Effect of Wind Turbine Blade Rotation on Triggering Lightning: An Experimental Study

Compared with other lightning targets on the ground, the most notable feature of a wind turbine is that the blades are usually in a rotating state when lightning strikes. To study the mechanism of blade rotation influencing wind turbine on triggering lightning, lightning discharge comparison tests based on a typical 2-MW 1:30-scaled wind turbine model with an arching high-voltage electrode were conducted under different modes of stationary and rotating blades. Negative polarity switching impulses of 250/2500 μs were applied to the arching electrode. The up-and-down method was adopted for 50% discharge voltage and the discharge process was observed. The experimental results showed that under the condition of a 4 m gap, the breakdown voltage decreases and the connection point of the leaders approaches the high-voltage electrode with increasing blade speed, indicating that the wind turbine’s blade rotation enhances the triggering of lightning. The analysis showed that the blade rotation could be altering the charge distribution on the blade tip, resulting in varied ascending leader development on the blade tip, which affected the discharge development process.


Introduction
With the development of wind power generation technology, the capacity of wind turbine units is gradually increasing. The length of wind turbine blades has increased to over 80 m, and the height of the blade tip from the ground has been increased to over 220 m, which increases the probability of wind turbines being struck by lightning [1,2]. This increase in lightning activity is especially significant in places where winter lightning activity occurs as reported in Japan and Europe during the winter season [3,4]. Wind turbine blades are likely in a rotating state when lightning strikes, and the ion distribution around the blade tips is similar to that around static high towers in strong wind conditions. The corona current of a static high tower varies with wind speed [5,6]; therefore, the blade rotation may affect the process through which lightning is triggered. Rachidi et al. analyzed the similarities between rocket-triggered lightning and tall wind turbines under rotary conditions and suggested that the wind turbines might generate lightning on their own [7]. On the basis of our review of the literature on wind turbines, model-based testing of lightning discharge in laboratory settings and lightning strike observations at actual wind turbine farms have been the main research methods.
In laboratory settings, researchers have conducted many model tests of lightning discharge; although the lightning in model tests cannot completely simulate the characteristics of natural lightning, such tests have some similarities in discharging regulation and they can be reproduced; moreover, the results can provide some reference values [8][9][10]. Radičević and colleagues first investigated the influence of wind turbine blade rotation on the characteristics of atmospheric discharge [11][12][13].
The tests were conducted in a high-voltage laboratory, and the up-and-down method was applied to determine the 50% breakdown standard switching voltage. The results indicated that, with a 2 m air gap, breakdown voltages were up to approximately 16% higher for a rotating scaled model than for a stationary scale model. Furthermore, it was recommended that under thunderstorm conditions, the rotational speed of wind turbine blades should not be decreased in an attempt to reduce the number of direct strikes on the wind turbine.
At actual wind farms, Montanyà et al. observed lightning by using a 3D lightning mapping array and high-speed video recorder. The results showed that under certain thunderstorm conditions, wind turbine blades can generate electric discharges at regular intervals of 3 s depending on the rotational speed. This periodic effect was not observed in static towers, indicating that the rotating effect may play a critical role [14]. Wilson et al. conducted an experiment in a wind farm and analyzed cloud-to-ground lightning data. The results indicated that wind turbines with a rotating blade tip height of 125 m have a larger attractive radius (276 m) than the expected (160-200 m) radius of stationary towers of similar height and an attractive radius equivalent to that of a nearby 231 m high radio tower [15]. Wang et al. used various lightning observation instruments to observe a wind turbine struck by lightning and a neighboring lightning protection tower; a comparison with a stationary windmill or tower of similar height revealed that the rotating windmill tended to have a higher chance of initiating an ascending leader. Thus, when a thunderstorm is overhead, the rotation of the windmill should be stopped [16].
In summary, the effect of rotation on the enhancement of lightning has been discussed and investigated. According to the scaled model test, the blade rotation weakens the lightning-triggering ability of wind turbines. However, this finding is inconsistent with observations at actual wind farms. Thus, there is no clear evidence that the number of lightning flashes increases significantly with the effect of rotation, and research on the influence of wind turbine blade rotation on lightning behavior needs to proceed further.
The present experimental study investigated the effect of rotation blades on triggering lightning in a wind turbine with a long air gap. Discharge tests in long air gaps (2 and 4 m) were conducted by applying a negative impulse of 250/2500 µs. The effect of blade rotation on the characteristics of lightning discharge and the breakdown path were investigated. Finally, the physical interpretation of the blade rotation effect on the lightning discharge process was analyzed to elucidate the influence of blade rotation on triggering lightning.

