Harmonic Modeling of the Series-Connected Multipulse Rectifiers Under Unbalanced Conditions

With the promotion of high voltage direct current system construction around the world, the series-connected multipulse rectifiers (SCMPRs) have been widely used in high-power rectifying occasions in the power system. Since the certain order of harmonics cannot be effectively eliminated under the nonideal conditions, many different phenomena, such as the unbalance of supply voltage and line/transformer parameters, the influence of background harmonics, and the nonequidistance of thyristor triggering pulses, may cause the noncharacteristic harmonics of SCMPR. However, the existing frequency-domain harmonic model cannot be used to analyze the harmonic characteristics of the SCMPRs under nonideal conditions. In this article, a general frequency-domain harmonic coupling matrix model (HCMM) is proposed. The proposed HCMM can consider most of the above imbalance factors and the model can explicitly illustrate the harmonic coupling relationship between the SCMPRs supplied voltage and the generated harmonic currents. The generality of the SCMPR model under nonideal conditions is improved accordingly. The HCMM is simplified by comparing and analyzing the effects of the transformer structure and parameters. Moreover, by analyzing the harmonic generation characteristic of the negative-sequence component, the HCMM can be further simplified. Finally, time-domain simulation and lab experimentation are performed to verify the accuracy and efficiency of the frequency-domain HCMM of SCMPRs. The results prove the generality of the proposed model and also illustrate the harmonic generating influence of unbalanced parameters from the transformer and supply voltage.