Until recently, lightning-strike monitors provided incomplete information. Recent systems deliver more useful and actionable data such as which turbine blade has been struck and the power in the strike.
Daniel J. Sylawa / Business Development Manager – Renewable Energy / Phoenix Contact USA, Inc.
Lightning risks to wind turbines vary by geographic area, time of year, and turbine location. In fact, there is a risk of lightning at every wind farm. To mitigate threats to wind turbines, the technology used to directly measure lightning include geolocation and direct-measurement systems. Examples of geolocation lightning-detection systems include weather-data-based lightning measurements and lightning detection sensors.
Weather-measurement systems use metrological data to predict the probability of a lightning strike in a given wind-turbine area. Lightning sensors perform the same function but use local measurements to determine the location of a lightning strike. These types of systems can provide the time of strike, location, amplitude (intensity), and sometimes polarity information. Another plus: The information is available remotely. However, weather-measurement systems cannot tell which turbine or blade was impacted by the lightning.
Direct-measurement systems that use the lightning down conductors on a wind turbine can provide information about its blades. Examples of this include event counters and card-measurement systems. Event counters record information on the time and number of lightning strikes. This information is available remotely. Card-measurement systems provide information only on the highest amplitude lightning current that has passed through the down conductor. This system cannot be monitored remotely and must be read by a technician during a service call to the wind turbine.
All these lightning-measurement systems have been used effectively in turbine applications. However, there is a trend in the wind-power industry to use effective asset management techniques to reduce operating costs. Ideally, wind operators would look to increase the intelligence of wind-turbine systems to reduce maintenance costs. One way to do so is by employing targeted inspections to the most suspect turbines. That would reduce the need for truck rolls due to lightning events.
The Faraday effect
Recent lightning measurement systems, such as the Phoenix Contact Lightning Monitoring System (LM-S), use sensors based on the Faraday Effect to provide a more comprehensive range of lightning data to manage assets. These sensors operate by transmitting a polarized light beam through a transparent dielectric material. When mounted on a turbine blade’s down conductor, per the Faraday Effect, an external magnetic field is generated by the lightning current, which travels down the conductor and rotates a light beam proportional to the current amplitude. A polarized output filter converts this into a measurable light signal. This system operates at the speed of light so additional information, such as a measured area under the curve is available to more effectively characterize the impact of the lightning strike. In addition to information such as the time stamp, peak current, and number of impulses, the system also records additional parameters such as specific energy, charge, duration, and slope of the lightning current and lightning direction.
For an installation on a wind turbine, lightning sensors are mounted within individual turbine blades and attached to lightning down conductors. Because blade designs differ with respect to installation of the down conductors, recommended mounting methods are available to handle differing blades designs of various manufacturers. Blade-mounted sensors allow measuring lightning impacts on individual blades. Fiber-optic communication between the sensors and the hub-mounted evaluation unit provides a high degree electrical and noise immunity from the high-energy environment of a lightning strike.
The hub-mounted evaluation unit acts as a data logger and data server. Direct visualization of the lightning data is then possible using a standard web browser. An FTP server allows pulling data from a higher level SCADA system for asset management. A “settable” trigger is available for immediate notification of high-impact events. The small number of components required to install the lightning measurement systems like the Phoenix Contact LM-S make installation suitable for new and retrofit wind-turbine applications.
The LM-S has been installed on wind turbines globally for managing blade assets. Data beyond the standard time stamp, peak current, and impulse count can be more effectively used to gauge blade damage. Additional parameters measured, such as charge and specific energy, are of particular value because they are associated with some mechanisms of lightning-induced blade failures. Beyond blade impacts, the full range of lightning parameters measured by the LM-S can be used to gauge lightning effects on other wind-turbine systems,
including bearings and SCADA components. Detailed lightning analysis can be an effective input to asset-management systems to optimize the inspection intervals for wind-turbine blades and appraise damage requiring immediate attention. This can result in performance improvements by reducing turbine downtime and increasing availability. These are important implications as wind turbines are built larger and located in more remote areas, such as offshore. What’s more, the system has been used to monitor lightning effects on building and structure applications throughout the world, including the Burj Khalifa in Dubai UAE, the world’s tallest building.