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Theme group 1 - Power and Control Systems

​On the electrical power side there has been a clear trend over the lifetime of modern wind power technology. The traditional Danish windmill was designed to operate with constant speed and as simple an electrical system as possible. While today's large turbines run exclusively with variable speed. The main reasons for this are the reduced mechanical stress that comes with being able to vary the speed at high winds, reduced noise from the turbine in low winds is also an important factor, and low speed in low winds also makes the turbine's ability to convert the wind's motion into mechanical rotation more effective.

To achieve variable speed operation electronic power converters are currently used in, these are connected between the generator and grid to disengage the generator's frequency/speed from the grid's fixed 50Hz frequency. Today the DFIG system (double-fed induction generator) dominates the market, but the trend is clearly toward full effect converters gaining market share. The main reasons are that the power electronics of the frequency converters become more cost effective and the system with full power effect converters have greater manoeuvrability, especially in the event of a fault on the power grid. Grid knowledge is becoming increasingly important as network companies are making ever increasing demands on wind turbines. Grid companies around the world describe their grid codes in official documents. They are available at a national and international level. It is essential that wind turbines meet these requirements in order to ensure safe operation of the power grid, especially in the future when we will have a much greater amount of wind power connected. Connection requirements are constantly evolving and wind power producers must be able to test their wind turbines to demonstrate compliance.

Issues within Power and Control Systems are:

  • Can new converter topologies with unique control be an advantage given the power quality, minimal bearing currents, ageing of the coil insulation and electronics components?
  • What does the optimal generator system look like, taking into account converter topology, power generation, controllability, power quality, maintenance and noise?
  • To what extent the choice of generator system, control of the system and fault scenarios affect the mechanical loads on the wind turbine?
  • How can the development of new magnetic materials affect generator structure, can new power semiconductor devices lead to improved converter technology?
  • Can high voltage DC technology be a good electrical system inside the turbine, in the collection network within a wind farm and for the transmission of the wind farm's total power to the national grid?
  • How can the demands of the grid to control the effect be met and thus support the frequency of the network through advanced control of wind turbine and/or wind farm?
  • Control of wind farms has been shown to be a promising solution for the integration of wind power into the grid. What control algorithms can be used and can they be implemented in a realistic way?

The following projects within the Centre fall under Theme group 1:

TG1-2 Models of electrical drives for wind turbines
TG1-4 Model verification and testing of wind turbines systems by VSC-based testing equipment
TG1-21 Electromagnetic Transient study of wind farms connected by HVDC

Theme group 2 - Turbine and Wind loads

​The length of the rotor blades of a modern 5MW wind turbine is more than 60m and weighs about 18 tonnes. The cost of production is about SEK 3.5 million representing 15-20% of the wind turbine's total cost. The dynamic forces created due to unsteady lift and drag forces are considerable. The time variation of the forces arises primarily because of the interaction between the flow around the rotor blades and time variations in the incoming wind. The characteristics of the incoming wind, i.e. its turbulence, depend on the surrounding environment (oceans, forests, mountains etc.) The turbulence of the incoming wind is dependent on the type of forest, such as northern or Småland coniferous forest, deciduous forest.

The cost per installed kWh for wind power has been halved between 1985 and 2000. Estimates show that 60% of this cost is due to upscaling, i.e. larger turbines. A limiting factor in building large turbines is the size of the rotor blades which are usually manufactured in GRP (Glass Fibre Reinforced Plastic). Today it is possible to manufacture rotor blades made of composite materials, CRP (Carbon Fibre Reinforced Plastic). Marströms Composite AB is a world leader in the use of CRP as a material in construction and manufacturing. Rotor blades manufactured in CRP are 40% lighter than comparable blades in GRP. A lighter blade has a major positive impact on the rest of the wind turbine: the loads on the tower, housing (with gearbox and bearings) and the foundation will be correspondingly less.

