Transient stability margin of variable versus fixed speed wind systems in electrical grids

Abstract

For networks with large scale integration of wind farms this paper demonstrates a significant difference between the transient behavior of Variable Speed Wind Systems that adopt doubly fed induction generators with a four quadrant acto-ac converter connected to the rotor windings @FIG) and a fixed speed wind system, where the conventional cage induction generator is used. The reason for this difference is explained, and it is shown that the phenomenon is due to the ability of the two dedicated rotor current regulators of the DFIG to control the electrical torque and reactive power quickly and accurately. This difference is clearly observed after a severe fault (short circuit) occurrence. It will be demonstrated that the adjustable speed generator (DFIG) enhances system stability in power networks.

Influence of the Variable-Speed Wind Generators in Transient Stability Margin of the Conventional Generators Integrated in Electrical Grids

“…To enable dynamic modeling of the DFI-based wind turbine under different wind speeds, it is important to understand the required configuration of the system and to consider that this kind of machine has to be fed from both the rotor and stator sides [10], [11]. A wind converter system with a DFIG and a bidirectional power converter is shown in Fig.…”

“…Considering the grid side of the converter operating at unity power factor, the reactive power at the stator of the DFIG varies with the stator currents as follows: (11) Since the stator transients are very fast, when compared with the rotor ones, it is possible to neglect them. This corresponds to ignoring the dc component in the stator transient current, enabling representation of only fundamental frequency components.…”

“…Alternatively, it is possible to omit the inner current control loop of the converters in transient stability studies due to the fact that the action of these controllers is much faster (milliseconds) than the speed and voltage-control loop of the DFIG [11]. Therefore, it is also possible to model the converters as a current source.…”

Performance analysis of a grid connected DFIG for non-optimum power tracking

“…However, variable speed doubly-fed induction generator wind turbines have gained importance because of their interesting features. Some of them are highlighted below [5]- [6].…”

Rotor Speed Stability Analysis of a Constant Speed Wind Turbine Generator

“…The study of transient stability of a constant speed WTG using dynamic simulation has been presented in (Rodriguez et al, 2002) with case study on Spanish system. The comparison of transient stability margin between constant speed and variable speed WTGs has been discussed in (Nunes et al, 2003). It has been shown that a constant speed WTG has a much lesser transient stability margin as compared with a variable speed WTG.…”