Lightning damages wood epoxy blades

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An investigation into two blade failures on NEG Micon 1.5 MW turbines in February at the Sustrum/Renkenberge wind station in Emsland, Germany, has concluded that both failures were related to lightning damage. The two turbines were dismantled in late March and parts of the blades were sent to their British place of manufacture in Southampton for detailed examination. The remaining 30 machines at the wind plant were scrutinise and it was evident that there had been a high incidence of lightning strikes, but they were deemed safe to go back on-line.

The blades for the NM1500/64 turbines were supplied by Aerolaminates, the rotor blade arm of Denmark's NEG Micon. The company bought Aerolaminates from British company Taylor Woodrow last year, along with the Wind Energy Group. Aerolaminates specialises in making wood epoxy blades and is described by NEG Micon as its "competence centre" for blade development and production. The company is expanding its production facilities into premises on the Isle of Wight.

Problems arose at the Sustrum/Renkenberge plant in early February when two of the 1.5 MW machines were put out of action due to blade failure. On February 5 a blade was completely destroyed and found in pieces on the ground, according to Friedrich Preissler-Jebe of NEG Micon Deutschland. The failure was traced to damage at the blade tip, which led to the assumption that lightning was the cause-especially as a neighbouring turbine had been hit. On February 16 a new blade failure occurred. NEG Micon Deutschland started an investigation and the plant was stopped.

Aerolaminates' detailed investigation confirms that the first blade was struck by lightning at the tip which led to internal arcing in the metal tip box, causing the blade to fail. The second blade too was affected by lightning, but the investigation also revealed a weld defect in the metal box. According to Aerolaminates, all remaining metal tip boxes on the site have been inspected for this defect and are found to be satisfactory. The company believes, however, that the high incidence of lightning strikes may be due to lightning being attracted to the metal tip boxes. It therefore plans to replace all tip boxes in the summer months with a composite box. The two failed blades are to be replaced and installed this month.

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