A major limiting factor for the expansion of wind power is the noise which often determines the location of both onshore and offshore wind farms. Flow-induced sound at the trailing edge of the rotor blade tip is the greatest source of noise with wind energy systems because of the rotor blade tip's high speed (210-250 km/h). A noise limit of 40 dB (A) applies. At the Department of Fluid Mechanics there is a lot of activity in aeroacoustics for aircraft. Large computer simulations are performed regarding how the sound is created by turbulence and how sound propagates in the air. Some calculations are also including sound-absorbing surfaces. This experience of sound calculations in the aviation field will be used to simulate the propagation of sound from wind turbines.

Icing is a problem in Sweden. ice build-up creates vulnerability in the form of falling ice. A major problem related to the rotor blades is production loss. Either in the form of stoppage or reduced effectiveness of the rotor blades. Even a small ice build-up has a major impact on the rotor blade as its shape is changed. This gives rise to altered flow around the rotor blades and the result is that the driving aerodynamic forces are reduced and thus also the energy produced. Ice build-up can also cause vibrations resulting in increased maintenance costs and reduced service life.

The wind creates the largest loads on a wind turbine. In the forest high turbulence and wind gradient are high compared to the conditions on the plains. Turbulence is also highly dependent on the height of the trees and the forest's condition. High turbulence and strong wind gradient give rise to large fluctuating forces and torque on the turbine. This leads to fatigue damage and shorter service life. Knowledge of the local wind conditions can be used to make a wise choice when placing wind turbines, which leads to a longer service life and higher reliability.

Issues within Turbine and Wind loads are:

  • How big are the dynamic loads? Amplitude and frequency?
  • How big are the forces on a wind farm in the forest? How decisive is the forest type? How do you choose an optimal location (siting) in a wooded area? What is the impact of the weather?
  • How do ice build-up forces affect the rotor blades and fatigue loads on a wind turbine? What is the impact on energy production? How does location (siting) and weather affect ice build-up?
  • How so the dynamic loads spread from the rotor blades to the gearbox and bearings?
  • What material should the rotor blades manufactured in? How does the choice of material and the rotor blade's weight affect the structure of the other parts of the wind turbine? New structures based on new materials can provide increased rigidity.
  • Smart rotor blades in composite: how much are the aerodynamic forces reduced by?
  • How is noise transported from wind turbines to the surroundings? How does the sound transmission affect the environment (meadows, forests, oceans, mountains)? How far is the sound transported in the marine environment?

The following projects within the Centre fall under Theme group 2:

TG2-1 Aerodynamic loads on rotor blades
TG2-2 Fatigue loads in forest regions
TG2-21 Triblade rotor blades preliminary study

Theme group 3 - Mechanical Systems and Structures

​The focus of thematic group 3 is in two main topics. Within the first main topic the focus is on the drivetrain, i.e. the mechanical system of a wind turbine that transmits power from the rotor's hub to the power electronics (generator) and can be described as a complex controlled electro-mechanical multi-body system subjected to a random load. The components of the drive train, such as bearings, shafts, gearboxes, brakes, clutch, and generator are mounted inside a nacelle on top of a flexible tower. With the design the random winds and external circumstances that give widely varying loads with large axial forces and bending moments in the main shaft have to be taken into account. The tower's flexibility and foundation and changes in load due to events in the power electronics (e.g. short circuit) need to be considered. Internal factors are the high gear ratio (for indirectly driven turbines) that set high demands for components and lubricants, manufacturing quality and low noise emissions. From a design perspective, the big engineering challenge is to ensure the reliability of the system and its functional components over the 20-year expected service life.

Within the second main topic the focus is on bearing structure which in this context includes all mechanical subsystems that interact, among other things, to bear the wind load on the generator and the tower down to the wind turbine foundations. In Stage 1 of the Centre's activities, the main part of the activities focuses on rotor blades, which themselves bear the wind load onto the rotor hub, but also the larger part of the wind turbine system as part of an optimisation study. Of key interest for the supporting structure is the choice of technical solution, material selection, dimensioning and performance.

