Investigations On The Electromagnetic Effects In Wind Turbines Struck By Lightning

With the increase of wind turbine capacity and wind farm dimension, the security of wind turbine operation is more and more important. Lightning is one of the mainest factors to affect the security of wind turbine operation. When a wind turbine is struck by lighting, the large impulse current is usually injected from the top of blade and flows through the inner conductor to the blade root. The lightning current then flows through the main shaft, bearing and nacelle conducting path to the tower top and injected into the grounding system by the tower foundation which is finally dissipated into the ground. In the transient process, the ground potential will be raised for the existence of grounding resistance which may cause backflash to the devices installed inside the tower. At the same time, electromagnetic fields will be produced inside the tower by lightning current and surge overvoltages will be induced on the signal lines and power lines by means of radiation, induction and coupling, which will destroy electric and electronic devices and hinder the safe operation of the wind turbine. Therefore, the purpose of this paper is to predict and analysis the transient characteristics of the wind turbine struck by lighting and to provide guidelines for lighting protection design to reduce losses of lightning damage for wind turbines.An integrative transient circuit model of wind turbine including lightning stroke, blade, bearing, tower and grounding system is established according to the lightning flowing path and the R, L, C and G parameters in the model are calculated. The wind turbine is equivalent to an electric network composed of series ofπtype circuits and the lightning stroke is considered to be the current source injected to the network. The transient responses of the wind turbine can be obtained by solving the circuit equation of the network. The nonlinear characteristic of soil ionization is taken into account and the influence of soil ionization is equivalent to the dimension increase of grounding electrodes. The grounding parameters are amended according to the situation of soil ionization and the transient grounding responses are solved dynamically. The effective methods of calculating the transient responses are proposed respectively for the circuits with constant parameters in case of perfect ground and with frequency-dependent parameters in case of imperfect ground. An investigation is made on the influences of lightning current waveform, coupling parameter, calculation step length, discretization manner, lightning stroke manner and grounding parameter on transient responses. An experimental study on lightning transient responses of a wind turbine system using a reduced-size wind turbine model is presented in this paper.Based on the transient current responses along the tower conductors, Maxwell's equations are solved by dipole technique. The formulas for calculating electric and magnetic fields are derived directly in time domain and the numerical solutions of electromagnetic fields are obtained by the discrete time and space. The influences of lightning current waveform and lightning current injected manner on electromagnetic fields are analyzed and the transient overvoltage induced in a loop is also calculated according to the law of electromagnetic induction.The electromagnetic field inside the tower is treated as incident field and the induced voltages and currents on the cable sheath are calculated numerically by solving the transmission line equations using the equivalent circuit method. According to circuit theory, the transfer impedance is applied to evaluate the difference voltage in the cable and the influence of grounding manner of cable sheath, transfer impedance, lightning current waveform and installation manner of the cable on the induced voltage is discussed. The results and conclusions obtained in this paper can provide the design references for lightning protection of practical wind turbine. According to the models and calculating methods presented in this paper, researchers can analyze the transient characteristics of the wind turbines and adopt the suitable protection measures for sensitive devices, elements and cables et al. Furthermore, according to the methods and results in this paper, the designers can optimize the lightning protection design of wind turbine system radically.