Developing complex turbines with simpler model-based designs
Steve Miller, Technical Marketing Manager, Physical Modeling, The MathWorks, Natick, Mass., mathworks.com
Simulating a new turbine piece by piece allows working out the bugs
where it’s easiest – in software.
The flow chart suggests how a simulation for a wind turbine’s mechanical and electrical systems would progress.
Wind turbines are more than a set of integrated systems working together for maximum output. They must also correctly interpret a range of environmental conditions and react accordingly. Furthermore, the expense of diagnosing and repairing these systems makes the testing phase a particularly important aspect of development. Testing cannot be done on full sized prototypes because of their size and cost. Such challenges can be met, however, by using a Model-Based Design philosophy throughout development. Such simulation partly replaces prototype tests or makes them more effective, and it allows maximizing the performance of the combined systems.
More complex all the time
There are more than 20,000 wind turbines in operation in Germany generating nearly 24,000 MW, roughly 7% of the electricity consumed there. The growth and development is made possible by constant technical enhancements. Better rotor-blade aerodynamics, more efficient generators, and improved supervisory control systems.
This enhanced technology presents engineers with new challenges. A wind turbine is a complex system in which a variety of subsystems must work together as efficiently as possible. The subsystems include mechanical devices such as rotor blades, gearboxes, hydraulic or electric drives for setting blade pitch angles, electrical yaw drives, along with the generator and equipment that surrounds it. What’s more, all of these are monitored by a complex supervisory control system that must respond in a specific way to varying environmental conditions –changing wind speeds in particular.
The circuit that might describe blade-pitch hydraulics would be part of a larger simulation that could include yaw drives, brakes, and electronic controls
Wind-turbine subsystems are often developed by different teams, sometimes different companies. In traditional development work, the designs may be created in separate software and simulation environments with requirements captured by separate methods. This can result in several problems.
Because the requirements have not been incorporated into the development process, it is difficult to compare the design with the requirements and specifications. Engineers are unable to determine if the changes made in development cycles still let the system meet requirements. Worst of all, requirements that are incorrect or incomplete will not be detected until the subsystems are combined in a final development phase when errors are expensive or impossible to fix.
An inability to integrate different designs early in development can result in poor designs. For example, if the teams developing the generator and the supervisory control system work separately, it is difficult to predict what will happen when the subsystems are integrated at the end. Engineers working in different software tools and simulation environments may not have the option of testing the integrated design in simulation. The result is that subsystems can only be tested together when hardware prototypes have been produced. Since the wide range of weather conditions and failure analyses prohibit exhaustive testing on hardware prototypes due to cost, safety, and feasibility, parts, and systems must be over designed (and therefore less efficient) to make sure the turbine does not fail.
The point here is the hydraulic pressure readouts to the right. Should these spike or show a reading too low, adjustments can be made in software before building physical prototypes.
Contradictory goals
Various control systems with goals that at times run counter to one another have to interact within the overall wind turbine. On occasion, the control for different subsystems may work against each other. For example, a monitoring system for the entire wind turbine must ensure it generates electricity as frequently as possible and can therefore operate economically. At the same time, it must protect individual parts from unnecessary wear and tear. It also must react to imminent power failures to prevent the turbine from becoming unstable and destroying itself. Normally, the generator is only switched on when the wind reaches a speed of 2 to 4 m/s. Lower wind speeds fail to generate enough power and unnecessarily wear turbine parts. In high winds, controls shut down the generator and set the rotor spin slowly so as to reduce load on the drive train. The system that controls the blade does so to keep the generator’s speed in a relatively narrow range so it can generate the maximum amount of power. At the same time, it must bring the turbine to a halt in a power failure.
Furthermore, proper yaw control keeps the turbine facing the wind. The yaw controller guides a system that has a non-linear behavior, which is also influenced by backlash in the gearbox and friction in its large ball bearings. The yaw controller also ensures the nacelle doesn’t turn in the same direction all the time. This keeps cables in the tower from twisting beyond their limit.
Smooth and continuous development
All controllers in a turbine can be simulated and tested as part of an integrated system at an early development stage by using Model-Based Design as an approach to development. Doing so has several advantages. Controller hardware can be tested before building hardware prototypes. Systems that must eventually work together– such as the pitch and yaw actuators – can be tested together and matched for best performance.
The schematic on the left starts with wind hitting the blades, S1 to S3. Blade load, top element, governs blade pitch, which further provides signals to controls in the nacelle. The model also includes tower effects. The turbine on the right looks crude but can show how the controls are pointing the unit or positioning the blades given the user’s assigned wind speeds and directions.
Wind turbine developers who use the design philosophy profit from a smooth and continuous development. The models and simulations are all in one environment and linked directly to the requirements and specifications. In addition, the specs can generate embedded the software required, straight from the model. Doing so simplifies communication between various teams and makes it easier to spot errors and problems concerning the integration early on.
From model to code generation
As a starting point in development, consider modeling a wind turbine entirely in software such as MATLAB and Simulink. Various blocks represent the physical system with its mechanical, electrical, and hydraulic subsystems along with actuators for the whole system, the pitch angle and yaw. These can be supplemented by models of aerodynamic effects and various inputs, particularly wind speed and direction.
Engineers can conduct system-level analyses with idealized models to select equipment and determine system requirements. Ideal models, such as various drive units, can be gradually refined and replaced by realistic models to determine system performance. For example, an idealized pitch actuator can determine the force an actuator will need, letting the engineer size a hydraulic cylinder. Developers can then add a more detailed model of the selected hydraulic unit in simulations. A yaw actuator’s model can start as a single ideal torque source, and incrementally refined to include four individual motors, a model of the mechanical system including a gearbox, circuit diagram, and other details. This gradual progression lets engineers test their design at each step.
Models for all the subsystems can then be combined and simulated early on in development. Other subsystems developed by separate teams can be gradually added to the overall simulation to test system performance. At each step, the tradeoff of model fidelity and simulation speed can be balanced so designers can iterate quickly and check for integration issues. For example, if focusing on a yaw controller, the engineer can use a detailed model of the yaw system and quickly substitute a lower fidelity model for the pitch system into the overall model. This keeps simulation times short while making it possible to check for integration issues between these two systems.
Different simulations can now be conducted using an entire system-level model. A 3D animation of the system and plots displaying different values of relevance can show turbine developers how the design reacts under varying conditions.
Documents of specifications and requirements connect directly to the model via bi-directional links using Simulink Verification and Validation. This lets designers check whether all requirements are still being satisfied at each stage of development.
Testing without physical prototypes
At the end of development, embedded C code is generated from the model for the supervisory control system. To test this control code and the controller hardware, hardware-in-the-loop tests can be used instead of physical prototypes of the wind turbine. The model of the physical system (mechanical, electrical, and hydraulic) can be converted into C code and downloaded onto a real-time computer. This can be connected to the hardware controller for testing. The hardware controller behaves as if it is connected to an actual wind turbine. Engineers can test the system with few limits and over a wider range of conditions than would be possible with a physical prototype. And, by using the same model of the physical system as used in the earlier phases of development, the engineer can verify that the generated code performs exactly as it did in the computer model.
The Model-Based Design allows testing the system and controller hardware before hardware prototypes are even made, as well as on-site power failures. This saves engineers from traveling to a turbine site to diagnose problems. The feature is particularly useful for turbines erected at remote locations.
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