Issues within Mechanical Systems and Structures are:

  • To which loads and movements is a drivetrain exposed under different operating conditions?
  • Which phenomenon is critical for drivetrain component service life? Does current knowledge about individual drivetrain components' lifetime suffice and to what degree of complexity must mathematical and computational models be developed to predict the service life of the components? Which level of detail is required for a model to be used successfully and effectively for different dynamic processes?
  • How important is the interaction between the internal dynamics of the various subsystems of a wind turbine? What motion in the nacelle is the result of the different aerodynamic loads and how does this proposal affect the drivetrain's dynamic environment? Is it necessary to include the tower and generator dynamics in the calculations for the study and analysis of bearings, fittings, axles and gearbox?
  • How can specific problems with unexpected bearing and gear damage be explained?
  • How detailed should a model of the drivetrain be to evaluate algorithms for “automatic” commissioning of control systems for wind turbines?
  • What can modern methods of model validation and model calibration of wind turbines contribute in gaining physical understanding and could this lead to modelling guidelines based on sound physical principles?
  • In the wind turbine the choice of technology solutions for components influences the mechanical environment for other components. How does this impact occur and how can the whole system be optimised overall and analysed by synthesis and simulation. How should the optimisation tool be designed to best support the development of wind turbines?
  • Are modern OMA methods sufficiently reliable to be used in monitoring the turbine's state of health?
  • What can be gained by introducing a probabilistic approach towards wind turbine dimensioning?
  • Can lignin-based composite replace or supplement fibreglass and/or carbon fibre composite and how can material combinations in the blades be used to enhance the damping properties?
  • How does the balance between maximising the annual energy production, system security, minimising peak weight, system maintenance costs and restrictions on noise vibration take place?

The following projects within the Centre fall under Theme group 3:

TG3-1 Wind turbine drive train system dynamics
TG3-2 Development of Compound Bearing Concept for Wind Power Applications
TG3-21 Freedyn and wind turbine system simulation
TG4-1 Validation of Wind Turbine Structural Dynamics Models

Theme group 4 - Offshore

Offshore wind farms promise to become an important source of energy in the near future: it is expected that by the end of this decade, wind farms with a total capacity of thousands of megawatts will be installed in European seas.

For Sweden, the Baltic Sea and the large lakes are the main priority areas, but also the west coast with deeper waters are interesting areas. The Baltic sea in particular has a great potential of installed wind power plants at sea levels of 5-30m. The west coast has a deeper sea level, which calls for research on floating wind power plants.

Research on wind turbine technology — rotors, nacelles, control systems — is knowledge intensive, while research on supporting structure, tower and foundations, remain design and materials intensive.

Issues within Offshore are:

  • The more expensive marine substructures (foundation based and floating)
  • The more expensive integration into the electrical network and in some cases a necessary increase in the capacity of weak coastal grids
  • The more expensive production and installation procedures and restricted access during construction owing to weather conditions
  • The more expensive operation and management due to limited access, which results in an additional penalty of reduced turbine availability and hence reduced output
  • The complex interaction of loads from wind, current, wave, ice
  • The harsh environment conditions effect on materials
  • The need for a decommissioning planhe need for a decommissioning plan

The following projects within the Centre fall under Theme group 4:

TG4-21 ISEAWIND – Innovative Structural Engineering Approaches for design of off-shore WIND power plant foundations

Theme group 5 - Maintenance and Reliability

The wind power industry has undergone and is still undergoing extensive development towards larger wind turbines. This means a large number of technical challenges and the results in many cases have left room for improvement even after the wind turbines have been installed. One of the causes of failure in wind turbines is that components of the turbines have been designed for prevailing operating conditions. Other causes of errors are flaws in the manufacture and installation of components. In many cases, the underlying mechanisms and phenomena leading to faults are not well understood.

Faults in wind turbines lead to direct costs for spare parts, maintenance equipment, personnel transport and maintenance to correct the errors. In addition there are indirect costs in terms of lost production. A reduction of these costs, and thus the cost of wind power in general, can be achieved by further improvements in the design of wind power components, which leads to an increased inherent reliability. The product development potential of individual critical components of the system is analysed. An approximate way of finding the most critical components is to study notional scenarios for the total maintenance costs of a wind turbine over its lifetime, where the expected service life for certain critical components varies.

