The aim of the work is to derive a steady state PQ-diagram for a variable speed wind turbine equipped with a Doubly Fed Induction Generator. Firstly, the dependency between optimal rotor speed and wind speed is presented. Secondly, the limitations in reactive power production, caused by the rotor current, the rotor voltage and the stator current are derived. Thirdly, the influence of switching from D to Y coupling of the stator is investigated. Finally, a complete PQ diagram for a wind turbine is plotted. It is concluded that the limiting factor regarding reactive power production will typically be the rotor current limit, and that the limit for reactive power absorption will be the stator current limit. Further, it is concluded that the rotor voltage will only have a limiting effect at high positive and negative slips, but near the limitation, the reactive power capability is very sensitive to small changes in the slip.The reactive power capability of a wind farm depends a lot on the capability of the wind turbines, although the impact of the grid should also be considered in a PQ diagram for the whole wind farm. The reactive power capability of the wind turbines depends on the type or concept for grid connection of the wind turbines.The first generation of commercial grid connected wind turbines in the 1980s was dominated by the fixed speed concept using squirrel cage induction generator, which was soon supplemented with a capacitor bank for reactive power compensation. Through the 1990s, different types of variable speed concepts became an increasing share of the market. According to Hansen et al., 2 the doubly fed induction generator (DFIG) concept was the most successful variable speed concept with more than 45% market share in 2002.For the synchronous generator, the boundaries defined by the field current limitation, the armature current limitation and the mechanical power limitation can easily be calculated from the ratings and the reactances. This is typically presented in a so-called PQ diagram. 3 A similar description of a DFIG is not found in textbooks. The scope of this paper is the reactive power capability of a DFIG, taking into account the influence of a typical speed control of a wind turbine. Other concepts are not considered here, and the impact of the grid including the step-up transformer is not included.The advantage of the DFIG concept is mainly that variable speed control is obtained with a minimum of power converter capacity. The connection of the main electrical components is shown in Figure 1, based on the description of a commercial wind turbine in Bolik. 4 The DFIG exchanges power with the grid through the stator windings as well as the rotor windings. The main part of the power passes from the generator through the stator into the grid, whereas only a fraction of the power is passed from the rotor windings through the power converter. Besides, a star-delta switch is used to increase the speed control interval as discussed below.To characterize the DFIG, the rotor speed should be taken...

Emphasis in this article is on the design of a co‐ordinated voltage control strategy for doubly fed induction generator (DFIG) wind turbines that enhances their capability to provide grid support during grid faults. In contrast to its very good performance in normal operation, the DFIG wind turbine concept is quite sensitive to grid faults and requires special power converter protection. The fault ride‐through and grid support capabilities of the DFIG address therefore primarily the design of DFIG wind turbine control with special focus on power converter protection and voltage control issues. A voltage control strategy is designed and implemented in this article, based on the idea that both converters of the DFIG (i.e. rotor‐side converter and grid‐side converter) participate in the grid voltage control in a co‐ordinated manner. By default the grid voltage is controlled by the rotor‐side converter as long as it is not blocked by the protection system, otherwise the grid‐side converter takes over the voltage control. Moreover, the article presents a DFIG wind farm model equipped with a grid fault protection system and the described co‐ordinated voltage control. The whole DFIG wind farm model is implemented in the power system simulation toolbox PowerFactory DIgSILENT. The DFIG wind farm ride‐through capability and contribution to voltage control in the power system are assessed and discussed by means of simulations with the use of a transmission power system generic model developed and delivered by the Danish Transmission System Operator Energinet.dk. The simulation results show how a DFIG wind farm equipped with voltage control can help a nearby active stall wind farm to ride through a grid fault, without implementation of any additional ride‐through control strategy in the active stall wind farm. Copyright © 2006 John Wiley &Sons, Ltd.

This paper introduces the power quality issues of wind power installations in a historic perspective, as the development from a few small wind turbines connected directly to the low voltage grid, to the present system with high penetration on the medium voltage distribution grids and two large offshore wind farms connected at transmission level. In this perspective, the power quality issues are divided into local issues particularly related to the voltage quality in the distribution systems and global issues related to the power system control and stability. Power quality characteristics of wind turbines and wind farms are described according to national and international standards, and measurements from wind farms are presented.

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Converter connected units are to a large extent replacing conventional synchronous machines. Most grid codes require reactive current injection from converter-controlled equipment in the power system during fault condition. At present, there is no standardized way of calculating the short circuit level in a system with a high penetration of power electronic converters. In this paper, three different methods (IEC60909, superposition and iterative) have been tested on a grid model of the West Danish power system, which has a high wind power penetration. The short-circuit currents at the short-circuit location calculated by means of these three methods show the results with a negligible difference based on the present-stage system model. However, this is because the converter-controlled wind turbines (WTs) at present constitute 7% of the installed capacity in the model. It is foreseen that the total contributed short-circuit current will be larger as the share of the converter-controlled WTs increases. The result achieved using the iterative method is more reasonable because the grid-voltage support from the converter-controlled WTs is represented in the iterative method but not in the IEC60909 or the superposition method.