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风力发电机组雷电暂态效应的研究
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摘要
随着风力发电机组单机容量和风电场规模的增大,风电机组的安全运行问题日益受到重视。雷击是影响风电机组安全运行的主要因素之一。当风电机组遭受雷击时,巨大的雷电流通常由机组的桨叶叶尖注入,沿桨叶内置导体注入叶尖根部,再经滑环、电刷或放电器等环节流过主轴和机舱导流路径进入塔筒顶部,由塔筒将雷电流导入接地装置并最终散入大地。在此暂态过程中,由于接地电阻的存在,地电位将被抬高,有可能对设备造成反击。同时,雷电流将在塔筒内部空间产生电磁场,通过辐射、感应和耦合等方式在塔内的信号线与电力线上以及机组控制系统内产生干扰和电涌过电压,损坏电力电子设备,对机组的安全可靠运行产生威胁。因此,本文旨在对风电机组遭受雷击情况下的暂态特性进行仿真分析,以期为机组的防雷保护提供可信的设计依据,从而降低机组的雷害损失。
     本文根据雷电流的流动路径,建立了包括雷击、桨叶、机舱、塔筒及接地装置在内的一体化的风电机组暂态电路模型,计算了电路中的电阻、电感、电容、电导(R、L、C、G)参数,将风电机组等效为由一系列π型电路组成的电网络,雷击被视为注入此电网络的电流源。通过对机组电路方程的求解便可得到机组受直接雷击时的暂态响应。文中考虑了土壤电离的非线性特征,将土壤电离的作用等效为接地导体尺寸的增加,在计算过程中根据土壤电离情况修正接地导体的电路参数,动态地求解电路响应。分别提出了计算理想大地情况下常参数电路的暂态响应和非理想大地条件下频变参数电路的有效方法。论文考察了雷电流波形、耦合参数、计算步长、分段方式、雷击方式和接地装置参数等因素对暂态响应的影响,设计了对实际风机按比例缩小模型的雷击试验。
     根据计算得到的塔筒上的暂态电流分布,采用偶极子法求解麦克斯韦方程,推导了塔筒内部空间任意点处的电场和磁场分布的时域计算公式,通过对时间和空间的离散化处理,得到了电场和磁场的数值解法;分析了雷电流波形、雷电流注入方式等因素对电磁场的影响,根据电磁感应定律计算了回路感应电动势。
     以塔内空间的电磁场作为入射场,用暂态等值电路法求解传输线方程,对电缆屏蔽层上的感应电压和电流进行了数值计算。根据电路理论利用转移阻抗计算了电缆芯线上的差模感应电涌过电压,并考察了电缆屏蔽层在塔底长距离路径上的单端与两端接地方式、电缆转移阻抗、雷电流波形及电缆敷设方式对过电压的影响。文中的计算结果和分析结论为实际风电机组的雷电防护设计提供了很好的参考依据,根据文中所建立的模型及相应的求解方法,研究者可对不同的风电机组进行暂态分析,并对敏感设备、元件、电缆等采取适当的保护措施。更进一步,在本论文的指导下,设计者可以从根本上优化风电机组的设计。
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.
引文
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