**1. Introduction**

Windmills have been around for centuries, operating as grain grinders and water pumps. The concept and technology behind windmills have been adapted to generate electricity, which in its new form is now called wind turbines (i.e., wind energy). Wind energy generation is now becoming one of the largest contributors to renewable energy generation, where the recent demand for renewable energy has seen its increasing growth in use as well as in physical size. In other words, wind turbines are getting taller, in order to accommodate the demand, by capturing wind through a larger blade swept area and converting it into electricity. Owing to this, wind turbines are now more prone to lightning strikes due to the fact of their increased structural height.

There are approximately 2000 thunderstorms at any given minute and about 100 lightning strikes per second worldwide [1]. This creates grea<sup>t</sup> risk for tall structures, such as wind turbines, to be struck by lightning, where the average electric current from a lightning return stroke is 30 kA [1]. This massive flow of current can heat up the leader channel air to between 25,000 ◦C and 30,000 ◦C (around five times the effective temperature of the sun) [1–3]. Lightning protection system (LPS) is composed of lightning receptor, down conductor, and grounding, and all elements must be well connected to pass the lightning current to Earth safely. Although wind turbines are installed with LPS, there are still cases where blades and whole turbines are destroyed due to the fact of lightning strikes. Considering the 20–25 year design life for wind turbine [3], it is worth safeguarding the turbines from lightning strikes, because the damage associated with it will cause the down time of the turbine operation, causing extra costs for maintenance and an shortage of electricity. This, therefore, may sugges<sup>t</sup> a need for improving the existing lightning protection systems for wind turbine blades.

#### **2. Lightning Discharges and Existing Lightning Protection Systems for Wind Turbine**

### *Mechanism of Lightning Discharge*

Lightning discharge from cloud to ground stems from a stepped leader initiated in a cloud and increases the electric field within its path. When a grounded object is in that electric field, it generates a leader towards the stepped leader, and it is called a connecting leader. If the downward moving leader has a negative charge, then the connecting leader is positive. If the downward leader is negative, then the connecting leader is positive [3].

As a stepped leader approaches ground level or the tip of the grounded structures, the electric field increases to such an extent that it discharges, and connecting leaders starts to propagate towards the downward leader in an attempt to connect, to equal the potential di fference. Taller structures generate longer connecting leaders due to the field enhancement caused by the accumulation of positive charge on the structure [1,3,4].

The stepped leader channel is at cloud potential, approximately 50 MV [1,3–8] and with the final connecting jump, a near of ground potential travels along the channel in the direction of the cloud, which is called return stroke. The flow of charge generates a large current with an average peak of 30 kA [1,3] to 80 kA [1]. Due to the rapid generation of heat of around 30,000 K [1,3] in the channel, a pressure is created of 10 atm or above [3]. In some instances, new charges from the cloud forming another electrical discharge called dart leaders, creating subsequent return strokes with an average peak current of about 10–15 kA [1,3].

Most negative cloud-to-ground flashes contain more than one stroke, generally 3–4 [1] and, in major cases, the first stroke is usually 2–3 [3] times larger than the following subsequent strokes. On the other hand, occasionally in multiple stroke flashes there is at least one subsequent stroke which is greater than the first return stroke [3].

### **3. Wind Turbine Blades and Its Protection Methods**

### *3.1. Wind Turbines and Blades*

There are two main types of wind turbines on the market nowadays: vertical axis and horizontal axis turbines. Due to the lower e fficiency, vertical axis turbines were not considered in this paper. Modern turbines are dominantly composed of horizontal axis models, since with rotor blade pitching, the speed of rotation and hence the power output can be controlled, and the blade aerodynamics can be optimized for maximum e fficiency. In most cases, the three-blade model is used as it has the highest efficiency in ratio of the number of blades and their overall weight.

At blade design, the actual shapes are very similar within commercial turbines, although, slightly di ffers by each manufacturer for the best possible aerodynamics according to company preferences [9]. Common characteristics are the hollow design, to reduce weight and the turnable rotor blade tip to help overspeed limitation [10]. Modern blades generally made of Fiber-Reinforces Composites such as carbon fiber and glass fiber with a matrix material of polyester resins or epoxy resins.

