| This paper focuses on the control strategies to improve the performance of the three-phase ac-dc PWM Voltage-Source-Converters (VSR) under unbalanced power supply. The problems such as negative sequences current, voltage and current harmonics and un-constant (un-smooth ) power flow from ac to dc side are discussed in this paper. The analytical solutions for negative sequence current and harmonic current elimination are obtained by fully feed-forward of negative sequence voltage and producing a non-harmonic current command of the inner-loop. Also the solutions for dc voltage harmonics are achieved resulting in controlling a smooth (constant) power flow from ac to dc side in spite of the unbalanced voltage conditions.Differs from the traditional unbalanced control method which makes VSR only operate in the unity power factor when eliminating dc voltage harmonics under unbalanced conditions[24][25], The newly proposed control strategy in this paper can not only eliminate the dc voltage harmonics but also control the average active/reactive power of the VSR separately. So the VSR can operate in any power factor.Internal model control (IMC) design concept has been introduced to the unbalanced control system of VSR. With IMC design concept, the unbalanced control system not only achieve zero steady state error of sinusoidal AC current , but also obtains strong robustness of the control system. Furthermore the unbalanced control system can be greatly simplified based on the IMC.Finally, The establishment of switching fanction simulation models, and the simulation results of static and dynamic performance are described in detail. And a laboratory prototype has been established to verify the discussions and analyses done in this paper. The experimental results and simulation results show great consistency. They all verify the correctness of the proposed control strategy.
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