**3. System modelling**

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

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‐

Doubly-fed induction generators are gaining popularity these days for several reasons. The primary reason for this is their ability to vary their operating speed, typically +/- 30% around the synchronous speed. The stator is directly connected to the grid and the rotor is fed from a back-to-back AC/DC/AC converter set as shows Fig. 1. The rotor side converter (RSC) controls the wind turbine output power and the voltage measured at the grid side. The grid side converter (GSC) regulates the DC bus voltage and interchange reactive power with the grid, allowing the production or consumption of reactive power. Then, DFIG can

PV mode refers to DFIG generating or absorbing reactive power (MVAr) to/from the distri‐ bution network in order to maintain the terminal voltage at a specified value. The minimum and maximum MVAr have to be specified in order to operate at a power factor between 0.9 leading and 0.85 lagging, otherwise the plant operators will be charged for violating the op‐ erational limit. In load flow studies DFIG is represented as a PV bus for voltage control

PQ mode refers to the DFIG generation at a fixed MW and a fixed MVAr. When DFIG re‐ al power generation varies, the reactive power will also vary to maintain a fixed power factor. This mode usually employs unity power factor operation (zero reactive power out‐ put). However, other power factor values can be specified (e.g., from 0.95 leading to 0.95 lagging) according to the system operator requirements. In load flow studies DFIG is rep‐ resented as a PQ bus for power factor control mode. In this study both control modes are

operate on voltage control mode (PV) or power factor control mode (PQ).

Excitation Limiters (OEL) and On Load Tap Changers (OLTC).

action between OLTC and OEL equipments.

mode [13].

226 Advances in Wind Power

considered.

**2. Doubly Fed Induction Generators (DFIGs)**
