Research on the parallel technique for the direct-drive wind power converter

At present, the parallel back-to-back PWM converters which adopt the parallel mode of sharing DC bus are used in megawatt-level high-power wind generation system. However, there will be circular current when the switch states of two parallel converters are not the same and this circular current will increase system wastage and decrease system efficiency. This paper adopts a novel control strategy that adds a zero sequence circuiting current (ZSCC) control loop to the total output current loop and ameliorates PWM pulse drive method of parallel converters. This control strategy assures that the on-off times of two parallel converters are totally the same and consequently the impact of circular current will be eliminate. The simulations and experiments show that the converter can satisfy the system demands of reliability, circular current control and the hardware implementation.