Test System Arrangement
Simulation tests of more than 400 lightning strikes were conducted outdoors in the ultra-high-voltage alternating current test base in China. Negative switching impulses of 250/2500 µs were obtained from a 7.5 MV, 750 kJ, 25-stage impulse generator. The discharge voltages were measured using a capacitive divider. The practical test field is presented in Figure 1.

Experimental Set-Up
The experimental set-up is presented in Figure 2. The switching impulses were applied to a high-voltage electrode to simulate a descending lightning leader. A typical 1:30-scale 2-MW wind turbine model was adopted in this experiment. The gap distances between the high-voltage electrode and wind turbine blade tip were 2 and 4 m. According to the theory of similarity, the distances of 2 and 4 m correspond to striking distances of 60 and 120 m, respectively. The discharge path was recorded using a camera (Canon 5D MarkII, Tokyo, Japan), and the breakdown voltages were measured using an oscilloscope (Yokogawa DLM2054, Tokyo, Japan).

High-Voltage Electrode
The high-voltage electrode selection is critical for investigating the influence of wind turbine blade rotation. To ensure that the distance between the rotation blade tip and high-voltage electrode was consistent, a special arching electrode was designed. The arching electrode mainly comprised a steel tube and a steel framework for preventing deformation of the steel tube. The steel tube was 159 mm in diameter and 5 mm in thickness. Considering an angle of 120° for each blade and the flashovers caused by the border effect, the arching electrode adopted an angle of 150°. Moreover, a reverse circular arc of 110° was arranged at the edge of the electrode. For the 2 m air gap, the radius of the arching electrode was 3.25 m, and the total length and weight exceeded 10 m and 260 kg, respectively. For the 4 m air gap, the radius of the arching electrode was 5.25 m, and the total length and weight exceeded 17 m and 700 kg, respectively. This design facilitated the generation of a quasi-uniform field above the blade by the arching electrode, which facilitated the development of streamers from the wind turbine model.

High-Voltage Electrode
The high-voltage electrode selection is critical for investigating the influence of wind turbine blade rotation. To ensure that the distance between the rotation blade tip and high-voltage electrode was consistent, a special arching electrode was designed. The arching electrode mainly comprised a steel tube and a steel framework for preventing deformation of the steel tube. The steel tube was 159 mm in diameter and 5 mm in thickness. Considering an angle of 120° for each blade and the flashovers caused by the border effect, the arching electrode adopted an angle of 150°. Moreover, a reverse circular arc of 110° was arranged at the edge of the electrode. For the 2 m air gap, the radius of the arching electrode was 3.25 m, and the total length and weight exceeded 10 m and 260 kg, respectively. For the 4 m air gap, the radius of the arching electrode was 5.25 m, and the total length and weight exceeded 17 m and 700 kg, respectively. This design facilitated the generation of a quasi-uniform field above the blade by the arching electrode, which facilitated the development of streamers from the wind turbine model.

High-Voltage Electrode
The high-voltage electrode selection is critical for investigating the influence of wind turbine blade rotation. To ensure that the distance between the rotation blade tip and high-voltage electrode was consistent, a special arching electrode was designed. The arching electrode mainly comprised a steel tube and a steel framework for preventing deformation of the steel tube. The steel tube was 159 mm in diameter and 5 mm in thickness. Considering an angle of 120 • for each blade and the flashovers caused by the border effect, the arching electrode adopted an angle of 150 • . Moreover, a reverse circular arc of 110 • was arranged at the edge of the electrode. For the 2 m air gap, the radius of the arching electrode was 3.25 m, and the total length and weight exceeded 10 m and 260 kg, respectively. For the 4 m air gap, the radius of the arching electrode was 5.25 m, and the total length and weight exceeded 17 m and 700 kg, respectively. This design facilitated the generation of a quasi-uniform field above the blade by the arching electrode, which facilitated the development of streamers from the wind turbine model.

