Page 10 - North American Clean Energy May/June 2019 Issue

wind power
Multiphysics Protects Wind Turbines when Lightning Strikes
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.
Modeling and analytical engineers are actively involved in the committees that form the International Electrotechnical Commission (IEC), which de nes 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 ow through a blade following
a lightning strike, and precisely where it will ow. e 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 Pitts eld, MA, USA. e 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 ber 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 e ects 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 a ect 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 signi cant di erences in carbon’s conductivity along di erent 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. e 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 signi cantly less than the realistic anisotropic case.
A time-domain wave equation for the magnetic vector potential was solved using multiphysics simulation. e results determined the associated currents, electric elds, and other values at those points, providing insight into the current’s overall behavior throughout the entire structure.
e isotropic case underestimates the amount of current traveling through the 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 bers. It is very conductive in the ber 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 bers in the carbon can be burned away through heating and/
or arcing (Figure 4). Carbon bears the primary structural
Figure 1. High-voltage generator.
10
MAY•JUNE2019 /// www.nacleanenergy.com