| Compared with traditional two-level converter,multilevel converter is provided with flexible control manner,low output voltage harmonic content,good economics,high efficiency and it is suitable for high voltage and high power output.Therefore,multilevel converter are widely applied in power quality management,ac motor speed regulation and dc transmission under high voltage and high power condition.According to whether there is a Shared capacitor on the dc side,the topology of multilevel converter can b e divided into cascade and dc side common capacitors.The chain multilevel topology has the advantages of easy to modularize and suitable for high power application.But the dc side of each power unit is an independent capacitor,this results in active power not being able to flow freely between the three phases and makes it difficult to realize the dc-side voltage equilibrium of each phase when there is a negative-sequence current component.Although a large number of diodes or capacitors for clamping are usually required in the dc side common capacitor multi-level topology,which is only applicable to low-voltage situations,the characteristics of the dc side common capacitor enable it to realize active powerexchange between each phases.In this paper,a new general multilevel converter topology is presented.The main converter of the topology is a chain structure and the slave converter is a dc side common capacitor structure.The output voltage of the chain structure is always vertical to the current by the coordinated control of the master and slave converter.The active energy exchange between phases is realized by the slave converter,which ensures the dc voltage balance of each phase.The paper optimizes the modulation strategy by eliminating the active power coupling fluctuation.SPWM with phase shift angle?/N is selected in this paper,and it is necessary to consider the unbalance dc-side voltage of each phase power unit in the chain structure caused by the power coupling between the carrier sideband harmonic voltage and harmonic current.To solve this problem,this paper gives two ideas:The first method is to optimize the carrier frequency so that the active power coupling is suppressed at the source.The second method is to carry out periodic rotation of the carrier to evenly distribute the power fluctuations to each unit so as to make the dc-side voltage of each power unit balance.Two methods are compared through simulation.In this paper,the topology of the new general multilevel converter is selected and applied to active power filter under unbalanced conditions.The equivalent circuit is transformed by establishing mathematical model.A simple instruction current extraction method is obtained by this equivalent circuit.This method does not need to extract the reactive power and each harmonic current separately,nor does it need to separate the positive and negative sequence components of the fundamental current.and real-time dynamic compensate the harmonic current,fundamental negative-sequence current and fundamental positive-sequence reactive current.In order to ensure the good compensation effect of the device,the voltage of the capacitance on the dc side is controlled from three aspects:The first layer is the overall dc-side voltage balance control.The active power of the grid inject in device is controlled by decoupling to ensure the balance of active power exchange.The second layer is coordinate control between master and slave converter.The output voltage of the chain part is always vertical to the current of the bridge arm so as to reach the active power balance of the chain part of three bridges.The third layer tracks our instruction values by giving each unit an active power correction.Based on the above theoretical analysis,this paper carried out simulation verification and prototype development.And the simulation results,system structure and hardware architecture of the device are given in detail.The advantages of the new general multilevel topology and the effects of the optimal modulation strategy are verified by simulation.we can see that instruction current extraction algorithm is simple,voltage control strategy is effective,system robustness is good,and has good engineering practical value for its application in active power filter. |
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