NTS News Center

Latest News in Testing, Inspection and Certification

NTS News Center - Latest News in Testing, Inspection and Certification

Lightning Protection of Wind Turbines

lightning protection for wind turbinesOver the past decade, we’ve all watched the world’s energy markets shift towards cleaner, alternative sources. Technological advancements in renewable industries like wind, solar, and tidal, have enabled us to make these potential solutions a reality. As research and development continue to improve efficiency, and drive down cost, these solutions will become more and more prevalent.

We still face considerable obstacles. Wind turbines, in particular, take on one of the largest hurdles; weather. Their fixed location poses unique challenges for turbine and blade manufacturers, which are expected to increase the longevity and reliability of their products without compromising effectiveness, or substantially raising costs. Lightning strikes can cause significant damage to turbines. This damage is extremely expensive to repair, and can shut a turbine down completely.

Severe storms are generally comprised of one or more cumulonimbus clouds, which can be several kilometers in height. As these clouds develop, warm air rises towards the top of the cloud. As the air rises, it becomes cooler. At the dew point, excess water vapor condenses into water droplets and forms the cloud.

When the air has risen high enough, the temperature can drop to -40 degrees Celsius –  water vapor within the cloud will freeze. As ice crystals and hailstones form, and become heavier, they fall through the cloud. When additional water droplets freeze onto the hailstone, small splinters of ice chip off, many of which are positively charged (electrically). Together, these can deposit a net negative charge. Vertical winds carry these smaller splinters upwards into the cloud, while the hailstone falls until it reaches warmer air.

This process causes pockets of electrical charge to form within the cloud, which creates strong electric fields. These electric fields allow charge from the surface of the earth to be “pulled” upwards. in an effort to become charge neutral (typically through tall objects and structures). As these charges get closer together, the electric field in the air further intensifies until it reaches levels of ~30,000V/cm.

When the field intensity reaches these levels, air begins to break down, allowing charge (in the form of current) to flow through the air. Sharp objects significantly intensify the electric field, forming corona (also called St. Elmo’s Fire). Charge from the lightning cloud begins propagating towards the earth. At some point, charge from objects on the ground begins flowing upwards. When these two flows meet, a conductive channel is formed, and lightning occurs.

During a lightning strike, currents of up to (and even greater than) 200,000A travel between the cloud and the object where the lightning channel formed. Wind turbine blades have a sharp, aerodynamic profile – this not only allows them operate efficiently; it also makes them extremely susceptible to lightning strikes. Without suitable conduction paths to safely carry such high current, this transfer of energy can devastate a wind turbine blade.damaged wind turbine blade

Most wind turbine manufacturers strive to make blades and turbines that are more reliable, and can withstand natural phenomena such as lightning. These designs attempt to provide “preferred” current paths for the lightning current; conductive meshes over the exterior of the blades, lightning receptors (preferred lightning attachment locations), and large down conductors, help carry high currents safely to ground.

All of these precautions require highly-detailed planning that includes proper grounding of conductors, shielding of signal wires, wire routing, providing parallel current paths, etc. Despite the complexity of these designs, the benefits are significant; the effects of lightning are not entirely negated, but damage can be significantly reduced – instead of a total blade replacement, getting the turbine up and running again becomes a maintenance and repair exercise.

lightning damage repair progressionThe repair may be straightforward, but it’s still physically demanding. Most modern wind turbine blades are several dozen meters in length. The turbine tower itself can extend 50-100 meters above the ground. Maintenance workers must climb the tower, or be carried up to the blade via a crane. The repair generally involves removing the exterior coating, prepping the damaged area for repair, repairing the laminate, and reapplying the exterior coating.

The physics of nature make lightning almost unavoidable. Therefore, manufacturers must build an effective lightning protection design into the blade.