Scaled Wind Turbine Model
The three-blade scaled wind turbine model used in the experiment was a modified version of a small commercial wind turbine. To meet the experimental requirements, several aspects of the wind turbine were modified including wind turbine blade, power transmission module, and support component.

Wind Turbine Blade
The blades were made of a glass fiber-reinforced polymer composite insulating material and had a length of 1.25 m. Installing a lightning protection system (LPS) on the blades can effectively ground the lightning current and decrease the risk of lightning damage to the blade. Specifically, when the lightning discharge strikes the receptor on one of the blades, a surge current propagates through a down conductor over the blade, metallic cabin, bearings, and metallic tower to the grounding system.
The experiment adopted a typical LPS comprising the following: The three-blade scaled wind turbine model used in the experiment was a modified version of a small commercial wind turbine. To meet the experimental requirements, several aspects of the wind turbine were modified including wind turbine blade, power transmission module, and support component.

Wind Turbine Blade
The blades were made of a glass fiber-reinforced polymer composite insulating material and had a length of 1.25 m. Installing a lightning protection system (LPS) on the blades can effectively ground the lightning current and decrease the risk of lightning damage to the blade. Specifically, when the lightning discharge strikes the receptor on one of the blades, a surge current propagates through a down conductor over the blade, metallic cabin, bearings, and metallic tower to the grounding system.
The experiment adopted a typical LPS comprising the following:

Power Transmission Module
To ensure that the line speed of the scaled wind turbine blade tips was consistent with that of the actual operational wind turbine blade tips, a power transmission module was designed in this study, as shown in Figure 3b. The continuous speed regulation of the three-phase asynchronous motor was realized using a frequency converter; the asynchronous motor drives the wind turbine blades to rotate through the belt drive. A three-phase induction motor was placed in a Faraday cage near the top of the tower. The power of the motor was 2.2 kW, and the belt length was 1 m. To ensure the safety of the external power source, a surge protector was connected in parallel with the input of the frequency converter. Equation (1) presents the formula for simulating the line speed of the actual wind turbine blade tips: where v (m/s) is the line speed of the blade tips, w (r/min) is the rotational speed of the wind turbine blades, and l (m) is the length of the blade. When an actual wind turbine was operated with a rated power of 2 MW, the line speed of the turbine blade tips was approximately 60 m/s. Therefore, when the wind turbine model simulated the rated operating condition of the actual wind turbine, the rotational speed w was 450 r/min.

Power Transmission Module
To ensure that the line speed of the scaled wind turbine blade tips was consistent with that of the actual operational wind turbine blade tips, a power transmission module was designed in this study, as shown in Figure 3b. The continuous speed regulation of the three-phase asynchronous motor was realized using a frequency converter; the asynchronous motor drives the wind turbine blades to rotate through the belt drive. A three-phase induction motor was placed in a Faraday cage near the top of the tower. The power of the motor was 2.2 kW, and the belt length was 1 m. To ensure the safety of the external power source, a surge protector was connected in parallel with the input of the frequency converter. Equation (1) presents the formula for simulating the line speed of the actual wind turbine blade tips: where v (m/s) is the line speed of the blade tips, w (r/min) is the rotational speed of the wind turbine blades, and l (m) is the length of the blade. When an actual wind turbine was operated with a rated power of 2 MW, the line speed of the turbine blade tips was approximately 60 m/s. Therefore, when the wind turbine model simulated the rated operating condition of the actual wind turbine, the rotational speed w was 450 r/min.

Support Component
To ensure the stability of the rotating wind turbine, a support structure was designed in this study. The tower comprised a seamless steel tube with a height of 8 m and diameter of 245 mm. The cross angle steel composed the base of the wind turbine model.

Selection of Applied Waveform
A reasonable voltage waveform in a scaled wind turbine discharge test can reflect the characteristics of actual wind turbine discharge processes. Many researchers investigating the lightning discharge of ground objects have extensively applied a short-head-time (20-250 µs) switching impulse to prevent a step leader from forming in the discharge process [8,10]. And according to IEC61400-24 [17], the 250/2500 µs switching impulse waveform was recommended to achieve the lightning attachment manners of wind turbine blades. In the present study, the following parameters were considered for selecting the voltage waveform: the pause time between step leaders and the average electric field variation of objects on the ground.
The average electric field variation of objects on the ground can be obtained through electric field simulations. The applied waveform was selected to ensure that the average electric field variation caused by the impulse voltage was consistent with the electric field variation at the corona inception moment under natural conditions. Based on this method, the impulse voltage waveform was selected as 320/2500 µs in a study by Xie et al. [18] and 200/2000 µs in a study by Li et al. [19]. In the present study, the applied waveform was adopted from [19], and the variation under natural circumstances was calculated using Dellera's leader progression model [20]. The settings of the simulation parameters are described as follows.

