**1. Introduction**

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224 Advances in Wind Power

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The wind energy industry is experiencing a strong growth in most countries in the last years. Several technical, economic and environmental benefits can be attained by connecting wind energy to distribution systems such as power loss reduction, the use of clean energy, postponement of system upgrades and increasing reliability. The doubly fed induction gen‐ erator (DFIG) is currently the most commonly installed wind turbine in power systems. DFIG can be operated in two different control modes: constant power factor and voltage control. In the power factor control mode, the reactive power from the turbine is controlled to match the active power production at a fixed ratio. When terminal voltage control is em‐ ployed, the reactive power production is controlled to achieve a target voltage at a specified bus. Many wind operators currently prefer the unity power factor mode since it is the active power production that is rewarded [1].

The integration of wind turbine in electricity networks still face major challenges with re‐ spect to various operational problems that may occur, especially under high penetration lev‐ el [2,3]. Among many problems, it can be highlighted the voltage instability phenomenon, a constant concern for modern power systems operation [4,5]. Voltage stability refers to sys‐ tem ability to keep voltage at all buses in acceptable ranges after a disturbance. This phe‐ nomenon is local and non-linear, characterized by a progressive decline in voltage magnitudes, and occurs basically due to system inability to meet a growing demand for re‐ active power at certain buses in stressed situations [6,7]. The phenomena involved in voltage stability is usually of slow nature (minutes or hours), being driven by the action of discrete type devices and load variations [7].

© 2012 Rorato Londero et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Rorato Londero et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Some papers have analyzed the impacts caused by the connection of wind generation on voltage stability using static models [8,9]. However, the exclusive use of static models is in‐ sufficient to fully describe voltage instability phenomenon, especially considering the actua‐ tion of dynamics equipments. Also, few papers have explored the differences in DFIG mode of operation [10,11,12]. Until now, no work has presented a study analyzing the impacts of different DFIG control modes on long-term voltage stability analysis, especially considering the dynamics aspects and interaction of equipments installed in the network, such as Over Excitation Limiters (OEL) and On Load Tap Changers (OLTC).

This chapter presents a study comparing the impacts caused by different control modes of DFIG wind turbine on long-term power system voltage stability. The study uses time do‐ main simulations and also includes the representation of Over Excitation Limiter (OEL) and On Load Tap Changers (OLTC). The analysis focuses on the two DFIG excitation control modes: constant voltage control and constant power factor control (unity power factor and leading power factor) with a 20% load increase. The impact of each control strategy is stud‐ ied and the resulting change in long-term system stability is quantified, as well as the inter‐ action between OLTC and OEL equipments.
