[en] Highlights: • A transfer function represents the wind energy conversion process with spatial effects. • A compensation wind speed represents the overall effect of all pitch angle controllers. • Simplest way to build equivalent model of complete wind energy conversion process. • Proposed model is identified and validated by measurements from an actual wind farm. • Example of using proposed model to convert forecasted wind speed into output power. The correct electric power fluctuation process of a wind farm is essential for the operation and studying of the power grid integrated with wind power. A complete wind energy conversion model, that should take both the spatial effects and the overall effect of all the pitch angle controllers into account, is required to convert the wind fluctuation into the electric power fluctuation. Although some equivalent models have been proposed in previous studies, there is no simple solution for equivalently modeling the non-synchronous actions of the pitch angle controllers in individual wind turbine generators. This study found the relationship between the overall effect of all the pitch angle controllers and the spatial effects of the wind farm and presented a theoretical derivation of the frequency-domain equivalent modeling method. The proposed modeling method is the simplest way to obtain the equivalent model of the complete wind energy conversion process of a wind farm with consideration of all the spatial effects and the overall effects of all the pitch angle controllers. The only input signal of the proposed equivalent model is the speed of the wind before entering the wind farm, which is called the “original incoming wind speed” and the only output signal is the total power of the wind farm. In this proposed equivalent modeling method, a discrete transfer function for representing both the wind energy conversion process, and all the spatial effects of a wind farm, can be obtained, first through a wind process below the rated wind speed. Second, a compensation factor for calculating a “compensation wind speed” of the original incoming wind can be obtained through a wind process that partly exceeds the rated wind speed. Using this compensation wind speed, a compensation power with negative values can be obtained to represent the total reduced power caused by the pitch angle controllers in individual wind turbine generators. A frequency-domain equivalent model has been identified and validated by field measurements of an actual wind farm. The normalized root-mean-squared error of the model is less than 8% over the entire wind process, and the maximum error of the power ramp rate in 10 min is less than 5 MW. Finally, an example is provided to demonstrate the online use of the proposed model to convert the forecasted wind speed into output power of a wind farm in ultra-short-term wind power forecast.

[en] A source of welfare, health, social and economic development, energy is essential to human life. Meeting world energy demand, restraining energy consumption in developed countries, and reducing global emissions of greenhouse gases (GHG) raises fundamental challenges for the future of our planet. Because the problem is so pervasive and because of the approaching 21. UN Climate Change Conference, COP 21, the four National Academies of science and technology in France and Germany decided to enhance their mutual cooperation and to organize two workshops on the 'energy transition'. This initiative is intended to provide a framework for sharing knowledge, anticipating future challenges, identifying areas for closer cooperation between our countries and generating advice for decision makers and society in general. The workshops defined priority areas for further collaboration. These include energy efficiency, grid infrastructure and smart grids, mobility, nuclear energy (fission): safety and waste management, fusion, renewable energies, energy storage as well as social and economic aspects of the energy transition. In view of the complex challenges, compounded by the political debate, and the different conditions and starting points for the energy transition in the two countries, the four academies also identified the following common lines of action. 1. Creating a sustainable energy system. The overriding aim of energy and climate policy in this century will be to create a sustainable system of energy supply, i.e. one in which GHG emissions are substantially reduced worldwide, while at the same time ensuring security of supply. This will require a long-term perspective, as well as a suitable balance between environmental goals, social expectations, and economic objectives. 2. Promoting collaborative policies. The academies welcome the stated intention of both governments to support effective energy and climate policies on a European level through the medium of a European Energy Union. This will make energy more sustainable, affordable and secure, and produce policies that are compatible with those implemented in other parts of the world. In this spirit, the academies particularly encourage the effort of both governments to reach an ambitious international climate agreement during COP 21 in Paris at the end of 2015. 3. Asserting the central role of science, technology and industrial development. Achieving an efficient energy transition resulting in a substantial net reduction of GHG emissions raises complex issues that are usually underestimated. They will require solutions that are fundamentally based on science, technology and industrial know-how. The academies thus support strong funding programmes for energy R and D, in particular for long-term basic research in crosscutting topics. Such programmes should focus on novel approaches and, if designed properly, would also help to improve the international competitiveness of European industry. 4. Raising public awareness of energy policies. To promote a systemic and shared understanding of the energy system, it is important to understand public expectations and enhance public awareness of energy issues and constraints. Ultimately, a European energy vision should evolve on which the European citizens can agree. 5. Enhancing French-German scientific, technological and industrial cooperation. Both France and Germany need innovation and breakthroughs in the energy field. They are fortunately endowed with a high degree of competence in the appropriate areas, but would both benefit from more intense scientific and industrial cooperation. Finally, the academies suggest the establishment of an academy led French-German consultative research committee which could discuss both research issues of mutual interest and priorities in connection with the energy transition. It could also provide a means for assessing developments in the two countries