~ National Lightning Safety Institute ~

Section 5.5.1

Abstract

The USA wind farm industry (WFI) largely is centered in low-lightning areas of the State of California. While some evidence of lightning incidents is reported here, the problem is not regarded as serious by most participants. The USA WFI now is moving eastward, into higher areas of lightning activity (1).

The European WFI has had many years experience with lightning problems. One 1995 German study estimated that 80% of wind turbine insurance claims paid for damage compensation were caused by lightning strikes (2).

Neither the European or USA WFI have adopted site criteria, design fundamentals, or certification techniques aimed at lightning safety. Such guidelines are necessary if lightning hazard reduction at wind farms is to be an achievable goal. (3).

Fig. 1. Lightning Effects to components of a wind power plant (4).

Lightning current parameter	Relevant component of the lightning strike	Effect	Endangered components
peak current I	first impulse current	potential rise of the wind power plant, voltage drop across cable shields	nacelle &power plant building, SCADA
specific energy	first impulse current	electromechanics, heating, evaporation	blades and bearings stressed by I
charge Q	long duration currents, first impulse current	melting	blades and bearings
average current steepness i/T1	subsequent and superimposed impulse currents	magnetic induction	SCADA
number of impulse currents n	subsequent and superimposed impulse currents	repeated H-field impulses	SCADA

Recent Case Studies

USA Experience

1. At one southwestern USA Wind Farm lightning damage exceeded $50,000 in the first year of operation. Damage occurred to blades, generator, controller, control cables, SCADA, etc. A Lightning protection retrofit at site by manufacturer included air terminals, TVSS products and additional bonding & grounding measures.

Further lightning damage occurred after the retrofit. A consulting engineering specialist in lightning mitigation was hired. Recommendations for enhanced grounding measures are being implemented. TVSS, air terminal, shielding, nacelle, blade treatment, and personnel safety recommendations are not being implemented at this time. (5)

2. Eighty-five percent of the downtime experienced by a second southwestern USA commercial wind farm was lightning-related during the startup period and into its first full year of operation. Direct equipment costs were some $55,000, with total lightning-related costs totaling more than $250,000. (6)

European Experience.

1. A 1996 European retrospective study was conducted of some 11,605 wind turbine years experience in Denmark and Germany. Very accurate operational records were available for analysis. General findings indicated:

a) lightning faults caused more loss in wind turbine availability and production than the average fault;

b) ranking in descending susceptibility to lightning damage were turbine control systems, electrical systems, blades, and generators;

c) the number of failures due to lightning increases with tower height;

d) wood epoxy blades have significantly less damage rates than GRP/glass epoxy blades. (7)

2. The German electric power company Energieerzeugungswerke Helgoland GmbH shut down and dismantled their Helgoland Island wind power plant after being denied insurance against further lightning losses. They had been in operation three years and suffered in excess of 800.000 German Marks damage. (8)

Design and Testing

Many USA lightning codes and standards are incomplete, superficial, and provide more benefit to commercial vendors than to those seeking relief from lightning's effects. Devices that claim to offer absolute protection abound in the marketplace, confusing specifying architects, engineers, and facility managers. Safety should be the prevailing directive (9)

The time to review possible lightning effects upon wind turbines is during the site selection and design stages. A lightning mitigation plan can be derived from a hazard design analysis. Then, a testing and verification program can provide validation and certification that the protective measures will function as engineered. Frequently, lightning problems do not receive consideration during the design stage. It then requires a specialized lightning safety engineer to analyze the effects of lightning during operations, and provide a rationale for "safety-through-redesign" modifications to the wind farm facilities.

Lightning Realities

Lightning prevention or protection, in an absolute sense, essentially is impossible. However, hazard mitigation and threat reduction are achievable through an understanding of the lightning phenomenon and preparation for its effects. Adoption of customized Safety Guidelines for Wind Farms (LSGWF) document offers a rational, systematic approach toward lightning safety. The general outline of a LSGWF should include:

• 1. Management Approval.
• 2. Personnel Training.
• 3. Site Analysis.
• 4. Threat Warning.
• 5. Safety Devices.
• 6. Testing and Certification.

The cost of enacting a comprehensive lightning mitigation hardware system for wind farms normally is some 0.75 - 0.50 percent of total capital costs.

Conclusion

A LSGWF document should be developed by wind industry participants. When applied, together with an understanding of lightning behavior, it will enable manufacturers and operators to have working criteria to apply to most any wind turbine design or location.

References

1. See USA Isokeraunic map in Uman, M.: 1986, "Lightning", Dover, NY, p. 57. See also USA wind map at WWW:http://nwtc.nrel.gov/html_docs/usmaps.html.
2. Hoppe-Kilpper, M. & Durstewitz, M. : 1995 :"Blitz und Uberspannungsschutz von Windkraftanlangen" -Institut fur solare Engergieversorgungstechnik (ISET), BMBF- Gesprach Blitzschutz von Windkraftanlagen, Bonn, 19.01.1995.
3. Wiesinger, J.:1996: "Lightning Protection of Wind Power Plants", Proc. ICLP, Florence, Italy, Sept. 1996.
4. op cit.
5. Mitigation Study performed by NLSI, 1996.
6. NLSI conversation with Site Manager, 1996.
7. Cotton, I and Jenkins, N, "Lightning Protection of Wind Turbines", UMIST, CEU Joule Project - JOR3-CT95-0052, Nov. 1996.
8. Knauer, R.:1995 "Wenn der Blitz plotzlich die Windmuhle lahmlegt", Stuttgarter Zeitung, No. 71, Wissenschaft und Tecknik, 25 March 1995.
9. IEEE Std. 1100-1992, "IEEE Recommended Practice for Powering and Grounding Sensitive Electronic Equipment", p.41.