MITIGATING THE EFFECTS OF LIGHTNING STRIKES ON WIND TURBINES
Background:
One issue that has always been apparent on Wind Farms is large 'scale blade and main component damage due to heavy lightning strikes largely due to imbalances between storm clouds and the ground, or within the clouds themselves. Most lightning occurs within the clouds. "Sheet lightning" describes a distant bolt that lights up an entire cloud base.
Considering the average lightning can produce energy in the region of 300 million volts (30,000 Amps) a single Lightning strike that hits a Wind Turbine may have the capability to cause high level blade and main component damage leading to long periods of asset downtime and high repair costs for asset owners.
Lighting is more prone to hit the leading edge and the outer blade surfaces towards the tips as this is where the highest amount of static energy is generated, modern Wind Turbines are designed in such a way that the flow of energy from a lightning strike starts at the blade tip, dissipating through the blade via the internal conductor built into the blade, and then to the main earthing point or the surge arresters that are designed to protect the pitch motors and other electrical components.
Effects:
Lightning strikes can cause damage to each of these components to different degrees leading to the following types of asset failure:
Blades:
This example damage to the polymer matrix composite, blade damage from a lightning strike can cause structural damage resulting in catastrophic failure and long-term outage.
Nacelle Electronics:
The electronic components that are within the lightning path between the blade tip and the main earthing point can experience damaging currents which can also cause damage to associated systems.
Bearings:
Lightning current and arcing can cause surface damage to bearing components, leading to long-term degradation and reduction in the bearing lifetime.
Nacelle Enclosure:
Lightning strikes can cause structural damage to Nacelle enclosures, leading to large scale asset downtime.
Mitigation:
During RFM Services Wind Turbine Inspections we ensure that the current lightning protection system on your Wind Turbine is sufficient and maintained correctly by analysing several factors which include:
Conductivity of the blade tip - Due to the high level of static energy that is generated at the blade tip, it is extremely important that an adequate level of conductivity is present, there are many solutions available to increase the conductivity at the blade tip such as the installation of a thin conducting mesh onto the top portion of the blade which would be connected to the internal conductor of the blade via the lightning receptors, this would increase the cross-sectional area allowing for better conductivity reducing the resistance on the blade leading to a reduction in blade damage caused by lightning strikes.
Adequacy of the Internal conductor of the blade – If the size of the Internal conductor of the blade is not sufficient then the internal conductor will not have the correct current carrying capacity in order to transfer the lightning current to the main earthing point the internal conductor of the blade needs to be able to withstand a peak capacity of 300 million volts/30,000 Amps with a potential resistance of 10,000 Ω.
Surge Arresters / Discharge Boxes – Due to the potential flow of current that can go through the blade and the hub section before flowing to the main earthing point, there is a risk present to the electrical equipment within the hub such as the pitch motors and the control equipment, the hub should have a minimum of 3 surge arrester boxes installed with a rating of 10 KΩ as the calculated current of a lightning strike is in the region of 30,000 Amps / 300 million volts, resulting in a potential resistance of 10,000 Ω, adequate surge protection helps to reduce asset damage and downtime due to electronic overload.