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

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

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 mode [13].

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 considered.