Thundercloud
The simulation parameters of the thundercloud are as follows [19,21]: • Suppose a unipolar negative charge (simplified condition) with a uniformly distributed extension of 10 km at a height of 2 km. • Suppose an equivalent unipolar charge of 8 C.

Descending Leader
For the method of describing the descending leader's channel, the charge density distribution equation proposed by Cooray and colleagues [20] was adopted, which is given by in which where z is the height of the descending leader tip in meters, H c is the height of the cloud in meters, I p is the return stroke peak current in kilo-amperes, and ρ s is the charge density in Coulomb per meter. The remaining coefficients were selected as follows: a 0 = 1.476 × 10 −5 , α = 4.857 × 10 −5 , b = 3.9097 × 10 −6 , c = 0.522, and d = 3.73 × 10 −3 . The speed of the descending leader was 2 × 10 5 m/s.

Calculation of Electric Field on the Receptor Surface
According to the aforementioned considerations and assumptions, the field variation (S c in kV/m/s) of the receptor on the blade tip at the corona inception moment was calculated using Equation (3) [19]: where t peek is the corona inception moment in the receptor on the blade tip and ∆t = 8 µs.
The finite element software COMSOL Multiphysics 5.0 (COMSOL, Stockholm, Sweden) was used to calculate the electric field, and the simulation steps of the descending leader were set using MATLABR2014a (MathWorks, Natick, MA, USA). The simulation parameters in the model were as follows: • For the scaled wind turbine model, a simplified model based on these parameters was arranged using the finite element software. The average field variation (S e in kV/m/s) at the receptor in the scaled model test was calculated using Equation (4) [19]: where E 90% and E 10% are the electric field strengths of the receptor in the scaled model when the arching electrode potential is 90% and 10% of the impulse waveform peak, respectively; t 90% and t 10% are the times at which the arching electrode potential is equal to 90% and 10% of the impulse waveform peak, respectively; and S c and S e are obtained from Equations (3) and (4). The field variations at various wavefront times are presented in Table 1. A comparison of the average electric field variation under natural lightning (S c ) conditions with that under various waveform front times (S e ) revealed that the negative 250/2500 µs switching impulse was the most similar to natural lightning among the four waveforms. This waveform was recommended by IEC61400-24 [17] for lightning protection tests of the wind turbine. Therefore, a negative voltage waveform of 250/2500 µs was selected for the test in the present study.

Test Procedure
The 50% breakdown voltage (U 50% ) was obtained through the up-and-down method with a 3%-4% difference in the expected discharge voltage between steps, and 40 strokes were applied for each arrangement. The scaled model test was conducted under the following three wind turbine operating conditions:

Test Data of Various Arrangements
All the tests in the present study were performed outdoors for a duration of one week in May 2016. Under all test conditions, the discharge struck the receptor directly. The test data were normalized using Equation (5): where δ is the atmospheric correction factor to eliminate the effects of temperature, humidity, and pressure during the discharge processes [22].
During the tests, the temperature ranged from 21.4 to 31.6 °C, the relative humidity ranged from 52% to 80%, and the atmospheric pressure ranged from 99.8 to 100.9 kPa. The test results are presented in Table 2. Here, σU50% and σTf represents discharge dispersivity, which equals the value of the standard deviation divided by the U50% and Tf of each arrangement, respectively.  Table 2, the U50% and Tf ratios under various rotating speeds are shown in Table 3. From Table 3, the U50% ratio was plotted to obtain the relationship

Test Data of Various Arrangements
All the tests in the present study were performed outdoors for a duration of one week in May 2016. Under all test conditions, the discharge struck the receptor directly. The test data were normalized using Equation (5): where δ is the atmospheric correction factor to eliminate the effects of temperature, humidity, and pressure during the discharge processes [22]. During the tests, the temperature ranged from 21.4 to 31.6 • C, the relative humidity ranged from 52% to 80%, and the atmospheric pressure ranged from 99.8 to 100.9 kPa. The test results are presented in Table 2. Here, σ U50% and σ Tf represents discharge dispersivity, which equals the value of the standard deviation divided by the U 50% and T f of each arrangement, respectively.

