A common-mode voltage reduction strategy for a DFIG with a three-level back-to-back converter

Since the application of power electronic devices became dominant in wind turbine systems, shaft voltage turned into a drawback of the AC generators as well as the AC motors in the modern AC drive applications. This voltage is a resultant phenomena of interaction between PWM strategies of power electronics converters and AC generator structure. Common mode voltage generated by PWM inverters and the parasitic couplings of the machine structure in high frequencies create a model for the system which leads to an induced voltage on the shaft. This paper investigates a shaft voltage remediation strategy for a doubly fed induction generator. A three-level back-to-back AC-DC-AC converter has been used in this system. This analysis is based on reduction of the maximum common mode voltage levels by removing related switching vectors from a switching pattern. Applying this technique leads to a 66 percent reduction of the common mode voltage of the rotor side converter which plays the key role in shaft voltage generation of the DFIG. The advantages of proposed method are: voltage balancing of DC link capacitors and controlling of the active and reactive power delivered by the wind generator. Mathematical analysis and simulation studies have been presented to verify this approach.