Load calculation and system dynamics

The MoWiT (Modelica for Wind Turbines) Library

At Fraunhofer IWES, a computational model for wind turbine load calculations for state-of-the-art onshore and offshore wind turbines has been developed. This model is programmed in the open-source object-oriented and equation-based language Modelica and is archived in a component-based library called MoWiT, which is available free of charge for academic use. The MoWiT library is able to perform aero-hydro-servo-elastic simulations of wind turbine systems, i.e. it comprises the following models in a time-domain coupled simulation environment:

• wind-inflow and aerodynamics (aero)
• waves, currents, and hydrodynamics (hydro - for offshore turbines)
• control system (servo)
• and structural dynamics (elastic)

The models are grouped according to wind turbine components. Due to the hierarchical programming in Modelica and the multibody approach, single components can easily be adapted or exchanged to model onshore, offshore bottom-fixed, or even floating offshore wind turbine designs.  

Real-Time Models and Real-Time Simulation

Despite the utilization of modern simulation methods, experimental hardware tests with real components during the design and certification process of wind turbines is still essential. In order to reduce the high logistic and financial efforts associated with full component testing, a combination of simulation models and specific testing facilities for individual components can be employed in a hardware-in-the-loop environment. MoWiT is used for hardware-in-the-loop tests with specific real-time simulators to allow for realistic and dynamic testing situations.

The realistic wind turbine (rotor) models can be run on an Opal-RT real-time simulator to obtain optimal I/O interfacing possibilities, maintaining strict real-time requirements with a time step of 5 ms. The aim is to couple the MoWiT rotor or full wind turbine simulation model with the IWES DyNaLab test rig for future testing campaigns.

In addition, a real-time-capable simulation model of a wind turbine can be utilized as a digital twin of the turbine, allowing for the live simulation of possible control maneuvers without damaging the actual wind turbine. The digital twin might also be used as a detailed observer model for controller development and/or verification, as well as for testing of newly designed control strategies. Furthermore, the real-time model can be used as a detailed observer model for future condition monitoring and/or structural health monitoring systems.

Floating Support Structures and Offshore Environmental Conditions

Floating support structures add more complexity to fully-coupled load analyses of wind turbine systems. The free motion of a floating wind turbine results in non-zero velocity and acceleration of the structure itself, which is especially relevant for hydrodynamic calculations (e.g. radiation). For station-keeping, a mooring line model is implemented in the MoWiT library. Furthermore, in addition to wind and aerodynamic models, offshore environmental conditions are implemented via wave, buoyancy, current, and ice models. This allows replication of many different sea states and effects, such as steady

• deterministic, or turbulent wind with or without shear or gusts
• linear airy, nonlinear stokes, or irregular waves following a Pierson-Moskowitz or JONSWAP wave spectrum
• breaking wave-induced, wind-generated, or sub-surface currents

Validation and Verification

Fraunhofer IWES actively participates in the verification and validation of coupled engineering-level modeling tools, which include models that consider the simultaneous loading from wind and waves as well as the interaction with the structural dynamics of the system and its control algorithms (aero-hydro-servo-elastic tools). Coupled tools are required in the modeling and design of onshore and offshore wind turbines.

Offshore Conditions: Hydrodynamics and Ice Loads

In addition to aerodynamic loads on the wind turbine, the offshore environment also implies hydrodynamic and ice loads on the offshore wind turbine system.

With respect to hydrodynamic loads, wave excitation, buoyancy force, and current impact have to be considered. The general approach for determining wave loads on a structure is to use the Morison equation. A more sophisticated approach, which also accounts for diffraction and radiation effects, is the MacCamy-Fuchs approach. The buoyancy force and righting moment are calculated depending on the wave surface elevation at the specific location and time, as well as the current position of a floating wind turbine system. Three different current types can be distinguished according to standards (IEC 61400-3) for determination of the current impact on the structure:

• sub-surface currents, which are generated by tides, storm surge, or atmospheric pressure variations;
• wind-generated, near-surface currents, which are no longer present more than 20 m below the still water level;
• near-shore, wave-induced surface currents, which run parallel to the coastline and cause shear forces due to the breaking waves.

