| Series-connected multi-pulse rectifier is widely used in large power alternating current motor speed regulation,battery charging and other occasions.The traditional series-connected multi-pulse rectifier may have difficulty in balancing the power quality at AC and DC link simultaneously when using the passive and active harmonic suppression method independently.Therefore,in this paper,the passive harmonic suppression method at DC link,the hybrid harmonic suppression method at DC link and the load adaptability are studied based on the analytical model of the series-connected 12-pulse rectifier.In order to determine the quantitative relation of the electric quantity between AC and DC link of the series-connected 12-pulse rectifier,and settle the theoretical foundation for the harmonic suppression thechnology at DC link,an analytical model of the series-connected 12-pulse rectifier with constant-voltage load is established.Through the description of the structure of the rectifier,the equivalent model of the rectifier and the equivalent current source waveform are obtained.According to the switching function method,working mode of the rectifier is analyzed,and the quantitative relationship between the load voltage and the input voltage of the isolation transformer is determined.By introducing the concept of the equivalent resistance,the analytical model of the rectifier is established.Using the circuit equation and ampere balance principle,the expressions of input current,input power factor and output current of the rectifier bridge are obtained.The simulation and experimental results show that the analytical model can describe the characteristics of the series-connected 12-pulse rectifier exactly,and has the merits of simple structure and strong adaptability.For the sake of reducing the harmonic content of the input current in series-connected 12-pulse rectifier,the passive harmonic suppression method at DC link is studied.This method is implemented by an injection transformer and a single-phase full wave rectifier circuit.According to Kirchhoff's current law,the working mode of the passive harmonic injection circuit is analyzed.The formation process of the 24 step wave input voltage of the isolation transformer is analyzed,and the quantitative relation among the input voltage of the isolation transformer,the injecting voltage harmonics and the load voltage is determined.From the perspective of the minimum harmonic content of the input voltage of the isolation transformer,the optimization design of the injection transformer is carried out.Using inductance equation and normalization method,the inductance voltage and input current are calculated.The simulation and experimental results show that the harmonic suppression effect of the passive voltage harmonic injection method at DC link is remarkable,and the capacity of the injection transformer is only 2.3% of the load power,the harmonic suppression cost is small.In order to improve the power quality at AC and DC link of the series-connected multi-pulse rectifier simultaneously,and simplify the structure of the system and reduce the loss,a hybrid harmonic suppression method at DC link is proposed.According to Kirchhoff's current law,the working mode of the hybrid harmonic injection circuit is analyzed.The formation process of the 36 step wave input voltage of the isolation transformer is analyzed,and the quantitative relation among the input voltage of the isolation transformer,the injecting voltage harmonics and the load voltage is determined.From the perspective of the minimum harmonic content of the input voltage of the isolation transformer,the injection transformer and the conduction angle of the switch are optimized,and the control circuit of the rectifier is designed.This method uses passive method to suppress low order harmonics in input current and increase the pulse number of load current,and uses active method to eliminate high order harmonics in input current.The simulation and experimental results indicate that,after using the hybrid voltage harmonic injection method at DC link,the power quality of the rectifier at AC and DC link is significantly improved,and the capacity of the injection transformer accounts for only 2.4% of the load power,the cost of harmonic suppression is small.In order to improve the load adaptability of the hybrid harmonic suppression method at DC link,the influence of the load parameter on the harmonic suppression ability of the rectifier is studied under the condition of large capacitance.Under the condition of resistance load and resistance-inductance load,the output current of the rectifier bridge and the primary winding current of the injection transformer are calculated according to Kirchhoff's current law.Using the capacitance equation and combining the Kirchhoff's voltage law and Kirchhoff's current law,the relationship between the load voltage and the load resistance is analyzed.Through the analysis of the necessary and sufficient condition for the rectifier working in 36-pulse rectifying state,the relation between the output capacitor and load voltage is obtained,and the selection method of the output capacitor under different load types is given.If the capacitor selection method proposed in this paper is used,the simulation and experimental results show that the hybrid harmonic suppression method at DC link still has good harmonic suppression performance under the condition of resistance load,resistance-inductance load and load mutation. |
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