Research On Low-frequency Current Ripple Reduction And Individual Phase Current Control Of A Cascaded Multilevel Converter

The cascaded multilevel converter(CMC)is well-known by possessing many advantages,such as higher voltage and power level,improved harmonic performance,modular structure,prone to redundancy and extension,and so on.It has become a research hotspot in recent years to integrate the CMC with the distributed power or battery in the fields of power electronics and renewable power generation.The CMC topology has been widely researched and used in the areas like power quality,motor drive,etc.,while the integration with batteries or distributed resources will bring some new problems and demands for it.For the CMC topology,the reduction methods of low-frequency current ripple at the submodule dc side are studied in this paper,a novel CMC topology is proposed which is current-ripple-free and simple-structured,and its individual phase current control(IPCC)strategy and operation performance under unbalanced grid conditions are investigated.Large amount of low-frequency ripple,which mainly contains the second harmonic component,will arise in the submodule dc side current of CMC.And it may deteriorate the performance of battery,or have an impact on the MPPT control of PV.In order to attenuate the low-frequency input current ripple,a dc-dc converter stage can be inserted between the distributed dc source and dc-link of each submodule H-bridge inverter.It can increase the control degrees of freedom,and realize the input current waveform control.For such kind of two-stage submodule circuit,a low-frequency input current ripple reduction is proposed in this paper based on the front-end dc-dc converter load current feedforward.It intends to control the submodule dc bus capacitor voltage to swing properly at twice the output frequency,making the dc bus capacitor supply nearly all the pulsating power.The implementation method is illustrated,and the operation performance analysis as well as the key parameter design principle is provided.The experimental results performed on a two-stage inverter prototype verify the validity of proposed method.Introducing the proposed feedforward path is able to significantly suppress the input current ripple with little impacts on the original system stability,steady-state tracking accuracy,and dynamic response.This control strategy is easy to implement,thus it is suitable to be employed in the CMC based on two-stage submodule structure.In order to further simplify the submodule structure of CMC,a single-stage submodule circuit is studied in this paper.Based on the power switch multiplexing technique,this circuit can realize both dc-to-ac power conversion and low-frequency input current ripple reduction with a lower number of switches.The equivalences on low-frequency ripple propagation and circuit model are firstly analyzed between this single-stage circuit and a conventional two-stage one.A control strategy is presented,which is capable of controlling the port performances of both the dc input and ac output.Then the similarities on device rating and dynamic response are revealed by making a contrast study.And finally the controller effectiveness and performance analysis validity are verified on the prototypes of two circuits.This single-stage submodule is much preferred in some applications which are sensitive to the power switch number and low-frequency input current ripple.Thus it can be considered to adopt as the submodule circuit in the CMC-BESS.On the basis of previous research,a novel CMC topology based on the single-stage submodule is presented in this paper by combining the switch multiplexing idea and cascaded multilevel technique.Without extra active circuits,this proposed CMC topology can operate with dc side current ripple-free.Hence this topology has a lower number of power switches and a simpler structure,making it quite suitable to be used as the power conversion system for battery energy storage equipment or distributed power generation.An IPCC strategy is put forward for the novel delta-connected CMC.It can redistribute the power among submodules and control the power flow inside a submodule,effectively realizing the functions of ac output power regulation,battery charge/discharge equalization,dc capacitor voltage balance,and low-frequency input current ripple reduction,etc.All the layers of this control scheme are individual and unaffected to each other.Then,the ac output harmonic characteristics and the system static reliability are analyzed.Finally,the simulation in PSCAD/EMTDC and experimental results on a prototype can verify the validity of the proposed circuit and controller.For the star-connected CMC under unbalanced grid conditions,an IPCC based on the optimal zero-sequence current separation(OZSCS)is presented in this paper.By adopting the IPCC method,each cluster of CMC can be viewed as an individual single-phase grid-tied inverter.Thus the CMC under IPCC can be adapted to the unbalanced grid conditions automatically.Without affecting other parts of the controller,the proposed OZSCS can eliminate the zero sequence component in three-phase current reference generated by IPCC when the power grid voltage is unbalanced through reconstructing the reactive current command.The output negative sequence current proportion and voltage stress of CMC are both in linear relationship with the grid unbalance factor.Finally,the validity of the proposed IPCC based on OZSCS is verified on both a PSCAD/EMTDC simulation model and an experimental prototype.