Research has shown that today's maintenance, both on land and at sea, is not optimal. It has been shown that there are large potential savings to be made by optimising maintenance decisions during the service life to lower the total costs (a) of maintenance activities and component failure and (b) costs due to product losses. This is an important factor in the choice and the optimal implementation of the most appropriate maintenance strategy. With corrective maintenance, a component is used until it breaks down and it is then repaired. With preventive maintenance, components are maintained at predefined intervals or based on their condition to prevent the component failing.

Parallel to optimising maintenance management is the continued improvement of the inherent reliability of wind turbines and their components, which is important to reduce the failure rate. This is particularly important in offshore and remote wind farms where the limited accessibility has a major impact on the ability to measure faults. To achieve greater inherent reliability of wind power components a profound understanding of the physical mechanisms underlying the faults is needed. The advantage is not limited to future turbine generations but the results can also be used to improve the operation of existing wind farms, e.g. through retrofits.

Issues within Maintenance and Reliability are:

  • What are the appropriate methods for effective condition monitoring, to be able to detect, predict and prevent errors and deficiencies in the drivetrain? Specifically, how can the control system for frequency converters be used for this?
  • How is data from vibration-based CMS and SCADA-based CMS used to create an effective maintenance plan?
  • How are effective methods created to predict the remaining service life of damaged components when using fault signatures?
  • How is maintenance optimised to maximise the utilisation of the remaining service life of damaged critical components?
  • Which mechanisms underlie current-induced damage on bearings and how can these be understood (modelled) and limited or even prevented?

The following projects within the Centre fall under Theme group 5:

TG5-1 Load- and Risk-based Maintenance Management for Wind Turbines
TG5-2 Characterisation and modelling of bearing current activity
TG5-21 Optimal maintenance of wind power plants

Theme group 6 - Cold Climate

​In Sweden today we have an increase in wind power projects in areas with cold climates. Even though at least 20GW of wind power is installed in cold areas internationally, knowledge is based more on experience than scientific evidence. So far, the focus of the cold climate research has been in forecasting, detection of ice and de-icing equipment. Cold weather and icing can cause production losses and reduced service life of the wind turbines and it is therefore important to gain more insight into how to use the machines in a cold climate.

Operational experience from wind power in the four northernmost counties shows that cold climate and icing causes low availability and lower efficiency during the winter. In winter 2010/2011 and 2013/2014, there were standstills which can be clearly linked to technical problems when operating at low temperatures. Much of the planned development is located in the region where the cold climate must be taken into consideration and the turbines have to be developed and adapted to the prevailing climate. Cold climate means an environment with risk of icing or temperatures below the operating temperature of the turbine. Today, there is uncertainty among suppliers of wind turbines concerning the possibilities of supplying plants with adequate technology for cold climates. If these problems cannot be resolved the rate of expansion of wind-generated electricity in the four northernmost counties will be compromised.

In terms of research the focus for cold-climate research has been on the forecasting and detection of ice and the development of de-icing equipment. In Stage 1, theme group 6 worked on initial studies of the effectiveness of the de-icing systems and models to simulate them. Equipment to indicate the ice has also been studied in smaller projects. De-icing systems, measurement techniques for the identification of ice and measuring equipment need to be developed to cope with the difficult environment that cold climate and ice causes. Looking ahead, there is the potential to move forward with these projects, and to include the study of loads and dynamics caused by ice.

Issues within Cold Climate are:

  • Which sensors are reliable and necessary for operating a wind turbine in a cold climate?
  • What is the best technique for “de/anti-icing”?
  • How is wind turbine production maximised in cold climates?
  • Which simulation tools are necessary to simulate icing and de-icing?
  • How should we model the dynamics and evaluate the loads due to icing?
  • How can availability and reliability be increased for wind turbines operating in cold climates?

The following projects within the Centre fall under Theme group 6:

TG6-2 Efficiency and influence of heating device on wind turbine blades
TG6-21 Increased reliability of heating systems on wind turbine blades