Most modern design approaches leverage a combination of numerical simulation and testing. Utilizing numerical simulation, electromagnetic models for wind turbine blades can be developed to analyze distributions between structural carbon, and surface protection layers. These models allow the determination of what is electromagnetically important, such as voltages or currents induced throughout the blade, including CFRP pultrusions, heater elements, surface protection layers, and down conductors. They capture critical design details such as material thicknesses, conductor routing, and receptor locations. Further evaluation exposes conductive materials and associated performance risks, such as arcing between blade elements, excessive current in structures, and induced transients into control systems.

After modeling is complete, one or several candidate protection designs is proposed, intending to conduct lightning current with lowest potential for damage or repair. In order for the model data to be considered high fidelity, it needs to be validated by replicating exactly the measurements taken during laboratory tests, and comparing them to the analytical data to determine correlation. This is typically done with high voltage strikes, high current physical damage testing and induced transient tests.

Lightning will always affect wind turbines. It’s complicated, and requires a well thought out design to reduce the severity of damage. If a good protection design is implemented, lightning damage can be reduced to a standard maintenance/repair operation, rather than a total loss.

Justin McKennon is a Senior Engineer and Manager of Simulation and Modeling at the NTS Pittsfield, MA location that specializes in lightning testing and protection services. Justin has a Bachelor’s and Master’s degree in Electrical Engineering from the University of Massachusetts, Dartmouth. He specializes in simulating and modeling the electromagnetic effects of lightning on wind turbine blades, electrical components, aircraft, and other structures. 


Seeing Beyond Boundaries: Industrial CT Scanning

Have you see the NTS sponsored white paper “The Basics, Common Applications, and 4 Tips to Maximize Results from Industrial CT Scanning Inspection” on the InCompliance EERC Resources page? Check it out today to learn about the history of CT scanning, X-ray and CT scanning imaging process, the difference between medical and industrial scanning, common applications and industry examples. Most importantly, learn how to maximize results with industrial CT scanning inspection! Click here to download the white paper. Click here to learn more about NTS non-destructive and CT Scanning services!

TorBots 1197 Robot Reveal

The TorBots 1197 revealed their latest robot creation on Friday, March 3rd at South High School in Torrance, California. The six week build period came to a close and students from all grade levels gathered to not only reveal their robot but to thank everyone that made it possible.

Throughout the year the TorBots team competes in a series of “games” where they must build a robot that is capable of doing certain tasks. This year’s game is titled FIRST Steamworks where the robot must collect gears, fuel (softball-sized whiffle balls) and construct a “flying” device. Not only does it have to collect those items but it must also have the capability to hoist itself up to “fly” away with the final product. Here’s a preview of what the students were able to construct:

This is all made possible by a group of mentors, sponsors and charitable donations. Mentors are either teachers or staff at the school, volunteers within the local engineering community or alumni of the TorBots program that simply want to lend a helping hand. Sponsors help, along with fundraisers, send the team to regional, state and national competitions. Donations can range anywhere from unused equipment, safety glasses or financial donations to help keep the program alive and well.

Last summer NTS donated several testing machines from the LAX facility that would help the team test the durability of their robots, including the one to be used in the FIRST Steamworks competition. Along with the testing equipment, safety glasses were donated to maintain the high safety standard that NTS promotes at its own facilities. Equipment and safety glasses weren’t the only items donated by NTS, Jerry Shu, a Test Technician out of LAX, donated his time and knowledge to the team. As an alumni of the program Jerry knows exactly what it takes to teach the students about mechanics but also understands what it takes to inspire young minds to come up with such amazing inventions.

An opportunity to combine a formal education and real-world skills is presented to these students and is made possible through various donations of time, equipment and money. Students fabricate frames, program software, engineer gears and mechanisms and learn to come together towards one common goal to succeed at the task at hand.

The TorBots 1197 team will be participating in their first Los Angeles Regional event March 23rd – March 26th at California State University of Long Beach. For more information on the First Robotics Competition click here.

For more information on the TorBots 1197 FIRST Steamworks competition please click here.

IPC Emerging Engineer Program

NTS is pleased to participate in the new IPC Emerging Engineer Program. Launched in 2016, this program provides professionals who are beginning their careers the opportunity to learn from dedicated industry volunteers.

