This review article aims to provide an overview and insight into the most relevant aspects of wind energy development and current state-of-the-art. The industry is in a very mature stage, so it seems to be the right time to take stock of the relevant areas of wind energy use for power generation. For this review, the authors considered the essential aspects of the development of wind energy technology: research, modeling, and prediction of wind speed as an energy source, the technology development of the plants divided into the mechanical and electrical systems and the plant control, and finally the optimal plant operation including the maintenance strategies. The focus is on the development in Europe, with a partial focus on Germany. The authors are employees of the Fraunhofer Institutes, Institute for Energy Economics and Energy Systems Technology and Institute for Wind Energy Systems, who have contributed to the development of this technology for decades.

In this article, an operational strategy and control concept for wind turbines (WTs) are described, which would allow them to actively contribute to black-start events after major power system outages. The approach is based on (a) a new generator/converter control strategy implementing a so called virtual synchronous machine (VSM) and (b) a number of modifications to the superimposed WT controller allowing for operation in black-start conditions. In order to operate stably even at very low active power levels and to cope with sudden changes in active power, due to switching of loads in the recovered grid, the rotor speed/pitch controller had to be redesigned. The extension of the operational range of the WT towards negative power, ie, power consumption, is discussed, which would allow the turbine to temporarily provide a controllable minimum load to conventional power plants until a sufficient number of consumers has been reconnected. The control system has been implemented and verified using two experimental power converters, each linked to a hardware-in-the-loop (HiL) simulator of a WT and connected to a real mediumvoltage laboratory grid. K E Y W O R D Sblack starting, frequency and voltage control, wind turbine, VSM, pitch control | INTRODUCTIONPower system outages are a major concern in many countries, and considerable effort is put in to avoid them. However, when multiple unexpected disturbances occur, power outages cannot always be avoided. In such situations, fast restoration (black start) of the power system is the key to minimize damage for people and economy. As examples, major power outages recently happened in Turkey in 2015 1 the US east coast, or in northern Brazil. 2 After a major power outage, a series of processes for black starting have to be carried out in order to restore the network operation step by step, normally without help by neighboring power grids. A recent review on power system restoration is given by Liu et al. 3 In general, after ramping up the so-called self-starting power plant units, the main transmission lines are restored firstly, and then additional power plants and loads are reconnected stage by stage. Especially in the beginning of this process, the grid area can be very weak in terms of both frequency and voltage deviations in response to switching events. For this reason, control actions to restore the active and reactive power balance must be carried out with high dynamics. 4 As initial self-starting power plants for black-start events, traditionally, hydropower or gas turbine plants are considered. 3 In recent years, also HVDC-converter stations are regarded as suitable means for black starts. 5

In this paper, a robust design method for current control is proposed to improve the performance of a three phase voltage source converter (VSC) with an inductorcapacitor-inductor (LCL) filter. The presence of the LCL filter complicates the dynamics of the control system and limits the achievable control bandwidth (and the overall performance), particularly when the uncertainty of the parameters is considered. To solve this problem, the advanced H? control theory is employed to design a robust current controller in stationary coordinates. Both control of the fundamental frequency current and suppression of the potential LC resonance are considered. The design procedure and the selection of the weight functions are presented in detail. A conventional proportional-resonant PR controller is also designed for comparison. Analysis showed that the proposed H? current controller achieved a good frequency response with explicit robustness. The conclusion was verified on a 5 kW VSC that had a LCL filter.

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Advanced testing methods are becoming more and more prevalent to increase the reliability of wind turbines. In this field, dynamometers that allow for system level tests of full-scale nacelles will play an important role. Operating these test benches in a hardware-in-the-loop (HiL) set-up that emulates realistic drive train modes is challenging because of the relatively low stiffness of the load machines’ drive trains. This paper proposes a control method for enabling the said operation mode. It is based on the idea that the HiL-controller overrides the present unrealistic dynamics and directly imposes desired realistic dynamics on the test bench. A solution for the control problem is given and applied in a design study with a generic wind turbine and a test bench model obtained from construction data of a real test bench. In the design study, the HiL-controller robustly imposes desired drive train dynamics on the test bench model. Despite measurement noise, unmodelled parametric uncertainty, and unmodelled delays, the first drive train mode is correctly reproduced. This is confirmed by a comparison with simulation results from a full servo-aero-elastic code. Furthermore, an implementation of the test bench model on a programmable logic controller showed the real-time feasibility of the proposed method