Analytical Modeling and Test Services for Lightning Phenomena Challenges in the Wind Turbine Systems and Structures
NTS’ Pittsfield, Massachusetts facility has long been involved in the wind power industry. Our participation in the IEC TC-88 PT 24 committee which is responsible for releasing the wind industry test standard IEC 61400-24 and the development of our analytical modeling capabilities enable our engineering department to provide the design evaluation and testing services your wind power products require.
Lightning Engineering and IEC 61400-24 Test Capabilities
NTS offers the following electromagnetic phenomena services for your wind power application:
- Protection Design for:
- Blades including traditional makeup, CFRP makeup, anti-ice/de-icing technology, electronic systems and control devices
- Supervisory Control and Data Acquisition (SCADA)
- Control Electronics
- Power Distribution
- Structural components including hubs, spinners, nacelle, mechanical drive train and yaw control systems, tower installations, grounding and equipotential bonding
- Numerical Simulation Services
- Blades with Candidate Protection Designs using COMSOL Multiphysics
- Blade, Hub, Nacelle, Tower and Earthing Installations to predict responses to lightning strikes and performance of protection designs
- Evaluation for protection devices (SPD, TVS, Shielding, etc.)
- Exposure assessments – Zoning (LPX) per IEC 61400-24
- Damage Risk Assessments on-site or off-site turbine inspections for incident investigation
- Retrofit Design Services
Protection Verification Services:
- Certification Test Planning and documentation
- Direct Effects Test conducted on blade specimens up to 15 meters in length, in accordance with Annex D of IEC 61400-24
- High Voltage Strike Attachment Test
- Initial Leader Attachment (Type A and Type B test methods)
- Swept Channel Attachment
- High Current Physical Damage Tests
- Up to 200 kA with 6 MJ and 300 C via arc entry and conducted current
- High Voltage Strike Attachment Test
COMSOL Analytical Modeling for Turbine Systems and Structures
NTS uses COMSOL as our preferred modeling environment. Our fully verified, industry standard modeling suite solves systems of differential and partial differential equations that comprise the materials and boundary conditions specified in the model.
The NTS modeling approach supports physics coupling, for example heat transfer and currents, and provides a myriad of options for customizing and developing models for just about any situation.
NTS has developed electromagnetic models for wind turbine blades, analyzing distributions between structural carbon and surface protection layers, determining transient voltages and currents to optimize lightning conductor locations, tolerances and more. We are able to accurately simulate the required IEC 62305 waveforms for all lightning protection levels (LPL).
Development and Replication Methodology
In general, models are built from decomposing CAD-level data into COMSOL native shapes. This allows the determination of what is electromagnetically important, such as the voltages or currents induced throughout the blade, including CFRP pultrusions, heater elements, surface protection layers, and down conductors. Models capture critical design details such as material thicknesses, conductor routing and receptor locations. The evaluation exposes conductive materials and associated performance risks such as arcing between blade elements, excessive current in structures, and induced transients into control systems.
NTS models are built to simulate physics via Maxwell’s equations and to replicate the test set up, i.e. return paths to the generator, etc. These are critical for initial model development. The replicated test setup results are compared with measurements taken, not tweaked to “match” the measurements.
Analysis and Validation Methodology
NTS engineers conduct analyses to evaluate current distributions for one or several candidate protection designs intended to conduct lightning current with lowest potential for damage or repair. In order for this model data to truly 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. Such tests typically include:
- High voltage strike attachment tests on a wing tip to determine likely attachment points, puncture possibilities, and internal streamer locations
- High current physical damage tests on a ~1m2 panel to determine current conduction efficacy of embedded lightning protection materials
- Induced transient tests on internal wiring harnesses to determine induced voltage/current amplitudes and to evaluate the potential for damage to installed electrical equipment
Comparisons are captured in a detailed Validation report that serves as the “Go or No Go” portion of the projects. If the model shows sufficient agreement with the measured data, it can then be said that the model is an appropriate representation of real test article. If the model does not showagreement, alternative modeling approaches can be taken, or the project can pause to reduce program risk. Experience to date has shown good correlation between model and measured data.
Validation and General Purpose
Once the model has been validated, it is returned to a general purpose setup. The boundary returns are used to remove any test setup specific artifacts that may have been included, and physics and boundary conditions are not changed. The model can then be manipulated as desired without undergoing further tests allowing for the examination of identified design changes warranted by initial model computations/data validation tests, and understanding transient levels on conductors and electronics, etc.
Fully Developed Models
The benefits of fully developed models are many. These models all for the collection of early lifecycle data allows for the examination of areas where measurements were or could not be taken and design better decisions can be made. Early life cycle modeling reduces certification risks, verifies design methodologies for future (similar) designs, and allows for similarity analyses on future designs to reduce testing needs.
The NTS modeling and analytical team, headed by Justin McKennon, works in tandem with our engineering services team, headed by Mike Dargi. Our team assists in the selection of materials and connection methods that are the most likely to be robust and require less repairs post-lightning strikes. We evaluate the protection design materials and features such as connections, SPL and ETH pads to sustain effects of multiple strikes.
In short, analytical models are shown to reduce project lifecycle testing costs, and provide insight for key design decisions.