With their extreme height and open-air locations, wind turbine systems are at high risk for damage from lightning strikes. To reduce this risk, exterior areas around a wind turbine usually have direct lightning protection, while electrical and control systems are guarded by a surge-protection system. These lightning and surge protection schemes are essential elements in maximizing a wind turbine’s availability and economic rate of return.

While exterior lightning protection systems reduce lightning risk, they do not eliminate all of it. Lightning is extremely variable in its impact and frequency. The high variable energy loads of a lightning strike can produce wear on lightning-protection elements. Most often, this is seen in wind turbine blades where lightning receptors must be replaced to prevent further blade damage. Inspection is the only method of determining the need for replacement. Balancing this need for inspection versus continued operation is one risk associated with lightning and wind-turbine operations.

Understanding and characterizing operational risks has been one of the objectives of the wind industry. From an operations and maintenance (O&M) perspective, real-time understanding of lightning exposure is one element of reducing wind-turbine operational costs. Analogous to condition monitoring systems for mechanical components, real-time lighting monitoring allows collecting and analyzing information on actual loads from lightning strikes. Although important in evaluating the operational status of turbine components, this information details component status more precisely and allows targeting lightning related maintenance in a more cost-effective manner.

In the past, installing lightning current measurement systems in wind turbines has been difficult or has provided a limited range of lightning information. However, new technology using polarimetric sensors is now available. It installs easily on wind turbines and provides a full range of real-time lightning information. These systems are suitable for new and retrofit applications.

Exposure zones and lightning parameters

The exposure levels of wind-turbine components to lightning are well documented [1]. Wind turbine blades in the LPZ 0A location are most vulnerable to direct lightning strikes. It is also difficult to determine a blade’s operational condition without direct inspection.

Complete characterization of lightning events, as represented in standards and reports, [2] includes the following parameters:

• Peak current
• Number of impulses
• Specific energy
• Charge and duration, and
  

  The graph is from calibrated laboratory equipment that produces and measures lightning current. In this case, the Iimp is 30 kA.

• Slope of the lightning current.

These parameters more closely represent the complete impact of lightning than just the peak current or number of impulses typically seen in practical lightning measurement.

Measuring current
Conventional methods for measuring lightning current and devising protection provides some background and contrast for more recent developments.

In research – For research, high precision, lightning-current measuring systems used to determine the physical parameters of lightning strikes usually employ current transformers, shunt resistors, and devices called Rogowski coils. These are capable of providing a full range of lightning-current measurement parameters. However, due to their complexity and cost, they have not found wide use in wind turbines.

For protection – Turbine operators commonly use two systems to analyze lightning strikes in wind turbine applications. One counts the number of strikes based on the magnetic field induced by the lightning surge current in the grounding conductor. Such information is typically limited to a counter’s response range. A second system uses magnetic cards that are also sensitive to the magnetic field of the lightning surge current in the grounding conductor. Analysis of the cards indicates the highest amplitude lightning current that has passed the grounding conductor.

Neither system, however, gives a full indication of the true lightning load experienced by a wind turbine.

Measuring polarimetric lightning current

The newest lightning measurement systems use polarimetric lightning-current sensors that use the Faraday Effect. The sensor functions by passing a linearly polarized light signal through a transparent dielectric material. When an external magnetic field permeates the material, the plane of polarization of the light signal rotates as a function of the strength of the magnetic field. The amount of light passing through an output filter depends on the degree of the light wave rotation. This creates a measurable and evaluable light signal. Sensors based on this principal can measure bidirectional current flow.

The polarimetric lightning sensor is mounted in a housing which is fastened directly on the conductor being monitored. To calibrate, the polarimetric lightning sensor must be located at a uniform depth in the magnetic field of the conductor.

Sensors based on this design have shown good agreement between actual and measured surge currents. Performance investigations have shown that measurement errors for a dynamic range of 5 kA < Iimp < 200 kA are significantly below 10%. Iimp is the impulse current of a strike.

Due to the fast response of the polarimeteric sensor, lightning current amplitude may be measured over the entire range of the lightning event. This data may be processed to provide a range of data that characterizes the lightning event, including peak current, number of impulses, lightning direction, specific energy, charge, and duration and slope of the lightning current.

For wind-turbine applications, the polarimetric lightning sensors are mounted in lightning down-conductors of the wind turbine blades using common cable ties. Associated measurement electronics are mounted within the turbine hub. Installating sensors in blades allows measuring lightning currents on the exterior areas most vulnerable to direct lightning strikes. Fiber-optic communication between the polarimeteric lightning current sensor and the measurement electronics provides a high degree of electrical isolation and noise immunity.

Real-time evaluation and access to lightning data

Real-time evaluation and remote access to lightning data are crucial to assessing the actual stress on the turbine caused by lightning strikes, and to trigger the required maintenance work or to avoid unnecessary inspections. Surge current amplitude, maximum current slope, and charge and specific energy of a lightning strike are analyzed and stored in the evaluation unit along with the date and time of the event. An evaluation unit may store up to 500 events.

For remote access, an Ethernet interface may be connected into the wind turbine network or accessed by wireless or cellular communication. Lightning data is

accessed via a standard web browser interface.

This type of interface may be used for set-up of the lightning monitoring systems or to export data for further statistical analysis and archiving.

A practical application for O&M

Monitoring lightning current loads in wind turbine systems was previously limited to counting the number of lightning events and possibly their maximum amplitude. Dynamic current characteristics were not monitored. However, both the charge and rise-time of the lightning surge current are important indicators of potential damage. The expected damage caused by a long-duration, low amplitude lightning strike can be as great as damage caused by a strike with many times the amplitude over a considerably shorter duration. Frequent lightning strikes with lower amplitude also create a high load for mechanical structures. Cumulative charge is a critical parameter to characterize these events.

With the installation of polarimetric, lightning-current monitoring in a wind turbine, it becomes possible to provide the continuous monitoring of dynamic current characteristics of lightning events. Data gathered – peak current, number of impulses, lightning direction, specific energy, charge and duration, and slope of the lightning current – can be used to assess lightning impact. Scheduling preventive maintenance in a more timely fashion for rotor blade, lightning receptors, and electrical system lightning surge-protection systems helps reduce unplanned wind turbine downtime. WPE

 

For further reading

1. Nager, M., Heckler, H., “Introduction to Lightning Protection of Wind Turbine Electrical Systems and Equipment,” IEEE The 8th International Conference on Emerging Technologies for a Smarter World, November 3, 2011, Hauppauge, NY.

2. IEC Technical Report 61400-24, “Wind turbine generator systems – Part 24: Lightning protection”, July 2002; under consideration: IEC 61400-24 Ed. 1.0 “ Wind turbines – Part 24: Lightning protection”

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Filed Under: Featured, Safety, Turbines
Tagged With: phoenixcontactinc
 

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