| Multi-pulse rectifier is a common interface of most high power applications.However,diode rectifiers are used to convert electric energy,which brings a lot of harmonic pollution while increasing the reliability.In order to solve the problems,a series-connected 18-pulse rectifier is proposed.The passive and Hybrid Harmonic Suppression Methods of rectifier are studied,the working characteristics of rectifier in unbalanced state is researched,which is of great significance to improve the input and output power quality of the rectifier.In order to improve the harmonic suppression performance of multi-pulse rectifier,a series-connected 18-pulse rectifier based on isolation transformer is studied.The phase-shifting transformer is designed,the equivalent model of rectifier bridge is established,the working mode of rectifier is analysed,and the relationship between input voltage and load voltage of rectifier is obtained.The simulation and experimental results show that the THD value of the input voltage is 6.44%,the THD value of the input current is 5.08%.In order to suppress the low order harmonics and improve the power quality of the 18-pulse rectifier,a passive harmonic suppression method at DC side of the rectifier is proposed.The mathematical relationship between the input voltage of the rectifier and the turn ratio of the injection transformer under each working mode is analyzed,the turn ratio of the injection transformer is calculated.Simulation and experimental results show that the THD value of the input voltage can be reduced to5.08%,the THD value of the input current can be reduced to 3.91%.In order to suppress the high-order harmonics of the 18-pulse rectifier,a hybrid harmonic suppression method at DC side is proposed.According to the working principle of the rectifier,the control circuit is designed.From the perspective of the minimum THD value of the input voltage of the rectifier,the turn ratio of the injection transformer is calculated.Applying the DC side hybrid harmonic suppression method to the series-connected 12-pulse rectifier,and the corresponding simulation is carried out.The simulation and experimental result s show that the THD value of the input voltage of the 18-pulse rectifier can be reduced to 1.9%,and the THD value of the input current of the 18-pulse rectifier can be reduced to 2.7%.The THD value of the input voltage of the 12-pulse rectifier can be reduced to2.21%,and the THD value of the input current of the 12-pulse rectifier can be reduced to 2.92%.In order to further analyze the harmonic suppression mechanism of the rectifier,the operating mode of the rectifier using passive harmonic suppression method under the condition of three-phase input imbalance is studied.According to the triangle theorem,the on-off state of the diode is analyzed,and the quantitative relationship between input voltage of rectifier and turn ratio of injection transform er is obtained.The simulation and experimental results show that the harmonic distortion rate of the input voltage and current of the rectifier will not change basically under the condition of three-phase imbalance,and the passive harmonic suppression method at the DC side can effectively suppress the non characteristic harmonics generated by the three-phase imbalance of the rectifier. |
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