Influence of Wind Turbine Blade Rotation on the Characteristics of Atmospheric Discharges
Derived from the U 50% and T f values in Table 2, the U 50% and T f ratios under various rotating speeds are shown in Table 3. From Table 3, the U 50% ratio was plotted to obtain the relationship between U 50% and the rotational speed in different air gaps and is shown in Figure 5.     Figure 5 show that with an increase in blade rotation speed, the U50% between the blade tip and arching electrode presents different trends for these two air gaps. In addition, no marked regularity was observed in Tf. Furthermore, the effect of blade rotation on the gap breakdown characteristic is detailed in the following.
For the 2 m air gap, compared with the static condition of the wind turbine, U50% increased slightly by 2.76% (medium blade speed) and 5.31% (rated blade speed). For the 4 m air gap, as the blade rotation speed increased, the U50% variation regularity differed from that in the 2 m air gap arrangement. Compared with the static wind turbine condition, the average U50% value decreased slightly by 1.04% (medium blade speed) and 2.74% (rated blade speed). In addition, as the blade rotation speed increased, Tf increased slightly by 0.32% (medium blade speed) and 3.95% (rated blade speed).

Breakdown Path Characteristics
In all arrangement cases, the discharge development path was recorded through long-time camera exposure. The typical discharge process of the rotating wind turbine with the 4 m gap is presented in Figure 6. Moreover, natural lightning striking rotating wind turbine blades in Japan is illustrated in Figure 7b [23]. The morphological characteristics of the breakdown path presented in Figure 6d are similar to those in Figure 7, indicating that the laboratory test could reflect the process of natural lightning striking the rotating wind turbine to a certain extent.
Through the analysis of a typical breakdown path, the method for determining the leader connection point is proposed as follows. According to the theory of discharge in long air gaps, streamer discharges develop from the head of the leader channel. Therefore, the development of ascending and descending streamer branches was observed proximal to the connection point of the leaders. In the interval between the two streamers, the apparent bending parts of the breakdown path represented the leader connection point.   Figure 5 show that with an increase in blade rotation speed, the U 50% between the blade tip and arching electrode presents different trends for these two air gaps. In addition, no marked regularity was observed in T f . Furthermore, the effect of blade rotation on the gap breakdown characteristic is detailed in the following.
For the 2 m air gap, compared with the static condition of the wind turbine, U 50% increased slightly by 2.76% (medium blade speed) and 5.31% (rated blade speed). For the 4 m air gap, as the blade rotation speed increased, the U 50% variation regularity differed from that in the 2 m air gap arrangement. Compared with the static wind turbine condition, the average U 50% value decreased slightly by 1.04% (medium blade speed) and 2.74% (rated blade speed). In addition, as the blade rotation speed increased, T f increased slightly by 0.32% (medium blade speed) and 3.95% (rated blade speed).

