Every year the world installed wind farms are growing due the increase of electricity demand, the growth of greenhouse gas and the rise of the fossil energy price. The use of DFIG wind turbine equipped with the three level back to back NPC converters could be the ways to reduce the installation prices. Moreover the choice of the MMC HVDC transmission increases the system reliability, and reduces the power losses.This thesis presents the simulation system to study the behaviors of the wind farm connected to the AC network using HVDC transmission system.We started develop the model of wind turbine, described it control strategies, and its behaviors have been simulated and analyzed. The back to back three level neutral point diode clamped converter model and it control strategies are presented.The research on the wind turbine using different LVRT techniques like the crowbar, the BESS and the braking resistor combined with fault current limiting. The protection characteristics of the techniques are modeled and simulated in PSCAD. The results show that for small wind turbine the crowbar and battery energy storage system can be used to protect the wind turbine during fault conditions. When the number of wind turbines is high and for big wind turbines the braking resistor combined with the fault current limiter can be used.The research on the HVDC transmission system using different LVRT techniques like the emulated short circuiting and the multilevel chopper has been described. The protection characteristics of the techniques are modeled and simulated in PSCAD. The results show that the emulated short circuiting can be used to protect the HVDC transmission system during fault at onshore grid. This system can be used to replace the telecommunication system. To protect the HVDC transmission system during onshore and offshore AC network fault the proposed multilevel chopper gives the best results. The impact of the dissipation of excess energy from wind farm in AC network voltage is reduced. During asymmetric fault the control of circulating current can be employed to damping the DC voltage fluctuations.The wind farm using doubly fed induction generator wind turbine model is developed and it control strategies are presented. The wind farm power flow controls during normal and abnormal operations are also presented. The results show that the proposed control methods are suitable.The MMC-HVDC transmission system model is developed and the control strategies for onshore and offshore converters are studied. According to the results the method used to model the MMC-HVDC converter reduces the simulation time, especially when the number of sub modules per arm is high. Therefore, we can assume that the model is suitable for the simulation.Aggregated five wind turbines using doubly fed induction generator which rotor is interfaced by the back to back of a three level NPC converter has been used to model the large wind farm. During the fault conditions, the wind turbine generator rotor current is controlled and limited to protect generator. The excess power from wind farm is dissipated in braking resistor in order to protect the converter against the DC link over-voltage. The HVDC system during disturbances at both the onshore and offshore AC networks remains connected to the grid thus it capable to mitigate the DC link over-voltage.The circulating current within the HVDC converters legs is controlled so that in fault conditions will limit the DC link voltage oscillation and then reduce the stress on power converters.