Unbalanced And Harmonic Voltage Compensation For Standalone Brushless Doubly-fed Induction Generators

Without brushes and slip rings,the brushless double-fed induction generator(BDFIG)can operate more stably and reliably,so that it is expected to be one of leading machines in the coming years for the standalone generation mode.The stability of frequency and amplitude of the power winding(PW)voltage is the main control target in standalone generation mode.Moreover,the standalone mode is sensitive to abnormal working situations,especially under unbalanced and nonlinear loads.The unbalanced and nonlinear loads can cause significant unbalance and distortion for voltage and current of PW.In PW voltage,the negative sequence component represents the effect of unbalanced loads,and the 3rd,5th and 7th harmonic components indicate the effect of nonlinear loads.In this thesis,the negative sequence voltage compensator,low-order harmonic voltage compensator and the dual-resonant controller(DRC)are proposed to reduce the unbalanced and nonlinear load influences.The control strategy proposed in this thesis has been fully verified on the 30 k VA BDFIG test platform in the laboratory.The main contributions of this thesis can be summarized as follows:(1)For the system under unbalanced loads,a negative sequence voltage compensator is proposed to minimize the impact of unbalanced voltage on the standalone BDFIG system.The proposed control strategy consists of two parts.The first part employs the direct voltage control strategy to control the frequency and magnitude of the positive sequence PW voltage.The second part is the negative sequence compensator in the machine side converter(MSC)to eliminate the negative sequence component of the PW voltage.The dual second-order generalized integrator is used to extract negative-sequence component of the PW voltage.The proportional-integral controllers are employed to obtain the reference of the control winding(CW)current that compensates for the negative sequence component of the PW voltage.And,the proportional-integral-resonant controller is applied to regulate the CW current in the positive reference frame.(2)For the system under three-phase nonlinear loads,the low-order harmonic voltage compensator is proposed to reduce the impact of low-order harmonic voltage on the standalone BDFIG system.The proposed control strategy consists of two parts.The first part is the direct voltage control strategy to control the frequency and magnitude of the positive sequence PW voltage.The second part is the low-order harmonic voltage compensator in the MSC to minimize the 5th and 7th harmonic components of the PW voltage.The multiple second-order generalized integrator is used to extract the 5th and 7th harmonic components of the PW voltage.The proportional-integral controllers are used to obtain the reference of the CW current that compensates the 5th and 7th harmonic components of the PW voltage.Finally,a proportionalintegral-resonant controller is proposed and designed,which can effectively control the CW current in the positive reference frame.(3)For the system under unbalanced nonlinear loads and single-phase nonlinear loads,the DRC is proposed,which can effectively reduce the influence of non-linear and unbalanced loads on the power winding voltage,and be with the advantages of simple structure and small calculation burden.The DRC composes of two parts: unbalance resonant controller and the harmonics resonant controller.The unbalance resonant controller is designed to compensate for the unbalanced and third-order harmonic components in the PW voltage.And,the harmonics resonant controller is employed to eliminate the 5th and 7th harmonics in the PW voltage.With the proposed DRC,no additional filters are needed to extract the negative-sequence and harmonic components of the PW voltage,which enhances the system response.