Today wind energy has the fastest growth among different kinds of renewable resources. Wind turbines are used to convert this energy to electrical energy. In this thesis, DFIG based wind turbines, as one of the most commonly used wind turbine structures, are studied and their behavior under network faults is evaluated and methods to improve LVRT capability is introduced. Increasing the wind power penetration level in power systems of many countries and regions may have a considerable effect on power system performance, therefore the relationship between wind turbine and networks and their problems in a the case of network connection is highly regarded and, utilities are looking for ways to ensure that these new types of power plants behave as much as possible as conventional power generating stations. Accordingly, to accommodate the specific operating characteristics of wind farms, transmission system operators (TSO) have generally established technical requirements and specific grid codes for wind farms to identify the conditions that wind turbines must meet to connect to the network. These Grid codes cover issues related to frequency operating ranges, reactive power capability, and voltage operating ranges under steady state and transient conditions. New grid codes stipulate that the wind farms, contribute to power system control (frequency and voltage), similar to conventional power plants and emphasize the behavior of wind farms in terms of abnormal performance of the network. One of the grid code requirements is LVRT capability which means, to ensure no loss of production for normally cleared faults, wind turbine must withstand voltage dips down to a certain percentage of the nominal voltage and for a specified duration, before they are allowed to disconnect and they should contribute to network stability because sudden disconnection of wind generators and especially large wind farms, can have negative effect on the network. Since the doubly fed induction generators (DFIG), are widely used in wind energy conversion, many technologies for improving LVRT capability of this type of generator have been proposed and studied in recent years.In this thesis, after the introduction of network faults, grid codes, and particularly the characteristic of LVRT, models and methods of controlling DFIG in normal and faulty conditions are reviewed and discussed and different existing ways to enhance LVRT capability is introduced. Proposed method in this paper, with minimal additional equipment and changes in control strategy, allows quick recovery of synchronization after fault clearance so that the turbine can be connected to grid very .... Keywords : Voltage Sag, Grid Code, LVRT Capability, Wind Farms, Doubly-Fed Induction Generator, Synchronization