Effects of the PWM carrier signals synchronization on the DC-link current in back-to-back converters

This paper presents a study about the synchronization effects of the PWM carrier signals of a back-to-back converter for grid connection of Wind Energy Conversion Systems based on Permanent Magnet Synchronous Generators. It is demonstrated by means of a spectral analysis that, with the proper synchronization of the carrier signals of both the rectifier and inverter stages, the rms value of the current through the DC-link capacitors is greatly reduced. As a result, the number of capacitors needed to build the back-to-back converter decreases, whereas its life cycle is expanded, so that the Wind Energy Conversion System becomes more cost effective. It is shown that the worst case occurs when the phase difference between both carrier signals is ±π/2, yielding the highest rms value of the DC-link capacitors current. In that case the harmonic with the highest rms value is located at twice the switching frequency. The theoretical analysis is compared with experimental results from a 10 kW back-to-back converter in order to validate the effects of the carrier signals phase shift on the DC-link capacitors current.