The motivation for this issue comes from a real need to have an open discussion about the challenges of control for very demanding systems, such as wind turbine installations, requiring the so-called “sustainability” features. It represents the characteristic to tolerate possible malfunctions affecting the system and, at the same time, the capability to continue working while maintaining power conversion efficiency. Sustainable control has  begun to stimulate research and development in a wide range of industrial communities particularly for those systems demanding a high degree of reliability and availability. The system should be able to maintain specified operable and committable conditions, and at the same time should avoid expensive maintenance works. For offshore wind farms a clear conflict exists between ensuring a high degree of availability and reducing costly maintenance.

Renewable energy can be produced from a wide variety of sources including wind, solar, hydro, tidal, geothermal, and biomass. By using renewables in a more efficient way to meet its energy needs, the EU lowers its dependence on imported fossil fuels and makes its energy production more sustainable and effective. The renewable energy industry also drives technological innovation and employment across Europe, as highlighted for the wind power conversion systems.

2020 renewable energy targets are settled. The EU’s Renewable Energy Directive sets a binding target of 20% final energy consumption from renewable sources by 2020. To achieve this, EU countries have committed to reaching their own national renewables targets ranging from 10% in Malta to 49% in Sweden. They are also each required to have at least 10% of their transport fuels come from renewable sources by 2020 [1]. All EU countries have adopted national renewable energy action plans showing what actions they intend to take to meet their renewables targets. These plans include sectorial targets for electricity, heating and cooling, and transport; planned policy measures; the different mix of renewables technologies they expect to employ; and the planned use of cooperation mechanisms.

A new target for 2030 is fixed. Renewables will continue to play a key role in helping the EU meet its energy needs beyond 2020. EU countries have already agreed on a new renewable energy target of at least 27% of final energy consumption in the EU as a whole by 2030. This target is part of the EU’s energy and climate goals.

Support schemes for renewables are available, which drive the technological innovation and employment in this framework. Horizon 2020 is the biggest EU Research and Innovation programme ever with nearly €80 billion of funding available over 7 years (2014 to 2020) – in addition to the private investment that this money will attract. It promises more breakthroughs, discoveries and world-firsts by taking great ideas from the lab to the market. Horizon 2020 is the financial instrument implementing the Innovation Union, a Europe 2020 flagship initiative aimed at securing Europe’s global competitiveness [1].

By coupling research and innovation, Horizon 2020 is helping to achieve this with its emphasis on excellent science, industrial leadership and tackling societal challenges. The goal is to ensure Europe produces world-class science, removes barriers to innovation and makes it easier for the public and private sectors to work together in delivering innovation.

Wind energy is perhaps the most advanced of the ‘new’ renewable energy technologies, but there is still much work to be done. Assessments of the research and technology developments and impacts have been highlighted by recent proposals within the Horizon 2020, with key benefits from both the scientific and industrial points of view.

Wind energy can be considered as a fast–developing multidisciplinary field consisting of several branches of engineering sciences. The National Renewable Energy Laboratory estimated a growth rate of the wind energy installed capacity of about 30% from 2001 to 2006, and even with a faster rate up to 2014.

The global wind power installations are 369,6 GW in 2014, with an expected growth to 415.7 GW by the end of 2015. After 2009, more than 50% of new wind power resources were increased outside of the original markets of Europe and U.S., mainly motivated by the market growth in China, which now has 101,424 MW of wind power installed. Several other countries have obtained quite high levels of stationary wind power production, with rates from 9% to 39%, such as in Denmark, Portugal, Spain, France, Ireland, Germany, Ireland, and Sweden in 2015. From 2009, 83 countries around the world are exploiting wind energy on a commercial basis, as wind power is considered as a renewable, sustainable and green solution for energy harvesting. Note however that, even if the U.S. now achieves less than 2% of its required electrical energy from wind, the most recent National Renewable Energy Laboratory’s report states that the U.S. will increase it up to 30% by the year 2030. Note also that, even if the fast growth of the wind turbine installed capacity of wind turbines in recent years, multidisciplinary engineering and science challenges still exist. Moreover, wind turbine installations must guarantee both power capture and economical advantages, thus motivating the wind turbine dramatic growth [1].

Industrial wind turbines have large rotors and flexible load–carrying structures that operate in uncertain, noisy and harsh environments, thus motivating challenging cases for advanced control solutions [2].  Advanced controllers can be able to achieve the required goal of decreasing the wind energy cost by increasing the capture efficacy; at the same time they should reduce the structural loads, thus increasing the lifetimes of the components and turbine structures.

