The paper analyzes a torque and DC-current ripple mitigation strategy in a modular axial-flux permanent magnet synchronous generator for multi-MW Wind Energy Conversion Systems (WECSs). The machine modules are aligned with one another and driven by the same shaft. Each module is connected to its own two-level converter and the converters are paralleled at the DC side. In order to reduce the DC-side current harmonics and the ripple in the resultant torque, a phase shift between the converters' PWM carrier signals is implemented. The converters are operated within the linear PWM modulation range. A mathematical analysis is carried out. A criterion for the best choice of the phase shift in two-and three-module configurations is developed. Theoretical results are compared with those from simulations in Matlab/Simulink, verifying the soundness of the method.

The paper concerns the study of multi-winding transformers (MWT), with inverter fed LV primary windings and one MV secondary winding connected to the mains. Typical application fields are medium power WECSs, consisting of modular components (multi-module generator and transformer). The transformer design and modeling aspects are considered, and its performances are estimated, considering inverter command parameters, current waveforms, copper and core losses, accounting for the effects due to harmonics. Index Terms-multi-winding transformers, transformer design, transformer model, inverter fed transformer losses.

The paper presents a method to reduce both the torque ripple and the dc current ripple in the machine side converter, in a multi megawatt Wind Energy Conversion System, based on a PM Synchronous Generator which has a multi modular structure. Each converter module consists of a three-phase, two level inverter, operating as a rectifier, with PWM sinusoidal modulation. The first aim of the method is exploiting not only the linear PWM region but also over modulation region, in order to allow a higher AC voltage, with the same dc link voltage. The ripple mitigation is gained by means of a suited shifting among the carrier signals and the control signals in the several modules. The paper describes the method, and discusses the criteria that can be adopted to choose the suited shifting angles. Some simulation results are reported.