4.ImpulseTestinaDownsizedModel

Figure 6 presents an example of the results of an impulse test using the 1/100 downsized wind turbine model with the proposed ring-shaped electrodes. As shown in the figure, spark-over successfully occurred between the two electrodes, showing that the lightning current was safely led to a grounding electrode through the outer down conductor.In another case shown in Fig. 7, the lightning struck backward of the wind turbine.

Thus, the result of the impulse test demonstrated that the proposed two ring-shaped electrode system is effective for lightning protection for electrical and electronic devices in a wind turbine.On the other hand, the result shown in Fig. 8 is an example of an unsuccessful case. In this case, where the gap between two rings, g, is less than the distance between the upper ring and the nacelle d, spark-over often tends to occur between the upper ring and the nacelle and the lightning current unexpectedly rushes to the inside or surface of the nacelle and the tower.

This may cause the breakdown or burnout of low-voltage and control circuits installed in a wind turbine. Moreover,Fig. 9 shows another type of an unsuccessful result,where the lightning directly struck backward of the nacelle in spite of the receptor on the blade.The graphs shown in Fig. 10 summarize the aboveresults. Graphs of positive and negative lightning currents clarify that the gap for the two rings should be designed to be lower than the distance between the upper ring and the nacelle.

Although the result shows that a lower gap may make for a safer operation, the best solution needs to be considered from standpoints including the method of fixation, the weight of materials, installation costs and the effect of flexural oscillation of the rings during rotation of the blades.

　　4.对模型进行雷击测试