground bond, potentially damaging equipment and posing a shock hazard.

I believe that this conjecture is unproven, theoretically and practically. The inductance of the equipment ground from tower to premises is actually significant, and it probably mitigates the coupling effect. There is no doubt in my mind that the personnel safety achieved by bonding towers to premises outweighs the lightning-coupling argument. The thought that an alternator insulation breakdown could result in a live tower is unacceptable.

Dr. Robert H. Wills, PE • Intergrid

I agree. If you do not connect, then you are trying to isolate. Isolating the turbine machinery ground and the tower from each other in a lightning strike is not viable, and could create just enough voltage difference to cause extensive damage. The main function of surge or lightning arrestors is to limit the voltage difference, which prevents blowing holes in the insulation, while draining energy to ground before the voltage spike reaches any sensitive equipment.

Robert W. Preus, PE • NREL

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