Variable Speed Wind Turbines for Power System Stability Enhancement Nayeem Rahmat Ullah, Student Member, IEEE, and Torbj¨ orn Thiringer, Member, IEEE (Invited Paper) Abstract—This paper investigates possible improvements in grid voltage stability and transient stability with wind energy converter units using modified P/Q control. The voltage source converter (VSC) in modern variable speed wind turbines is utilized to achieve this enhancement. The findings show that using only available hardware for variable-speed turbines improvements could be ob- tained in all cases. Moreover, it was found that power system sta- bility improvement is often larger when the control is modified for a given variable speed wind turbine rather than when standard variable speed turbines are used instead of fixed speed turbines. To demonstrate that the suggested modifications can be incorporated in real installations, a real situation is presented where short-term voltage stability is improved as an additional feature of an existing VSC high voltage direct current (HVDC) installation. Index Terms—Reactive power, static synchronous compensator (STATCOM), transient stability, variable speed wind turbine, volt- age stability. I. INTRODUCTION I NSTALLED wind power generation capacity is continu- ously increasing. Wind power is the most quickly growing electricity generation source with a 20% annual growth rate for the past five years [1]. As wind energy is increasingly integrated into power systems, the stability of already existing power systems is becoming a concern of utmost importance. Therefore, wind generators should react responsibly. Wind generators should not degrade the stability of the existing power system, but should, if possible, contribute to increased system stability. For example, the focus during the last years is the continued grid-connection of wind energy installations at certain grid-voltage disturbance levels, so as, to avoid voltage dips and regional power deficits when wind energy units are disconnected. A natural next step is now to utilize control of the active and reactive power of modern wind turbines to further enhance grid and wind energy installation interaction. Power electronic-based reactive-power compensators like static var compensator (SVC) and STATic synchronous COM- pensator (STATCOM) can control the voltage quickly and con- tinuously, unlike mechanically switched capacitors/reactors [2]. Several technical papers show the applicability and effect of these power-electronic-based var compensators on improved Manuscript received July 12, 2006; revised October 19, 2006. This work was supported by E.ON Sverige AB. Paper no. TEC-00313-2006. The authors are with the Department of Energy and Environment, Division of Electric Power Engineering, Chalmers University of Technology, Gothenburg 41296, Sweden (e-mail: nayeem.ullah@chalmers.se). Digital Object Identifier 10.1109/TEC.2006.889625 power system stability [3]–[7]. In [2], several utility applica- tions of these devices can be found. Today, variable speed wind turbines have become more com- mon than traditional fixed-speed turbines [8]. In 2004, the world- wide market share of these wind turbines was approximately 60% [9]. The variable speed wind turbines are either doubly fed induction generators (DFIG) or full power converters [8]. From a power system point of view, these configurations are interest- ing because the power electronic interface isolates the generator characteristics from the rest of the power system. Only the con- trolled converter characteristic is seen by the grid [10]. For the standard DFIG system, this is only true for a timescale of 100 ms and longer. Since a grid-side converter of variable speed wind turbine is a dc/ac voltage source converter (VSC), it can be seen as a STATCOM from a hardware point of view. This “wind turbine STATCOM” is already connected to the grid, albeit for its original purpose of handling only active power mainly to transmit energy from the turbine to the grid. In addition, for the DFIG system, the generator can also be utilized to produce or consume reactive power through the magnetization provided by the rotor-side converter. Obviously, this could further improve the impact on power system stability of modern wind turbines, and increase the value of wind energy installation, particularly, if this can be done without additional cost. An important issue is then the choice and extent of control modifications. The purpose of this paper is accordingly to investigate these grid reinforcing possibilities (voltage and transient stability im- provements) that could be achieved by variable speed wind turbines with power electronic converters, and to give some ex- amples in order to quantify the improvements. In addition, it presents a real situation to demonstrate the implementation of these ideas. II. WIND TURBINE SYSTEMS STUDIED A. Functionality Three functions of wind energy converter systems (WECS) are studied: induction generator characteristics (traditional fixed-speed turbine, system A); constant power factor opera- tion (system B); and variable power factor operation (system C). System B represents the standard control of a variable speed system and system C represents a modified control of a variable speed system. The capability diagrams of the three systems are shown in Fig. 1. The capability diagram of system B is shown with a bold horizontal line in the right diagram, the case when the unit is operating at power factor of unity. 0885-8969/$25.00 © 2007 IEEE Authorized licensed use limited to: King Mongkuts Institute of Technology Thornburi. Downloaded on October 15, 2008 at 23:50 from IEEE Xplore. Restrictions apply.