2017 Emerging Engineers including Rene Michalkiewicz of NTS and her mentee Tayler Swanson a student at RIT in NY (3rd and 4th from right)

To learn more about this program, visit the IPC website.

Evaluation of Accelerometers for Pyroshock Performance in a Harsh Field Environment

NTS Santa Clarita had the pleasure of assisting Anthony Agnello and Robert Sill of PCB Piezotronics, Inc., Patrick Walter of Texas Christian University, and Strether Smith with the pyroshock testing needed for their white paper “Evaluation of Accelerometers for Pyroshock Performance in a Harsh Field Environment”. Click here to review this white paper.

Click here to review the capabilities at NTS Santa Clarita.

Methodology for Transient Thermal Analysis of Machine Gun Barrels Subjected to Burst Firing Schedules

Authors: Ryan Hill and Logan McLeod

This work presents a method for simulating the heating of machine gun barrels during burst firings. The method utilizes a two-dimensional axisymmetric finite element model which solves the highly transient convection input on the bore wall, conduction through the barrel, and convective and radiative cooling on the outside wall. The transient input is derived from a coupling of a lumped-parameter interior ballistics code with a one-dimensional compressible flow model which includes the discharge of the combustion product gas behind the projectile. This transient convective boundary condition can be repeated as desired for arbitrary firing schedules. Finally, an example simulation is performed on a small caliber machine gun and compared with experimental data.

Fill out the form below to download a PDF version of this white paper.

Analysis of Inter-Chamber Energy and Mass Transport in High-Low Pressure Gun Systems

Authors: Ryan Hill and Logan McLeod
Two assumptions are often made by lumped-parameter codes in the analysis of dual-chamber guns: (1) that the gas enters the large chamber at the propellant flame temperature, and (2) that the flow between the chambers is that of an ideal gas. An investigation on the effects of these two assumptions was performed by creating three lumped-parameter codes: one that maintains the two assumptions above, and two that conserve flow energy of the gas instead of maintaining a constant temperature. Of the latter two models, one uses an ideal gas and the other uses a noble-abel gas for the flow calculations. In this work, the noble-abel gas equation of state will be discussed in detail as well as its implications to the gas flow. Then, descriptions of the three approaches to the gas flow model will be presented, followed finally by a comparison of simulation results from the three models.

Fill out the form below to download a PDF version of this white paper.

Modeling of Ordnance-Induced Pyrotechnic Shock Testing

pyro shock plateAuthors: Logan McLeod and Santina Tatum

Design of an ordnance-induced pyrotechnic shock test to meet a specific acceleration based Shock Response Spectrum (SRS) test requirement for a given test article has traditionally been an empirical process. Based on experience, the test engineer will determine a potential test configuration and then, through a trial-and-error process, modify the test parameters and configuration until acceptable SRS levels have been achieved. As a complement to this approach, National Technical Systems (NTS) has developed an explicit finite element based modeling approach to simulate an ordnance-induced pyrotechnic shock test. This tool may be used to assist with test configuration design for particularly challenging test requirements or to streamline the process of arriving at acceptable test levels during the calibration phase of a test program.

While others have recognized the value of modeling ordnance-induced pyrotechnic shock, the majority of these efforts have utilized more traditional linear implicit finite element based approaches. The implicit approach suffers from two major challenges: determining a suitable spatio-temporal force/pressure distribution on the resonating plate induced by the explosive charge detonation; and accounting for non-linear material response such as plastic deformation in the primary resonating plate which commonly occurs during an ordnance-induced pyrotechnic shock event. The explicit approach inherently overcomes both of these challenges.

The NTS-developed explicit finite element modeling approach for ordnance-induced pyrotechnic shock testing will be presented along with model predictions for specific test configurations. Predicted results will include the acceleration-time history and corresponding SRS levels for a given location on the mounting shelf. Test data for these test configurations will be presented for comparison with model predictions. Post-processing of the model results in order to facilitate comparison with measured test data will also be discussed.

Fill out the form below to download a PDF version of this white paper.