Currently, wind energy systems due to maturity in technologies are among the most promising renewable energy resources. Several countries have now considered deployments of wind farms as a part of sustainable energy solutions in energy development plans. Three types of fixed-speed, partial variable-speed, and fully variable-speed topologies are commonly used in wind turbine-generators. Recent high-power wind turbine generators mainly use variable or partial variable-speed topologies to achieve higher efficiency. Direct-drive wind turbine generators, as an efficient alternative for high power wind power units, have been of interest of researches during recent years. In a direct drive topology for a wind power unit, the gearbox is eliminated and the low-speed shaft of the turbine is directly connected to the generator. This topology improves the efficiency of the wind power units and it also reduces the maintenance costs which are significant due to modular structure of wind farms. The main challenges for deployment of direct-drive topologies are development of low-speed (less than 100 rpm) high power generators with suitable power electronics interfaces for capturing and delivering the maximum wind power to a grid. This thesis deals with topologies for direct-drive wind turbine-generators which address the above challenges. In the first part of the thesis, a poly-phase axial flux machine will be investigated as a promising solution for low-speed high-torque generators. Due to high-power density, axial flux machines can be used in direct-drive wind power units with limited spaces. The thesis investigates the dynamic model of an axial flux machine and develops a generalized control method for poly-phase machines based on conventional qd frame of reference that have been widely used in literatures. In the second part of the thesis, applications of newly developed modular multi-level converters for grid integration of wind units using poly-phase axial flux machines will be presented. In the proposed topology for capturing and tracking maximum wind power, the output voltage of the poly-phase axial flux machine is rectified to an almost ripple-free dc voltage via using series connected three-phase diode-bridge rectifiers. Then, the dc voltage is converted to a multi-level pulse-width modulated ac voltage via using a modular multi-level converter. The control provision that is used for the proposed topology is to adjust the rotor speed of the generator via control of real power flow in the converter. Meanwhile, injected reactive power at the grid-side converter has been also adjusted via using a feedback control loop that uses modulation index as an input control signal. The significant features of the suggested topology for direct drive wind turbine generators are: i) using a poly-phase machine in conjunction with diode interfaces which hugely reduce ripples and the required filter capacitor; ii) using a multi-level topology that reduces the switching frequency which in turn improves the efficiency of the converter especially at high-power applications. The performance of the suggested topology has been tested via simulation of a study system including a 9-phase permanent magnet axial flux machine and a power electronic interface including a 3-stage diode rectifier and a 5-level converter. Keyword: Direct drive wind power; Modular multi level converters; Permanent magnet axial flux machines; Poly-phase axial flux machines.