Thermal Effect Of The Lightning Induced Arcs And The Resultant Mechanical Burst Mechanism Of Wind Turbine Blades

In the recent years wind generation has gained significant development.With increased unit capacity and height of the wind generators,the rotating wind turbine blades may be more than 100m high and easily get lightning strikes.It would be very costly to repair or change the blades once it experiences a lightning-induced fault.Except for optimal design of the receptor-down conductor protection system,how to guarantee the mechanical strength of the blades is another significant factor as to withstand the lightning strikes.In this respect,the damage mechanism of the blade materials and the mechanical structures is a key issue to be studied so as to properly choose blade materials and design the blade structure from the view angle of lightning protection.This thesis focuses on the damage characteristics of the blade materials as well as the mechanical burst mechanism of the blade structure,the main advances of which are given as follows.To reveal the mechanical burst mechanism under the lightning strikes,the development characteristics of the lightning induced arcs flowing through the blade surface and the sealed chamber should be studied in the first stance.Based on the swept channel test improved from IEC 61400 24 2010,plate model of real blade was adopted to study the electrical weak areas and arc paths.The results showed that,the weak areas normally gathered in the rear sandwich structure,especially at the intersection between the rear sandwich structure and the main beam,as well as the wrapped glass fiber near the blade tip.A lightning risk evaluation method considering the breakdown points number and the breakdown area scale was proposed as to quantitatively characterize the striking risk of the blade skin.It can also be concluded that,the lightning induced arcs may consist of two kinds,namely,air arcs and surface arcs.Under the negative test voltage,the arc path tended to develop along the surface of the blade materials.If the peak value of the impulse voltage was increased,the surface arc portion would be enlarged.The blade materials would get severe damage due to thermal impact of the lightning induced arc passing through the blade skin.The damage characteristics of the blade materials were obtained with large current experiments,in which extremely fine metal wire was used to inject limpulse current inside blade materials.An evaluation method for the performance of the core materials under lightning strikes was presented based on three key indicators including the electrical strength,the heat resistance ability and the mechanical strength.It can be concluded that,the electrical strength of the balsa wood is the worst among the three samples,but its heat resistance ability and mechanical strength turns to be the best.The electrical strength,the heat resistance ability and the mechanical strength of the PVC sample are all inferior to those of the PET sample,hence,PVC is not preferably advised for use in the blade manufacturing.The damage mechanism of the blade core materials was studied by simulations based on COMSOL and ADF.The influence of the porous material parameters such as porosity and permeability on the damage feature of the sandwich structures was also obtained.The results gained above can serve as theoretical reference in designing the blade core materials.When lightning induced arcs develop inside the sealed blade chamber,the thermal,electromagnetic and airflow impact may result in severe destruction of the blade structure.Based on the large current test platform with current peak at 150kA,large current was injected into blade chamber and the mechanical burst characteristics of the blade structure plus its dynamic process were obtained,showing the principal damage types as the trailing edge cracking and web breaking.The blade usually started to crack at the trailing edge near the breakdown point,and then extended gradually to both the tip and the root directions of the blade.If the down conductor was placed between the two webs,the damage extent of the trailing edge was less serious than the case with the down conductor placed outside the back web,however,the rear web and the area between the two webs would also be damaged.If the breakdown point was close enough to the tip of the blade,the trailing edge began to crack from the position near the blade tip,and the damage in the tip direction was much more severe than that of the root direction,which would even endanger the tip receptor of the blade.A modified MHD model for the lightning induced arcs considering arc current path and the non-uniform distribution of the current density was established accordingly,and the airflow and gas pressure distribution were calculated by COMSOL.The simulation results showed that,the pressure on the trailing edge of the blade is very high,which is the main cause of the trailing edge cracking.The bonding area of leading edge and the area between the rear web and the skin are also a high risky area of damage.Using epoxy resin glue to fill the bonding areas into a circular corner can effectively reduce the pressure of the bonding areas.The length of lightning induced arc inside single web structure blade is often larger than that of double web blade.Under the same current waveform,the pressure in bonding areas of sigle web blade is larger than that of double web blade,so sigle web blade is not as good as double web blade from the angle of lightning strike mechanical defense.