Analysis On Lightning Stroke Characteristic Of The Large Scale Wind Turbine Blades

Wind power generation has been developing vigorously in the last ten years because of its cleanliness without any pollution and sustainable utilization. The flourishing development of wind power industry goes with the risk of lightning damage on wind turbines. The enlargement of wind turbine capacity and size increase the risk of lightning damage for a wind turbine. Wind turbine blade is most apt to be damaged by lightning stroke and lightning protection of modern wind turbine blades present a number of new challenges due to the use of new materials. It is necessary to analyze lightning stroke characteristic of the large scale wind turbine blades such as lightning attachment distribution on the blade and lightning transient characteristics to provide reference for lightning receptors arrangements, protection zones dividing and insulation coordination.Firstly,damage forms and mechanisms of blades were analyzed and shortcomings of existing lightning protection measures were discussed. Secondly, the formula for the number of lightning flashes according to IEC was introduced and problems in calculation were also analyzed. An improved calculation method was proposed. Then, a simplified model for the wind turbine was established and the connection process between downward leader and upward leader was simulated based on 2D random model. The connection process forms between downward leader and wind turbine were discussed. The interval distributions of strike distance and the final jump distance were counted. The locations of lightning attachment on the blade were analyzed. Finally, a complete model of the wind turbine blade, tower and grounding device were established in EMTP. Blades equipped with lightning down conductor, internal wire and carbon fiber laminate were analyzed. The main results of each part are as follows:Typical types of lightning damage at the blades mainly account for thermic and mechanical effect. The movement of the rotor blades may result in shift of lightning attachment point from one blade to another or swept stroke phenomenon. The rotation of wind turbine blades may also affect the accumulation of self-produced space charge. The pollution of blades may increase the probability of surface flashover.The IEC formula cannot take the rotation of wind turbine blades into consideration as well as the annual number of downward flashes and upward flashes.The calculation method proposed in this paper is in good agreement with actual situation because it has taken the rotation of wind turbine blades and upward flashes into consideration. When wind turbines are situated in mountainous regions, the structure's actual height above ground level should be replaced with the effective height.The 2D random model could well reflect the connection of downward leader and wind turbine blades and there are diversiform forms in connection process. The interval distributions of strike distance and the final jump distance show the characteristics of wide range which are related to lightning current amplitudes. The majority of lightning damage is concentrated at 4m inboard the blade tip, which accounts for 98.50% of the total lighting stroke numbers. The statistical results are in agreement with factual observation data. Lightning attachment distribution may be affected by rotation angle of the blade, which is more concentrated at the blade tip with the increasing of rotation angle.The voltage drop across the down conductor may reach up to order of MV; the voltage difference between the lightning down conductor and the inner wire decreases along the blade, but electric field between the lightning down conductor and the inner wire is several thousand kV/m which is easy to cause internal arcing;the voltage difference and electric field between the lightning down conductor and the carbon fiber laminate when connected at tip and root of the blade is far less than connected only at tip of the blade. In order to prevent side flashover, it is necessary to increase of the separation distances between the lightning down conductor and the inner wire and connect the lightning down conductor and the carbon fiber laminate at tip and root of the blade.