Although wind turbines can be developed in both vertical–axis and horizontal–axis configurations, the industrial and technological interest focusses on horizontal–axis wind turbines, which represent the most commonly solutions today in the produced large–scale installations. Horizontal–axis wind turbines have the advantage that the rotor is placed atop a tall tower, with the advantage of larger wind speeds that the ground. Moreover, they can include pitchable blades (i.e. they can be oriented with respect to the wind direction) in order to improve the power capture, the structural performance, and the overall system stability. On the other hand, vertical–axis wind turbines are more common for smaller installations. Note that proper wind turbine models are usually oriented to the design of suitable control strategies that are more effective for large rotor wind turbines. Therefore, the most recent research focus considers wind turbines with capacities of more than 10 MW [3].

Another important issue derives from the increasing complexity of wind turbines, which gives rise to more strict requirements in terms of safety, reliability and availability. In fact, due to the increased system complexity and redundancy, large wind turbines are prone to unexpected malfunctions or alterations of the nominal working conditions. Many of these anomalies, even if not critical, often lead to turbine shutdowns, again for safety reasons. Especially in offshore wind turbines, this may result in a substantially reduced availability, because rough weather conditions may prevent the prompt replacement of the damaged system parts. The need for reliability and availability that guarantees the continuous energy production thus requires sustainable control solutions [2].

These schemes should be able to keep the turbine in operation in the presence of anomalous situations, perhaps with reduced performance, while managing the maintenance operations. Apart from increasing availability and reducing turbine downtimes, sustainable control schemes might also obviate the need for more hardware redundancy, if virtual sensors could replace redundant hardware sensors. These schemes currently employed in wind turbines are typically on the level of the supervisory control, where commonly used strategies include sensor comparison, model comparison and thresholding tests. These strategies enable safe turbine operations, which involve shutdowns in case of critical situations, but they are not able to actively counteract anomalous working conditions. Therefore, recent research directions have been oriented to investigate these sustainable control strategies, which allow to obtain a system behaviour that is close to the nominal situation in presence of unpermitted deviations of any characteristic properties or system parameters from standard conditions (i.e. a fault). Moreover, these schemes should provide the reconstruction of the equivalent unknown input that represents the effect of a fault, thus achieving the so–called Fault Detection and Diagnosis tasks [3].

The need of advanced control solutions for these safety–critical and very demanding systems, motivated also the requirement of reliability, availability, and maintainability over the required power conversion efficiency. Therefore, these issues have begun to stimulate research and development of sustainable control (i.e. fault–tolerant control), in particular for wind turbine applications. Particularly important for offshore installations, Operation and Maintenance (O & M) services have to be minimised, since they represent one of the main factors of the energy cost. The capital cost, as well as the wind turbine foundation and installation determine the basic term in the cost of the produced energy, which constitute the energy “fixed cost”. The O & M represent a “variable cost” that can increase the energy cost up to about 30%. At the same time, industrial systems have become more complex and expensive, with less tolerance for performance degradation, productivity decrease and safety hazards. This leads also to an ever increasing requirement on reliability and safety of control systems subjected to process abnormalities and component faults [2, 3].

As a result, the Fault Detection and Diagnosis tasks, as well as the achievement of fault tolerant features for minimising possible performance degradation and avoiding dangerous situations are extremely important. With the advent of computerised control, communication networks and information techniques, it becomes possible to develop novel real–time monitoring and fault–tolerant design techniques for industrial processes, but this also brings challenges. Several works have been proposed recently on wind turbine Fault Detection and Diagnosis, and the sustainable (fault tolerant) control problem has been recently considered with reference to offshore wind turbine benchmarks, which motivated this issue [3, 4].

In this way, by enabling this clean renewable energy source to provide and reliably meet the world’s electricity needs, the tremendous challenge of solving the world’s energy requirements in the future will be finally enhanced. The wind resource available worldwide is large, and much of the world’s future electrical energy needs can be provided by wind energy alone if the technological barriers are overcome. The application of sustainable controls for wind energy systems is still in its infancy, and there are many fundamental and applied issues that can be addressed by the systems and control community to significantly improve the efficiency, operation, and lifetimes of wind turbines.

[1] Global Wind Energy Council. Wind Energy Statistics 2014. Report, 2014.

[2] Blanke, M.; Kinnaert, M.; Lunze, J.; Staroswiecki, M. Diagnosis and Fault–Tolerant Control; Springer–Verlag: Berlin, Germany, 2006.

[3] Simani S. Overview of Modelling and Advanced Control Strategies for Wind Turbine Systems. Energies, October 2015, 10, 12116–12141. (This article belongs to the Special Issue Wind Turbine 2015)

[4] Odgaard, P.F.; Stoustrup, J. A Benchmark Evaluation of Fault Tolerant Wind Turbine Control Concepts. IEEE Transactions on Control Systems Technology 2015, 23, 1221–1228.

Article provided by
Dr. Silvio Simani 
University of Ferrara
IFAC Technical Committee 6.4 – Safeprocess