Carbon fiber generally has good braking and elasticity characteristics, with sti ffness not far from steel, although it is the most expensive material component among the possible choices. Also, in regards to lightning protection, it requires special considerations due to the fact of its material properties which are similar to a semiconductor, creating issues with lightning attachment and flashovers on the surface of the blade.

Glass fiber, on the other hand, has lower ratings in almost all the characteristics mentioned before, but it is considerably cheaper, and it acts as an insulator. Manufacturers tend to use it with more expensive but high-quality epoxy resins to enhance the required physical properties of it [10,11]. Although, the blade is nonconductive, it still attracts lightning due to the fact of its height; therefore, lightning protection is necessary.

### *3.2. Lightning Protection in General*

Lightning protection systems for wind turbines are based on International Electrotechnical Commission (IEC) IEC 61400-24. According to this standard, the lightning protection levels (LPLs) have been set in accordance with the probability of minimum and maximum expected lightning currents, I to IV. The maximum protection, LPL I levels should not be exceeded with a probability of 99% for negative flashes, meanwhile, for positive flashes it is below 10% [12]. The parameters for LPL II and III–IV are the reduced values of LPL I by 75% and 50%, respectively.

The rolling sphere method (RSM) was used to identify the locations of the air termination system on a given structure. The method assumes that there is a spherical region with a radius equal of the striking distance located around the tip of the oncoming lightning leader to a structure. Owing to that, the RSM method demonstrated on a wind turbine with 20 m radius (LPL I). This radius, *r*, is in relation to the peak current *I* of the first stroke. According to the IEEE, the equation is:

$$r = 10l^{0.65} \tag{1}$$

There are many di fferent proposals regarding the calculations of the radius for the rolling sphere in relation with the peak current, but the suggested values for each protection level are set by the standards [7] where for each LPL and radius, there is a corresponding minimum peak current value which, against the protection level, it gives protection.

### *3.3. Protection Methods for Blades*

There are four main types of lightning protection methods developed as recommended and outlined in IEC 61400-24 [7]. The methods are as follow:


Regardless of the methods, the main function [12–14] of the lightning protection on the blades is:


With insulator-based materials blades, such as glass fiber composites, the conductors can be placed outside of the blade to divert lightning from the blade surface, also, can be placed inside, with air-terminations at specific point outside of the blade. When carbon fiber composites are used, a layer of conducting material is placed over it which can then carry the current to the blade root. With both cases, sliding connectors are used to carry the current from the blade to the hub towards the ground [12–14].

For the earth termination and down conductors, it has to carry the lightning current safely to the ground where common materials are aluminum, steel, and copper. In general, air termination and for down conductor, the cross-section of at least 50 mm<sup>2</sup> is recommended [7,12–14].

### *3.4. Lightning Damage to Wind Turbines and Blades*

According to many field observations and studies [15–18], wind turbines receive significant amounts of lightning attachments during their designed lifetime, mostly on rotor blades. The damages caused mostly from unsuccessful attachments on air terminations or from induced voltages from electric and electromagnetic fields. The highest percentage of damages occurred on the control system, although, on some cases, the damage were simple interruptions. Meanwhile the damage caused on blades are 11%, it often corresponds with severe damage. The damages associated with lightning are generally blade rupturing and burnout, wire melting, surface cracking and delamination, lightning receptor vaporization, and loss [19–25].

The most popular lightning protection model used nowadays for large turbines consist of an internal down conductor and metal receptors or air terminators penetrating the surface of the blade to serve as desired attachment points. These two systems are then connected together inside of the blade to carry the lightning current to earth. The receptors are installed at nearby the tip of the blade or placed at equal distance from each other alongside the blade from the root to the tip.

One of the main issues with this type of protection is that since the receptors are small compared to the blade planform area, it decreases the efficiency of the attachment of the lightning, causing damage on the surface of the blade [21–25].

Considering the distribution of the lightning attachment and damage along the blade, it can be seen that majority of the attachment occurs at the tip, and the percentage decreases as the distance increasing from the end of the blade. As it can be seen, around 60% of the total damage was located in the last meter of the turbine blade, and 90% of the total damage occurred in the first4m[26].

