Electromagnetic Calculation of a Wind Turbine Earthing System

Lightning protection systems (LPS) for wind power generation is becoming an important public issue since installations of wind turbines (WT) have greatly increased worldwide and their generating capacities are approaching 150 GW (GWEC 2010). WTs are often struck by lightning because of their open-air locations, special shapes and very high construction heights. As well as seriously damaging blades, accidents where low-voltage and control circuit breakdowns frequently occur in many wind farms. An earthing (grounding) system is one of the most important components required for appropriate LPSs in WTs and wind farms. Japan, in particular, suffers from frequent and heavy lightning strikes, an example being the notorious „winter lightning” found in coastal areas of the Sea of Japan (Yokoyama 2002). Indeed, many turbines in Japan have been hit by lightning, and winter lightning poses a specific threat due to its intense power and electric current that are much higher than the world average (Shindo 2009, Natsuno 2010). Furthermore, due to its narrow landmass, wind turbines in Japan tend to be constructed in mountainous areas with high resistivity soil. Thus, earthing is one of the most important issues in Japan for protecting wind turbines from lightning. Although IEC 61400-24:2010 („Wind turbine Generator System – Part 24: Lightning Protection”) indicates a methodology for protection against such accidents, a standardised solution remains to be established. The earthing issue for a WT described in the IEC 6140024 is completely subject to IEC 62305-3:2006 („Protection against lightning – Part 3: Physical damage to structures and life hazard”), which handles lightning protection for a general structure including a house and a building. The foundation and earthing system of a WT are generally much smaller than those of an ordinary building that is equivalent in height to a wind turbine. Furthermore, the lightning level for a WT is much higher than that of a normal house that has an equivalent foundation to a turbine. The smaller earthing system of a WT may not have enough capacity for lightning protection compared to conventional equipment. In this chapter, the authors will discuss, using electromagnetic calculations, possible unexpected cases for the requirement regarding „the minimum length” of an earth electrode defined in IEC 61400-24 and IEC 62305-3.