Breakdown Path Characteristics
In all arrangement cases, the discharge development path was recorded through long-time camera exposure. The typical discharge process of the rotating wind turbine with the 4 m gap is presented in Figure 6. Moreover, natural lightning striking rotating wind turbine blades in Japan is illustrated in Figure 7b [23]. The morphological characteristics of the breakdown path presented in Figure 6d are similar to those in Figure 7, indicating that the laboratory test could reflect the process of natural lightning striking the rotating wind turbine to a certain extent. After the leader connection point was determined, the length of the ascending and descending leaders in each arrangement of lightning discharge tests was measured using Autodesk Design Review 2013 (Autodesk, San Rafael, CA, USA). As shown in Figure 6b, the ascending and descending leaders developed from the arching electrode and blade receptor, respectively, and terminated at the leader connection point. The breakdown path is depicted using a multiline segment. The red line segment represents the descending leader and the blue line segment represents the ascending leader. The ascending and descending leader lengths at various blade rotation speeds under the gap breakdown conditions are presented in Table 4.  After the leader connection point was determined, the length of the ascending and descending leaders in each arrangement of lightning discharge tests was measured using Autodesk Design Review 2013 (Autodesk, San Rafael, CA, USA). As shown in Figure 6b, the ascending and descending leaders developed from the arching electrode and blade receptor, respectively, and terminated at the leader connection point. The breakdown path is depicted using a multiline segment. The red line segment represents the descending leader and the blue line segment represents the ascending leader. The ascending and descending leader lengths at various blade rotation speeds under the gap breakdown conditions are presented in Table 4. Through the analysis of a typical breakdown path, the method for determining the leader connection point is proposed as follows. According to the theory of discharge in long air gaps, streamer discharges develop from the head of the leader channel. Therefore, the development of ascending and descending streamer branches was observed proximal to the connection point of the leaders. In the interval between the two streamers, the apparent bending parts of the breakdown path represented the leader connection point.
After the leader connection point was determined, the length of the ascending and descending leaders in each arrangement of lightning discharge tests was measured using Autodesk Design Review 2013 (Autodesk, San Rafael, CA, USA). As shown in Figure 6b, the ascending and descending leaders developed from the arching electrode and blade receptor, respectively, and terminated at the leader connection point. The breakdown path is depicted using a multiline segment. The red line segment represents the descending leader and the blue line segment represents the ascending leader. The ascending and descending leader lengths at various blade rotation speeds under the gap breakdown conditions are presented in Table 4.  Table 4, the relationship between the leader length and rotational speed with various receptors was plotted (Figure 8), and the effect of blade rotation on the leader length was analyzed in detail.  From the results in Table 4, the relationship between the leader length and rotational speed with various receptors was plotted (Figure 8), and the effect of blade rotation on the leader length was analyzed in detail. For the 2 m air gap, compared with the static wind turbine condition, the length of the ascending leader decreased slightly by 1.21% (medium blade speed) and 3.61% (rated blade speed), and that of the descending leader increased slightly by 4.08% (medium blade speed) and 12.24% (rated blade speed). For the 4 m air gap, as the blade rotation speed increased, the leader length variation regularity differed from that in the 2 m air gap arrangement. Compared with the static wind turbine condition, the length of the ascending leader increased slightly by 1.54% (medium blade speed) and For the 2 m air gap, compared with the static wind turbine condition, the length of the ascending leader decreased slightly by 1.21% (medium blade speed) and 3.61% (rated blade speed), and that of the descending leader increased slightly by 4.08% (medium blade speed) and 12.24% (rated blade speed). For the 4 m air gap, as the blade rotation speed increased, the leader length variation regularity differed from that in the 2 m air gap arrangement. Compared with the static wind turbine condition, the length of the ascending leader increased slightly by 1.54% (medium blade speed) and 2.77% (rated blade speed), and that of the descending leader decreased slightly by 6.73% (medium blade speed) and 9.61% (rated blade speed).

Summary of the Test Results
To verify the objectiveness of the test results, they were compared with those of a study by Radičević [13]. In Radičević's study, the test was conducted under three wind turbine operating conditions (v = 0 m/s with one blade straight up, v = 31 m/s, and v = 50 m/s; v is the linear velocity of the blade tip, which was calculated using Equation (1)) with the same impulse voltage waveform and test method used in the present study. As the rotation speed increased, the U 50% of the scaled model increased by 2.85% (medium blade speed) and 9.41% (rated blade speed) relative to that of the stationary model, as shown in Figure 9a. In the present study, for the 2 m gap, three wind turbine operating conditions were selected (v = 0 m/s with one blade straight up, v = 33 m/s, and v = 60 m/s). As the rotation speed increased, the U 50% of the rotating scaled model increased by 6.17% (medium blade speed) and 9.39% (rated blade speed) relative to that of the stationary model. The results in Table 2 are plotted in Figure 9b. In conclusion, the results of the present study exhibited a similar trend to those in the study of Radičević; specifically, when the blades were rotating, an increase in U 50% was recorded, and the connection points of the leaders from the arching electrode and corresponding blade approached the blade tip. 2.77% (rated blade speed), and that of the descending leader decreased slightly by 6.73% (medium blade speed) and 9.61% (rated blade speed).