The impact of sea ice loads on the global dynamics of Offshore Wind Turbines (OWTs) is being investigated at Fraunhofer IWES. The research environment, industry, and certification authorities are currently lacking an understanding of this aspect. Thus far, they all have been relying on the ice experts (with little or no background in turbine dynamics), whose methods for ice-load calculation are not fully transparent and not always compatible with design and certification methodologies for OWTs. Fraunhofer IWES cooperates in the field of sea ice with RAMBOLL, TUHH within the SeaLOWT project, and also external experts from NTNU and TU Delft.

Control for Automated Load Calculation

The Fraunhofer IWES baseline controller was designed and implemented in order to support the load simulation. The controller is programmed in Fortran 90 using the external DLL interface convention defined by DNV-GL Bladed 4.3. The baseline controller features both collective pitch control and individual pitch control capabilities including both partial- and full-loading operation modes. Three load reduction methods, namely active drivetrain damping, tower fore-aft and side-to-side damping, are used. The basic supervisory control procedures including normal stop, emergency stop, overspeed monitor, and grid loss are implemented.

Applied Load Calculation in Industry and Research Projects

An excerpt for applied load calculation in industry projects:

• analysis of relevant tower movements for an onshore wind turbine
• control development for load reduction for an onshore wind turbine
• training in requirements for load analysis
• assessment of the sea ice loads for offshore wind turbines
• load assessment for a floating offshore prototype
   

An excerpt for applied load calculation in research projects:

• load analysis for the design of a 20 m and an 80 m rotor blade
• comparison of different active/passive load reduction strategies for rotor blades
• load analysis for the design of monopiles for different North Sea sites
• reliability-based optimization of floating wind turbine support structures
• validation of tools for load analysis of wind turbines
• assessment of wind turbine loading from wind fields with non-Gaussian wind increment statistics

Simulation Framework in Python

The "Model Wrapper" can either use a Modelica package of a MoWiT model directly, and, if required, modify some parameters, or can use the package as input for a DLC (design load case) script in Python, to set the simulation parameters according to DLC definitions in IEC or GL standards.

Depending on the number of cases and models, the "Simulation Manager" has to execute one or more simulations. By specifying the number of processors larger than 1, multi-processing in a pool and thus running simulations in parallel is possible. The parallel simulation option is very useful when running DLCs or optimization algorithms.

Automated Optimization

The Python framework can also be used for optimization processes. Currently, optimizers from Platypus(link to external Playtups webpage) are implemented, but other optimizers such as from OpenMDAO or self-coded optimization programs may also be used.

The multi-processing option is very useful for optimization applications. This allows several individuals of one generation within a genetic optimization algorithm to be simulated in parallel, for example.

Applications for optimization processes include controller optimization for load reduction of wind turbines and design optimization of floating support structures for offshore wind turbines.

A generic, state-of-the-art 7.5 MW wind turbine model was designed at Fraunhofer IWES with a special emphasis on the blades.

The wind turbine model specification contains:

1. The IWT-7.5-164 report, which summarizes all technical data necessary to implement numerical turbine models in different aeroelastic tools and serves as a baseline for detailed component design and research.

2. Multiple files with detailed structural,  geometrical, and aerodynamic design of the blades; structural and geometrical design of three monopile support structures - one for the onshore and two for the offshore environment; nacelle housing and spinner 3D design.  

3. Three aeroelastic models implemented in Bladed V4.9, HAWC2 and MATLAB Simulink S-function.

The wind turbine design was funded within the framework of the joint projects Smart Blades (FKZ 0325601A/B/C/D) and SmartBlades 2 (FKZ 0324032A/B/C/D). The monopile offshore support structures were designed by Ramboll within the projects: SeaLOWT (“Impact of Sea Ice Loads on Global Dynamics of Offshore Wind Turbines”, FKZ 0324022B) and TANDEM (“Towards an Advanced Design of Large Monopiles”, FKZ 0325841B). The Smart Blades, SmartBlades 2, SeaLOWT and TANDEM projects were funded by the German Federal Ministry for Economic Affairs and Energy (BMWi) on the basis of a decision by the German Federal Parliament.

Contact for further information:  iweswindturbine@iwes.fraunhofer.de

Wind turbine definition and numerical models                                             

Dipl.-Ing. Philipp Thomas
                                                                       

Blade structural design

M.Sc. Malo Rosemeier
 

Blade aerodynamic design

Dr. Bernhard Stoevesandt 
 

Controls

Dr. Tobias Meyer
 

Support structures design

Dr. Eng. Mareike Leimeister  

The IWES Wind Turbine IWT-7.5-164 is distributed under the GNU General Public License v3.0