Research On Cascaded H-Bridge-based Grid-Connected PV Systems

Cascaded H-bridge(CHB)converters have the advantages of modularity,scalability,and flexible control,which have been widely used in lots of fields,such as reactive power compensation,medium-voltage high-power motor drive,and battery energy storage system(BESS).In this dissertation,CHB converters are applied to large-scale photovoltaic(PV)systems and BESS-PV systems,where the power balance and optimization of DC-DC stage are studied from the points of topology architecture and control strategy.Firstly,the power imbalance in CHB-based grid-connected PV systems is discussed.Based on the simplified equivalent circuit,the inter-module power factor,the inter-phase power factor and the relative inter-phase power factor are defined.Then combined with the traditional inter-module and inter-phase power balance control strategy,the quantitative analysis of operation range limited by the inter-module and inter-phase power imbalance is conducted.By defining the power path and power nodes,the nature of the power imbalance in CHB-based grid-connected PV systems is introduced.Besides,the necessary and sufficient condition for the system power balance is obtained.According to the necessary and sufficient conditions,the topologies with power balance capacity are derived.The simulation and experimental results of the tranditional CHB-based grid-connected PV systems verify the correctness of the theoretical analysis and the feasibility of the control strategy.The common magnetic link is one of the methods to achieve inter-module and inter-phase power balance.In this dissertation,the CHB+MAB-based grid-connected PV system is studied.Then the equivalent circuit and average model of MAB converters are analyzed.Referring to the derivation procedure of Dual Active Bridge(DAB)converters,the quantitative relationships between the soft-switching range,the reactive current ratio and the voltage gain M_v,the phase-shift angle d are derived.Using CHB and QAB converters,the interphase CHB-QAB(IP CHB-QAB)converter and same-phase CHB-QAB(SP CHB-QAB)converters are proposed,compared and analyzed.In addition,the parameters of the QAB converter in PV systems are optimized so that the DC-DC stage is with wide ZVS range and small reactive current ratio.Finally the simulation results of the overall system and the experimental results of one QAB converter prove the correctness of the theoretical analysis and the power balancing capacity of the system.The common DC bus is another way to achieve inter-module and inter-phase power balance.In this dissertation,a CHB-based grid-connected PV system with a common DC bus is proposed.By taking advantage of the LVS ports of three-port DC-DC converters,the LVS DC bus is formed to achieve the power exchange among the inverter modules.Then the inter-module and inter-phase power balance mechanisms of the proposed topology are introduced.Through analyzing the operation principle and gain characteristics of the three-port resonant DC-DC converters,the quantitative relationships between the PV current ripple,the switching frequency range,the modulation strategy and system parameters are derived.Based on the above analysis,the system parameter can be optimized,which can reduce the PV current fluctuation and decrease the switching frequency range.Besides,the cross-control strategy is adopted,where the CHB converter controls LVS voltage and the resonant DC-DC converters control the PV voltage and MVS voltage.The simulation and experimental results verify the inter-module and inter-phase power balancing capacity of the proposed topology and the feasibility of control strategy.Gonsidering the asymmetry forward and reverse power flow in the BESS-PV system,an optimized topology-hybrid DC-DC type CHB-based BESS-PV system is proposed,where unidirectional and bidirectional DC-DC converters are adopted in the DC-DC stage.Cost and power loss models of unidirectional and bidirectional DC-DC converters in the proposed topology are established and compared with CHB+DAB topology.A control strategy based on reactive power compensation and optimized distribution is adopted to improve the proportion of unidirectional DC-DC converters and expand the operation range of the proposed topology in the case of reverse power flow.Finally,the simulation and experimental results verify the effectiveness of the control strategy.There are 84 figures,14 tables and 142 references in this thesis.