Summary of the Test Results
To verify the objectiveness of the test results, they were compared with those of a study by Radičević [13]. In Radičević's study, the test was conducted under three wind turbine operating conditions (v = 0 m/s with one blade straight up, v = 31 m/s, and v = 50 m/s; v is the linear velocity of the blade tip, which was calculated using Equation (1)) with the same impulse voltage waveform and test method used in the present study. As the rotation speed increased, the U50% of the scaled model increased by 2.85% (medium blade speed) and 9.41% (rated blade speed) relative to that of the stationary model, as shown in Figure 9a. In the present study, for the 2 m gap, three wind turbine operating conditions were selected (v = 0 m/s with one blade straight up, v = 33 m/s, and v = 60 m/s). As the rotation speed increased, the U50% of the rotating scaled model increased by 6.17% (medium blade speed) and 9.39% (rated blade speed) relative to that of the stationary model. The results in Table 2 are plotted in Figure 9b. In conclusion, the results of the present study exhibited a similar trend to those in the study of Radičević; specifically, when the blades were rotating, an increase in U50% was recorded, and the connection points of the leaders from the arching electrode and corresponding blade approached the blade tip. The results differed markedly from those of the 4 m air gap test; in that test, as the blade rotation speed increased, the U50% between the corresponding blade tip and high-voltage electrode exhibited an increasing trend and the leader connection point approached the arching electrode, which means that the ascending leader developed more adequately.
A comparison of the test results at the different gap lengths revealed that the trend in U50% and the connection points of leaders varied as the blade rotation speed increased, indicating that the physical lightning discharge processes at different gap lengths vary under different blade rotation conditions. The specific situation is analyzed in the following section.

Effect of Blade Rotation on the Lightning Discharge Process
In the standard atmospheric environment, if the initial field strength is 10 kV/cm, the average speed of electrons in the air gap is approximately 0.1 m/μs, whereas that of the positive ions is approximately 10 −4 m/μs. In the present study, the maximum speed of the blade tip was The results differed markedly from those of the 4 m air gap test; in that test, as the blade rotation speed increased, the U 50% between the corresponding blade tip and high-voltage electrode exhibited an increasing trend and the leader connection point approached the arching electrode, which means that the ascending leader developed more adequately.
A comparison of the test results at the different gap lengths revealed that the trend in U 50% and the connection points of leaders varied as the blade rotation speed increased, indicating that the physical lightning discharge processes at different gap lengths vary under different blade rotation conditions. The specific situation is analyzed in the following section.

