Lightning safety is a key concern for maintenance crews at wind farms, as most farms are located in wide open, lightning-prone areas and include turbine structures hundreds of feet high. And while tall structures are especially prone to lightning because they provide a conductive channel to the ground, some experts suspect that wind turbines may even have a higher risk of a lightning strike than most tall structures.
Turbine height also means it takes a considerable amount of time to get safely to the ground. Getting down from a tower and driving away from a wind farm is the ultimate safety goal. Considering the amount of time needed to completely evacuate a wind farm, significant advance notice of bad weather is required.
A system that detects lighting strikes and that also has the capability to provide alerts to on-site maintenance crews is a proven way to keep wind farm employees safe.
Wind energy organisations that have on-site maintenance staff, including operators, turbine manufacturers and contract maintenance crews, typically have well-established lightning safety practices. However, there are still various misguided practices that are not effective and very dangerous:
- Watching for lighting and listening for thunder: Lightning strikes can occur miles outside of a storm. Relying on your senses instead of adhering to advanced warnings allows lighting to “sneak up” on a team before there is adequate time to respond;
- Monitoring radar images on a PC or phone and guessing whether or not the storm includes lightning: This method for tracking lightning produces two opposite and undesirable results – false alarms and unidentified lighting in the area;
- Looking at delayed lightning on free websites: The average life of a thunderstorm cell is 20 minutes and lighting information from free websites is delayed by 30-60 minutes. Lightning can be on top of a team before it shows up on the delayed website. When crew member safety is on the line, it is worth the price of subscribing to a service that provides real-time updates.
The misguided lightning safety approaches listed above force wind farm operators to gamble with safety. Instead, wind farms should rely on a combination of a real-time lightning detection network with immediate alerting capabilities and all-clear notifications as an effective technique for keeping wind farm personnel safe.
Wind farm lightning safety best practices
To stay on top of changing weather conditions that can endanger employees and impact operations, wind power professionals need tools that allow them to launch a pre-emptive strike against lightning.
Constantly monitoring approaching thunderstorms and tracking lightning activity can be quite a burden. Wind farm operators cannot afford to cut corners; at the same time, they cannot spend their time glued to a weather map.
To effectively observe impending lightning, wind farm personnel should use a real-time weather service that allows them to program alerting parameters set specifically to meet their needs. Determining the alert radius can depend on the amount of time it takes to get down from a tower and at least get to the safety of a vehicle.
There are two alerts that are essential for wind farm crews to receive notice that lightning is moving into the area:
- Heads-up alert: Many wind farms have this alert go off when lightning is within 50 or 60 miles of the farm. It puts crews on notice that severe weather has entered the area and that they should be on heightened alert;
- Evacuation alert: Once lightning gets closer, an evacuation alert should be broadcast. A typical parameter for evacuating a wind farm when bad weather is 30 miles away.
Once a storm has entered the area, wind farm operators need to determine when operations can return to normal. Figuring out how to get crews back into the field without putting them in harm's way is a delicate task. While operators want to resume work as quickly as possible, sending crews back to work too early puts them in danger.
Lightning flash versus stroke
A complete lightning discharge is known as a “flash,” while the high-current pulses in a flash are known as “strokes.” Strokes can travel down the same channel or have different ground contact points. Typical flashes have three-to-four strokes.
Does polarity matter?
A cloud-to-ground lightning stroke can lower positive or negative charge to the ground. More than 90 percent of lightning is negatively charged, but the less common positive strokes typically have larger peak currents that can increase the probability of asset damage or fire ignition.
Receiving an all-clear alert is an effective way of maximising productivity while ensuring employee safety. A common practice is to send crews back after lightning has not occurred within 30 miles for 15 minutes. Some operators take extra caution and wait until lighting has been absent within a radius of 60 miles for 15 minutes.
In addition to advanced alerting capabilities, wind farm operators greatly benefit from maps that display real-time lightning data. A visualisation of lightning activity combined with radar can provide a good sense of when storms are approaching. This especially holds true when a second line of storms is developing that hasn't yet entered the warning parameter.
This capability is very helpful for maintenance scheduling, as operators can avoid starting activities that will have to be suspended due to bad weather. Pairing these detailed weather maps with animated lightning strike data shows whether lightning is intensifying or subsiding
While it is best to have the crews first and foremost responsible for their own safety, centralised monitoring can act as a backup for field personnel. Personnel at a central facility can monitor weather activity in proximity to wind farms and call crews about impending weather issues.
Viewing forthcoming and current lighting activity is crucial for maintaining safety on a wind farm, but it is also important to look back at recent lightning activity in the area in order to take more preventative measures. Weather services such as Telvent DTN MxVision WeatherSentry Online Wind Energy Edition provides access to detailed data for the previous 72 hours and the opportunity to archive the data for future reference.
Details such as time, latitude and longitude allow users to quickly pinpoint strikes to the wind farm even when it occured overnight or over the weekend. This allows operators to document, analyse and report on lighting activity in the area for damage assessment, analysis, planning and insurance claims.
The approach described above is a proven, effective way to keep wind farm maintenance personnel safe. Implementing a complete operational weather solution is also a cost-effective way of managing crew scheduling and reducing maintenance costs. These systems are easy to implement and manage, and allow wind farm operators to mitigate weather disasters.
Don Leick is the product management director at Telvent DTNt commercial weather services. He is responsible for understanding the needs of customers, including wind farm operators, and delivering solutions.
This article is a shortened version of one that appeared in the May issue of Renewable Energy Focus USA magazine.