The surge current flow inside the tower may create a large inductive current in low-voltage circuits such as control, measurement and communication devices. Thus, the conventional grounding system is potentially weak for the protection of low-voltage circuits inside the wind turbine. By contrast, the proposed system has two ring-shaped electrodes of several meters diameter, one of which is vertically attached to the nose cone and the other laterally placed on top of the wind tower lying just below the nacelle.
   The pair of rings is arranged with a narrow gap of no more than 1 m to avoid mechanical friction during rotation of the blades and the nacelle’s circling. When lightning (here, suppose the current is positive)strikes a blade, the lightning current reaches the upper ring from a receptor through a conductive wire installed on the blade.
    Then, the electric field between the two rings becomes high and finally sparks over and the lightning current flows downwards. The current propagates along the lower ring and grounding wire,which is arranged outside the wind tower rather than inside, and is safely led to a grounding electrode sited far enough away from the grounding for the tower.

    2.新型雷電防護系統(tǒng)的提出
    圖1 是常規(guī)機組防雷接地系統(tǒng)與概念性防雷系統(tǒng)的說明。通常雷電流是從葉片接閃器通過葉片內(nèi)部的導(dǎo)雷電纜到達變漿軸承，并且通過變漿軸承向輪轂、主軸在經(jīng)過偏航軸承通過電纜傳導(dǎo)至接地網(wǎng)。強大的雷電流在塔筒內(nèi)會產(chǎn)生巨大的雷電電磁脈沖，如果接地線靠近動力和低壓控制電纜，則會產(chǎn)生巨大的影響。
    因此，常規(guī)的接地系統(tǒng)可能加劇雷電對機組低壓控制系統(tǒng)的影響。相反，這種系統(tǒng)利用安裝在輪轂上的環(huán)形電極，并且與葉片內(nèi)部的電纜連接形成導(dǎo)雷通道；另一個環(huán)形電極安裝在靠近機艙底部的塔筒上，兩個環(huán)形電極分別處于不同的水平軸和垂直軸上，兩個電極間保持一個不到1m的放電間間隙，保持這個間隙的目的在于避免有輪轂和塔筒發(fā)生偏轉(zhuǎn)時可能造成的摩擦以及防止環(huán)形電極碰撞造成損壞而影響機艙偏航。當(dāng)有較大雷電流通過葉片到達環(huán)形電極時，由于間隙間存在較高的電場形成兩個電極間的觸發(fā)，使雷電流通過兩個環(huán)形電極進行放電，這個放電的過程會形成可見的電流通道，由于電流時通過塔筒的外表面而不是內(nèi)部的導(dǎo)線，所以也會減小雷電電磁脈沖的強度，雷電流會通過塔筒直接入地。
    3. Downsized Model of Wind Turbine
    To verify the effectiveness of the proposed lightning protection system, the author conducted a trial test using a 1/100 downsized model that on a 1/100 scale accurately simulated an actual 2 MWwind turbine with a hub height of 60 m and a blade radius of 39 m (therefore, the hub height of the model is 60 cm and the blade radius 39 cm, as shown in Figs 2 and 3). The blades of the model are made from nonflammable ABS resin and the nacelle and  tower from PC iron.
    The ring-shaped electrodes of the model are of 4 mm φ copper wires, and the diameter of the upper and lower electrodes are 5.4 and 7.7 cm, respectively. On the surface of the blades, 2 mm φenamel wires are strained to simulate receptors and down conductors. Also, as the outer down conductor, 2 mm φenamel wires drop down from the backside of the lower ring to the ground plate which is 20 cm distant from the base of the wind tower. The gap between two rings, g,and the distance between the upper ring and the nacelle,d, are design variables in the model. A detailed structure is shown in Fig. 4 in a CAD drawing and a photo of a prototype. Simulated lightning impulses with a wavefront of 1.3 μs, wavetail 49 μs, altitude 664 kV were generated using a 800 kV and 5 kJ impulse generator, as shown as Fig. 5.

　　3. 按比例縮小的試驗用風(fēng)機模型
　　為了驗證這種防雷系統(tǒng)的有效性，作者做了一個試驗性的測試。用1:100的比例模型，精準(zhǔn)的建立一個機組高度為60m，葉輪半徑為39m的風(fēng)力發(fā)電機組模型（模型的機艙高度為60cm,葉輪半徑為39cm，如圖2、3所示），模型葉片采用阻燃的ABS樹脂材料制成，機艙和塔筒采用鐵皮制作。兩個電極采用4mm的銅線制作，輪轂和塔筒上的環(huán)形電極直徑分別為5.4cm和7.7cm。在葉片表面采用2mm的搪瓷線模擬葉片接閃器和引下線（導(dǎo)雷電纜），同樣，塔筒上的環(huán)形電極采用搪瓷線連接到接地板上，長度約20cm，兩個電極間的距離經(jīng)過精確地計算，保證間隙不會阻礙葉輪的旋轉(zhuǎn)。通過CAD制圖工具制作出如圖4所示的實際結(jié)構(gòu)，采用800kV和5Kj脈沖發(fā)生器模擬雷電流脈沖，采用1.3/49μs沖擊波形，脈沖電壓為664 Kv,如圖5所示。

　　4. Impulse Test in a Downsized Model