Even though there are many different designs for the lightning protection of blades, there is still potential room for improvement. On the interception of the lightning to the air terminations to increase the effectiveness of the captured lightning flashes and on the down conduction part with the connections of different parts to conduct the current safely to earth.

### *3.5. Blade Model for Investigation*

The blade to be inspected was based on an existing model, currently the largest turbine on the market Vestas V164-9.5MW [9], at present, produced for offshore, although the company is in the process for an onshore model with similar dimensions [27]. For this study, the length of the blades was only considered for the simulation. Although their lightning protection systems are compliant with IEC 61400-24 standards, the exact lightning protection system employed by the blade's manufacturer is not available in the public domain. However, as briefly discussed in Section 3.3, any wind turbine blades should be protected and complied as per methods proposed by IEC 61400-24 standards [7]. For a structure this size, approximately with a tip height around 200 m above sea level, the number of strikes can be estimated considering the lightning density in Europe (between 0.1 and 42 flashes per year per km2) [5,7].

The height of structure greatly affects the number of flashes predicted on the structure. Based on the regular expected turbine lifetime, what is generally predicted to be 20–25 year, it is very likely that the turbine will be hit at least once during its lifetime. Without any protection, the blade will most likely be destroyed. If the base cost lies between GBP 0.6–0.8 million per MW for an onshore turbine, and generally around 13% of these blades are [28], therefore, the estimated price for losing one blade would be roughly GBP 300,000 on the aforementioned model, not calculating the replacement, transportation, and power outage caused costs. From this, it is clear that wind turbines require adequate protection against lightning strike nevertheless of their location, since even if it is estimated with the lowest density, over the expected lifetime the turbine will be struck at least one or two times.

### **4. ANSYS Workbench Implementation**

### *4.1. ANSYS Workbench*

Nowadays, engineering problems are becoming genuinely complex, relying only on theory, and. physical experiments are not practical anymore. Furthermore, deriving those with hand calculations are rather complex and time consuming. Analysis Systems (ANSYS) is one of the most reputable engineering software analysis packages available on the market and is used by many companies and research facilities around the world. The software is based on finite element analysis (FEA) to solve complex problems in single or multiphysics environment.

The basic principle of the method is that the domain or object is divided into elements with discretization. The distribution of the elements is called mesh, and the points connecting the elements are nodes. When the mesh is generated, an equation is generated for each element regarding with the solvable physics or method of analyzation. The elemental equation is than assembled to a global equation to describe the behavior of the body as a whole [29].

### *4.2. Blade and Protection Implementation*

As briefly discussed in Sections 2 and 3, the wind turbine is a grounded structure, hence, the lightning current as a result of return stroke will then be passed safely to the ground through hub, nacelle, tower, and tower footing at the ground level. Hence, when lightning strike on the lightning receptor installed on a blade, the ground is elevated to the highest tip at the time of strike due to the blade tip being at its highest point at a time. Thus, this assumption is also used by many other lightning researchers around the world [1,3,4,8,10,14,15,19,20,26] and also for this study. Owing to that, single blade was examined without any attachment to rotor and nacelle. The ANSYS Workbench version 18.2 was used to carry out the simulation of the lightning protection of blade. The available software license was for Academic Research, which restricts the meshing node number to 300,000 which corresponds to around 40,000 elements depending on the meshing algorithm chosen. As it was mentioned earlier, the size base was taken from an existing model (Vestas V164-9.5 MW. The turbine had approximately 80 m long blades; in the model it was extended slightly to represent a potential future size. As shown in Figure 1, the hollow blade design can be seen from what was modelled in ANSYS DesignModeler. The model measurements were 85 m long, 5 m wide, 2.6 m depth, 10 times base-to-tip ratio, and 0.015 m wall thickness. The current was applied at point A, meanwhile, points B and C were specified as 0 V.

**Figure 1.** Wind turbine blade for simulation.

The blade material was chosen to be E-glass fiber-reinforced polyester with the necessary values set manually [30,31] to serve as an insulator-type blade, the lightning conductor was set to the copper parameters taken from standards [12] with a 50 mm<sup>2</sup> round cross-section as the minimal specified

area. For evaluation, one of the recommended method by IEC 61400-24 [7] was considered where this method was used previously for smaller turbines, although in this project, it was examined for larger turbine blade.
