Optimal Modulation Strategy Based On Three-level Doubly-fed Wind Turbine

With the gradual depletion of fossil energy,new energy generation has attracted more and more attention because of its clean and recyclable advantages.Among them,wind power generation has less adverse impact on human production and life and is suitable for development according to local conditions.However,in the wind power generation system,doubly fed motors have a lower demand for the capacity of the converter and have a higher market recognition.However,common-mode voltage is a problem in both two-level converter and three-level converter systems.High common mode voltage will cause shaft voltage and shaft current,long time operation will affect the service life of the motor,great harm to the motor.In this paper,the mathematical models of the three-level doubly-fed motor and the grid-side converter are established,and the control strategies of the machine side and grid-side converters are deduced respectively.Aiming at the characteristic that the common-mode voltage reduction of the two-level converter needs to flip the one-phase switching state of the three-phase,four control schemes are proposed according to the difference between the flipping phase and the injected zero-sequence voltage,which are easier to implement than the traditional scheme.In order to reduce the common-mode voltage of three-level converter,an improved reference voltage decomposition method is proposed in this paper,which can achieve three-level common-mode voltage reduction at two levels,and effectively reduce the complexity of control.In addition to common-mode voltage,midpoint potential unbalance exists in midpoint clamped three-level converter.according to the difference of injected zerosequence voltage,two ideas of precise control and fuzzy control are proposed in this paper,which can effectively suppress midpoint potential unbalance.In order to achieve midpoint potential balance control and common-mode voltage balance control at one side of the converter,the reference voltage decomposition method is further optimized,and three optimal modulation strategies are proposed which can achieve common mode voltage balance control and common-mode potential balance control at the same time,which have certain practical value.Finally,the effects of different schemes on the system performance are compared through simulation,and the effectiveness of the proposed scheme is verified on the hardware-in-the-loop simulation platform.