No operator wants to deal with the consequences of a wind turbine fire. But the fact remains that fire mitigation costs money. Operators must decide whether it’s worth investing in fire safety products, taking into consideration their likelihood of experiencing a fire.
One of the best ways to do that is to identify the turbines in a fleet that carry the greatest risk. That way, operators can maximize their investment in fire protection by concentrating on high risk sites.
Fortunately, organizations like DNV GL have studied the cause and effect of wind turbine fires. New data from DNV GL, published in 2019, identifies design features in wind turbines that may increase fire risk. We discuss some of the turbine features operators can look out for in their fleet.
In a study of 61 turbine fires, DNV GL found that fires are more common in turbines with uptower transformers. In fact, more than half of all the fires studied occurred in turbines manufactured by two OEMs. Although the study does not name the OEMs, it does note that both offer turbines with transformers uptower.
The reason that uptower transformers increase fire risk is because transformers require medium voltage. “Medium voltage uptower is a risk factor,” explains Sally Wright, Principal Wind Turbine Engineer at DNV GL.
Moreover, uptower transformers increase the risk that a fire will be catastrophic. Fires that start uptower typically destroy the entire nacelle and even parts of the tower. By contrast, downtower fires can often be contained. Downtower fires damage equipment and can cause smoke damage, but they don’t typically result in complete loss of the turbine.
Many modern turbines feature carbon blades. Turbine manufacturers introduced carbon blades, because they are lightweight. That’s a benefit during both construction and operation of the turbine. Typically, lighter blades increase turbine output and extend the turbine’s working life. They can also withstand higher wind loads.
However, carbon is a conductor of electricity. That’s a problem when it comes to lightning protection. Given the height of wind turbines, lightning strikes are relatively common. Turbine manufacturers must be able to retrofit lightning protection systems, as they introduce new blade designs. That’s a formidable design challenge. It can be done, but operators should be vigilant about quality control when dealing with new blade designs. It’s important to recognize that carbon blades increase fire risk associated with lightning.
Speaking of lightning, the best way to protect against lightning strikes is to provide a path to the ground. When a turbine gets struck by lightning, the current must travel through the turbine and into the ground. It’s safest to give the current a continuous path to the ground. Most modern turbine designs feature a continuous path.
However, some designs still have spark gaps or non-continuous paths. Lightning protection systems with spark gaps do not perform as well and are more difficult to maintain. All lightning protection systems require some maintenance to protect against fires. However, it’s important to understand the benefits and drawbacks of different lightning protection systems.
Splices in Cabling
Turbine towers require hundreds of feet of cabling. Sometimes, multiple sections of cabling must be spliced together during construction. Any time you have a splice, you introduce the potential for electrical arcing. Bad splices lead to high resistance connections, which produce excessive heat and can cause fires. Fires in electrical cabling are not particularly common, because cabling material does not catch fire very easily. Melting is a lot more common.
To prevent heat damage and fires in cabling, minimize splicing where possible. Avoid pre-cabling sections of the turbine tower and splicing them together. Instead, assemble the sections of the tower during construction, then run one continuous cable.
If you have a site where bad splices are a problem, you can use infrared detection to identify hot spots. You can also install arc fault detection, which will open breakers early, before there’s enough heat to start a fire. If the bad splice can be contained in a small area, you may be able to implement a gaseous fire suppression system, targeting that particular part of the cable. However, gaseous fire suppression systems cannot flood the entire tower, because the volume of the space is too great.
Wind turbine condition monitoring systems continue to improve. Historically, these systems did not monitor bearing temperature. Bearings would overheat, leading to fires. Now, bearing temperature monitoring is much more common.
Similarly, arc fault protection is becoming a standard, especially in larger turbines. Arc fault protection interrupts medium voltage current within a fraction of a cycle. By contrast, overcurrent protection requires several cycles, which could be long enough to ignite a fire.
What happens next?
Turbines will continue to get larger, which means the stakes are getting higher. Not only does a total loss cost more in terms of capital equipment, but operating losses increase too, because each turbine is producing that much more power. Adoption of safety technology must increase alongside advancements in capacity.
We could also see insurance start to catch up with technological changes. For example, insurers of gas turbines already provide different pricing and incentives based on the technology present at different sites. Look for wind power to follow suit, as the industry continues to mature.
Want to learn more about how turbine design impacts fire risk? We recently hosted a 60 minute webinar with experts Sally Wright (DNV GL) and JP Conkwright (Eastern Kentucky University).