Research On The Dual-Output MMC Topology Applicable For 10kV Distribution Network

With the fast development of the Distributed Energy(DE)and increasing requirements in electricity quality,problems in the DN,e.g.,the unbalanced load in feeder,the long recovery time,the decreased reliability and quality of power supply,are becoming more and more salient.The problems mentioned can be solved with soft open point(SOP),which essentially is a power electronic converter interconnecting two or more feeders in the DN.The converter can be divided into two types.The first is the topology with AC/AC transformation,which is difficult to expand for multi-end interconnection application.The second type is the topology with AC/DC/AC transformation,and this type is easy for multi-end interconnection with DC bus.To satisfy the power demand,SOP is always based on MMC structure.SOP applicable for the three-end DN interconnection are discussed in this paper.The specific application is in a 20kV AC end and two lOkV AC ends with ?/6 rad phase angle difference.Topologies for the 20kV AC end with overmodulation or less sub-modules have been researched and analyzed,and a topology selection suggestion has been proposed.For the 10kV AC ends,the topologies with less sub-modules are mainly concerned in this paper,and it has been found that the related domain is under progress.Therefore,the topologies for two lOkV AC ends are primarily discussed in this paper.First,the principle of the three-phase nine-arm MMC topology is analyzed in this paper.The mathematical model was derived subsequently,and the close-loop control scheme has been established.A simulation model has been built to verify the stable performance and the controllability of the topology.Second,since there are still unsettled problems about the three-phase nine-arm MMC,e.g.,the precise model of arm current is covered,and the unbalance load for different arms under various output,a dual-output topology based on branched-arm structure is proposed in this paper.The principle and equivalent circuit of the proposed topology is analyzed,and the mathematical models of the output current and the circulating current are derived.A modulation wave generation method is proposed for the topology,and the relationship of the modulation index m between the sub-module number rate k and the two AC ends phase angle difference ? is revealed.The close-loop control scheme is designed based on the mathematical model and the modulation wave generation method.The stable performance of the proposed topology is verified with simulation model.Third,the enery balancing strategy for the proposed topology is analyzed.With the derivation of sub-arm energy mathematical model,the necessity of energy balance between sub-arms is revealed.And a PI-based energy controller is proposed to solve the problem.The effectiveness of the energy controller is verified by simulation.Then,the logical processing based fundamental frequency sorting algorithm is adopted as the capacitor voltage balancing strategy.The relationship of the algorithm' s convergence speed between modulation index m and power factor angle ? is analyzed,and the applicable power factor angle range is revealed.The correctness is verified by both simulation and experiment.Finally,a 5-level single-phase prototype is established to verify the stable performance of the dual-output topology based on branched-arm structure and the property of the proposed sub-arm energy controller.The results confirm the feasibility of the proposed topology and the effectiveness of the proposed sub-arm energy controller.