Study On Lightning Transient Of Offshore Wind Turbines

The growth of energy demand and the requirement of energy transformation have led to the rapid development of offshore wind power generation.As a continuous increase in the capacity and size of offshore wind turbines,the operation risk becomes higher.Nowadays,lightning strike has constituted a disastrous factor that menaces the reliable operation of offshore wind turbines.Therefore,the research on lightning protection of offshore wind turbines has been widely concerned.When an offshore wind turbine is struck by lightning,a large lightning current will be injected from the attachment point on the blade and flow through the inner conductor of the blade to the moving contact part.Subsequently,the lightning current will be conducted to the tower body and dissipated into the sea water and sand through the monopile foundation.During this travelling process of lightning current,the lightning transient is caused in offshore wind turbine,which results in the transient potential rise and high current distribution.Meanwhile,the transient electromagnetic fields are also produced inside the tower.Focusing on these lightning transient effects,this paper makes an investigation on the lightning transient response characteristic of offshore wind turbine in the two manners,i.e.calculation analysis and experimental measurement.According to the complete travelling path of lightning current on an offshore wind turbine,the modeling method is proposed for the tower,blades,moving contact part and monopile foundation,respectively.In consideration of the travelling wave behavior of lightning current,the long current-carrying conductors are segmented.The tower body is divided into a discrete multiconductor system with a subdivision along its height and circumference.The electrical parameters of the coupling branch unit can be calculated by Neumann formula and average potential integration.Each coupling branch unit is replaced by a coupling π-circuit and the equivalent circuit of the tower can be set up by these integrating π-circuits.In a similar way,the electric parameters of the blades are determined and an equivalent circuit including three blades is built.For the sake of engineering simplification,the moving contact part is modeled as a parallel capacitance-resistance circuit.The coefficients of self-resistances and mutual resistances are calculated by Laplacian method for the multiconductor system of monopile foundation and the corresponding circuit model is obtained by constructing the grounding resistance network.After all the equivalent circuits are connected according to the travelling path of lightning current,a complete transient circuit model is presented for the offshore wind turbine.On the bassis of the circuit model,the lightning transient calculation is carried out and the transient responses are obtained for the offshore wind turbine.In terms of the current distribution,the calculation formulas of the transient electromagnetic fields are derived from Maxwell equations.The experimental measurement of impulse responses is also made by a reduced-scale offshore wind turbine.The validity of the circuit model and transient algorithm is verified by comparing the measured results with the calculated ones.By setting the number of injection points of lightning current at the tower top,the distributions of the transient potentials and currents are calculated for an actual offshore wind turbine.The influence of peak value and wavefront steepness of lightning current on transient potential is investigated by examining the transient responses on the offshore wind turbine under the different lightning current waveforms.The transient calculations are also performed in the cases that single blade or multiple blades of offshore wind turbine are struck by lightning.Based on results of transient responses,the distances of back flashover are estimated inside the tower.According to the cumulative probability density function of lightning current,the results of lightning transient responses are probabilistically weighted and so the statistical values of transient responses are obtained.Finally,the transient electromagnetic fields are calculated and the spatial distributions of the electromagnetic fields are obtained at different heights in the tower.