Effect of Blade Rotation on the Lightning Discharge Process
In the standard atmospheric environment, if the initial field strength is 10 kV/cm, the average speed of electrons in the air gap is approximately 0.1 m/µs, whereas that of the positive ions is approximately 10 −4 m/µs. In the present study, the maximum speed of the blade tip was approximately 0.6 × 10 −4 m/µs [24]. Therefore, the electron speed was higher than the blade tip speed by four orders of magnitude, and the influence of the distribution of electrons under blade rotation can be ignored. However, the maximum speed of the blade tip and the positive ion speed were of the same order of magnitude, indicating that blade rotation influenced the distribution of positive ions. The lightning discharge process of the wind turbine blades under stationary and rotating conditions is analyzed in detail as follows.
As shown in Figure 10, a negative 250/2500 µs waveform was applied to the high-voltage electrode, and the blade tip presented relatively positive. When the blade rotation speed was w = 0, air ionization occurred because of the high electric field strength. Because of the high drift velocity of the ionized electrons, the electrons rapidly entered the receptor of the blade tip. Positive ions remained near the blade tip and were distributed along the highest electric field with a drift velocity (µE). When the blade rotation speed was w > 0, the distribution of the ionized positive ions near the blade tip was influenced simultaneously by the electric field and blade rotation. Because of the blade rotation, the corona charge q c was distributed in layers with a density of ρ c above the blade tip. The discharge process was altered by the corona in two aspects. First, the electric field strength (E ρc2 ) of the tip area was weakened by the positive ion region, thereby inhibiting the formation of new coronas and leaders. By contrast, the outside electric field strength (E ρc1 ) was enhanced outside the positive ion region, which was conducive to the development of the ascending leader. The lightning discharge process of the wind turbine blades under stationary and rotating conditions is analyzed in detail as follows. As shown in Figure 10, a negative 250/2500 μs waveform was applied to the high-voltage electrode, and the blade tip presented relatively positive. When the blade rotation speed was w = 0, air ionization occurred because of the high electric field strength. Because of the high drift velocity of the ionized electrons, the electrons rapidly entered the receptor of the blade tip. Positive ions remained near the blade tip and were distributed along the highest electric field with a drift velocity (μE). When the blade rotation speed was w > 0, the distribution of the ionized positive ions near the blade tip was influenced simultaneously by the electric field and blade rotation. Because of the blade rotation, the corona charge qc was distributed in layers with a density of ρc above the blade tip. The discharge process was altered by the corona in two aspects. First, the electric field strength (Eρc2) of the tip area was weakened by the positive ion region, thereby inhibiting the formation of new coronas and leaders. By contrast, the outside electric field strength (Eρc1) was enhanced outside the positive ion region, which was conducive to the development of the ascending leader.  When the gap distance was short (2 m in present test), the main effect was that the positive space charge altered the discharge process by increasing the outside electric field strength (Eρc1). When the blade rotation speed was w > 0, the positive space charge density decreased, and the enhancement effect of the outside electric field strength (Eρc1) was mitigated. Therefore, with the increase in blade rotation speed, it was more difficult for ascending leaders to develop, the leader length became shorter, and the breakdown voltage was higher. When the gap distance was long (4 m in present test), the enhancement effect of the positive space charge accumulation on the outside electric field strength (Eρc1) was clearly decreased. Under this condition, the main effect was that the positive space charge  [14]). E ρ : electric field strength of the gap; E ρc1 : electric field strength outside of the tip area; E ρc2 : electric field strength of the tip area; E w=0 : overall intensity of the applied electric field under w = 0 r/min; E w>0 : overall intensity of the applied electric field under w > 0 r/min; ρ c : charge density near the blade tip; v: the drift velocity of the positive ions.
When the gap distance was short (2 m in present test), the main effect was that the positive space charge altered the discharge process by increasing the outside electric field strength (E ρc1 ). When the blade rotation speed was w > 0, the positive space charge density decreased, and the enhancement effect of the outside electric field strength (E ρc1 ) was mitigated. Therefore, with the increase in blade rotation speed, it was more difficult for ascending leaders to develop, the leader length became shorter, and the breakdown voltage was higher. When the gap distance was long (4 m in present test), the enhancement effect of the positive space charge accumulation on the outside electric field strength (E ρc1 ) was clearly decreased. Under this condition, the main effect was that the positive space charge altered the discharge process by reducing the electric field strength (E ρc2 ) of the tip area, thereby inhibiting the formation of new coronas and leaders. When the blade rotation speed was w > 0, the positive space charge density decreased, ascending leaders developed more easily, the ascending leader length was longer, and the breakdown voltage was lower.
The test results and theoretical analysis demonstrated that when the wind turbine blades were rotating, the positive space charge distribution at the tip area differed from those observed under the stationary condition. The blade rotation altered the breakdown voltage and ascending leader length and influenced the attached lightning capacity of the wind turbine. The effect of the blade rotation on the attached lightning capacity varied with the gap distance.

Conclusions
This paper presents the lightning discharge behavior in rotating wind turbines, which was determined through scaled model tests with 2 m and 4 m air gaps. The test results reflected some discharge behaviors that occur in actual lighting discharge processes. On the basis of the findings, the following conclusions can be drawn:

1.
In the 2 m air gap test, with an increase in the blade rotation speed, the breakdown voltage between the blade tip and arching electrode presented a slightly increasing trend, and the connection points of leaders from the arching electrode and rotating blade moved slightly closer to the blade tip.

2.
In the 4 m air gap test, with an increase in the blade rotation speed, the breakdown voltage between the blade tip and arching electrode presented a slightly decreasing trend. Moreover, the ascending leader developed more adequately, and the connection points of leaders from the arching electrode and rotating blade moved slightly closer to the arching electrode.

3.
In the 4 m air gap test, the blade rotation altered the charge distribution on the blade tip region and favored the initiation of lightning, which is in agreement with observations at actual wind farms. However, for the 2 m air gap test, a small-scale test was insufficient for extrapolating the test results to practical cases because of the nonlinear discharge characteristics of long air gaps.
To obtain more practical results, large-scale tests are warranted.

4.
It is reasonable to consider that there is a difference in the triggering of lightning depending on whether the blades are rotating. Numerical software methods must be applied to account for blade rotation for predicting possible attachment points on wind turbines.