By Justin McKennon
As the world moves to reduce its dependence on fossil fuels, the global market for wind turbines is growing, projected to reach around $70 billion dollars annually in the next few years. While wind power on such a scale is a great achievement, another powerful force of nature is preventing the industry from reaching its full potential: lightning.
Lightning strikes are the single largest cause of unplanned downtime in wind turbines, responsible not only for the loss of untold megawatts of power, but also for huge operation and maintenance costs.
Wind turbines are particularly susceptible to lightning strikes because of their great heights, exposed locations, and large rotating blades. Lightning can wreak havoc, both directly and indirectly, on virtually all wind turbine components, including blades, control systems, and other electrical components. Repair is not only expensive but also physically challenging, given the logistical constraints.
Figure 1. High-voltage generator.
Modeling and analytical engineers are actively involved in the committees that form the International Electrotechnical Commission (IEC), which defines the lightning levels and situations that blades must endure. Industry regulations, such as IEC 62305, require wind turbine manufacturers to incorporate lightning protection designs into their blades. For maximum protection, it’s essential to know how much lightning current is likely to flow through a blade following a lightning strike, and precisely where it will flow. The problem is that simple assumptions about the behavior of lightning current often lead to inaccurate conclusions.
Deep Insights into Lightning Current
One of the most complete lightning simulation laboratories in the world is located in an 18,000-square-foot facility in Pittsfield, MA, USA. The space features 14- and 25-foot tall lightning generators capable of generating as much as 2.4 MV (Figure 1).
Traditional wind turbine protection schemes consist of a surface protection layer (SPL) covering the lightweight, high-strength carbon fiber composite blades. Often, the SPL consists of a conductive mesh meant to safely carry lightning current from the point where it “attached to” (e.g., hit) the blade, and then from the root to the ground.
Figure 2. Geometry of the thin aluminum surface layer protection (SPL) placed on top of a carbon stack.
Understanding a carbon stack’s ability to carry various amounts of current, along with other factors such as likely attachment points and puncture possibilities, is no small task. Given the cost to physically test these blades, some of which are 70 or more meters long, the numerical modeling of lightning effects has become a crucial part of the design process.
Given the complexity of the physics involved, it’s easy to make the wrong assumptions, which can affect the accuracy of the models.
Simulation Reduces Overengineering
One common (but improper) assumption is that the carbon stack’s conductivity is the same in all directions, when in reality there could be significant differences in carbon’s conductivity along different directions.
Figure 2 shows the geometry of a carbon stack placed 5 mm below a 500-μm-thick SPL mesh made from an aluminum sheet, whose conductivity is set according to experimental measurements. The carbon’s conductivity is also set according to experimental values; both its idealized isotropic and realistic anisotropic behavior have been considered in the simulation model.
Figure 3. Simulation results showing that the amount of current in the SPL in the idealized isotropic case is significantly less than the realistic anisotropic case.
A time-domain wave equation for the magnetic vector potential was solved using multiphysics simulation. The results determined the associated currents, electric fields, and other values at those points, providing insight into the current’s overall behavior throughout the entire structure.
The isotropic case underestimates the amount of current traveling throughthe SPL, implying that more current is traveling in the carbon and not in the SPL (Figure 3). Carbon is made up of many layers of individual fibers. It is very conductive in the fiber direction, but getting current into and out of the carbon is very challenging. If too much current has to pass through an interface between the carbon and something else, many of the individual fibers in the carbon can be burned away through heating and/or arcing (Figure 4). Carbon bears the primary structural loads, and damage here greatly reduces the lifetime of the blade and, in some cases, can lead to catastrophic blade loss. More current in the carbon is something engineers want to seriously avoid.
Figure 4. Simulation results showing the current density on a sample wind turbine blade made of several carbon stacks.
The isotropic case grossly overestimates the amount of current in the carbon because it ignores the very real orientation-dependent resistances in the carbon (Figure 5). Such an overestimate would introduce additional challenges that are not present, thus slowing down the development process, and leading to an overengineered product.
Figure 5. A plot demonstrating the current levels in the isotropic and anisotropic carbon cases.
Reliable Results for Business Decisions
The ability to rapidly simulate and turn around models greatly reduces program risk, and allows for engineering level data to be obtained almost on-demand. In many cases, critical data simply cannot be measured on real test articles, which requires simulation and analysis to fill in these holes. Rather than spending considerable amounts of time and money fabricating complex test articles, simulation helps to analyze the physical phenomena, drastically reducing the problem scope for these projects.
Justin McKennon is the Chief Engineer and Manager of Engineering and Simulation at NTS Lightning Technologies. Justin also is the chair of the SAE Commercial Space Committee for the SAE. Justin has a Bachelor’s and Master’s Degree in Electrical Engineering from the University of Massachusetts Dartmouth. He has published dozens of papers on the use of simulation tools, and is widely regarded as an expert in electromagnetics and numerical analysis.