| Megawatt-level wind turbines are currently the mainstream models deployed in the wind energy industry.Due to their height and location,they are prone to lightning strikes,causing damage,shutdowns and huge losses.Studying the mechanism of lightning attachment,interception and ablation of blades as well as the corresponding protection methods is not only an essential issue in wind power industry,but also has important theoretical significance and application value.To study the interception efficiency of lightning protection system in wind turbine blades,this paper establishes the 3-D electrostatic field model of lightning and blades,analyzes the lightning attachment and interception characteristics on the blades.The influence of the down-conductor insulation on upward leader is studied.The selection method of insulation index of down-conductor cables is proposed.The transient ablation characteristics of the glass fiber reinforced plastic(GFRP)is studied.The ablation mechanism under different lightning waveform components and amplitudes is revealed.The lightning electromagnetic transient model of the electric heating circuit in the blades with de-icing function is built.The influence of lightning parameters on lightning-induced overvoltage is analyzed and the corresponding protective method is provided.The specific research contents and conclusions are as follows:1.The lightning attachment and interception characteristics and failure mechanism.The electrostatic field model of downward leader and the blade at the moment of "last-jump" is built.The electric field on the blade surface of GFRP blades is calculated.The influence of the size,quantity and installation location of disc-shape receptors on the electric field distribution is analyzed.The influence of the downward leader positions and rotation angles of blades on interception area is studied.It is concluded that the installation of disc-shape receptors on the 0～10m area from the blade tip,especially the 0～5m area,will achieve the preferably interception effect.2.The method for selecting the down-conductor cables.The lightning damage of blades investigated from wind farms is classified.It is noted that the insulation level of the down-conductor cables is the key issue to suppress the upward leader.The electrical field model of down-conductor cables with different rating voltages at the moment of "last-jump" is built.The voltages between the insulation layer of 1-35kV cables are calculated and consequently the definition of breakdown factor KP is defined.The relationship between the rating voltages and breakdown factor KP are quantitatively analyzed.The selection method of down-conductor cables by considering the breakdown factor Kp>2,redundancy and economy comprehensively is proposed to determine the insulation level of down-conductor cables.A real blade sample is made to verify the selected cable can suppress the upward leader generating from down-conductor cables and increase the interception efficiency of receptors.3.Ablation mechanism of lightning damage on GFRP blade.To study the lightning damage mechanism of lightning current on GFRP blade,an ablation experimental method is proposed with the multi-components impulse current by setting leading copper wires on the surface of GFRP samples or cross GFRP samples is proposed.An experimental platform is established to carry out the proposed experimental method.The creeping discharge and penetrating discharge are investigated.Through SEM observation and analysis,it is found that high temperature ablation leads to the pyrolysis of the composite matrix resin,sublimation of glass fibers,pyrolysis produces gas-induced stress waves,leading to the separation of GFRP laminates and glass fiber fracture.A thermal-electrical coupling prediction model for lightning ablation damage of GFRP was developed,the comparison found a good agreement between simulation and experimental results,and the model can be used for damage area prediction.Through data fitting,the function relationship between the ablation damage area of GFRP and the amplitude,charge and specific energy of lightning current was deduced along the creeping discharge and penetrating discharge.The physical process of the lightning damage of the GFRP is revealed by simulating and calculating the ablation damage history at different moments.4.Protection method of the overvoltage between the dowconductor and the heating circuit of de-icing blades.By investigating the structure characteristics of de-icing blades and working principle of the heating circuit,the lightning electromagnetic transient models including lightning protection system,carbon fiber heating network and the power cables are developed.With these models,the overvoltage between the dowconductor and the heating circuit are calculated with difference waveforms of lightning current.The equations of the overvoltage and lightning current parameters are derived from the simulation results.The influence of the surge protective devices(SPDs)number and installation locations on suppressing the overvoltage between the dowconductor and the heating circuit is studied quantitatively.A preferable plan is proposed through calculation,as follows,installing 2 sets of T1 SPDs on the joint of live line and the carbon network and 1 set of T1 SPD on the joint of earthing line and the carbon network will achieve better protection effect against the overvoltage between the downconductor and the heating circuit. |
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