Control Strategy Of The Multi-sources Grid-connected System Based On Modular Multilevel Converters

In renewable energy generation system and energy storage system, the dc sources generally operated in series or parallel. The dc sources generally connected with power grid through power conversion system which constructed the multi-sources grid-connected system. The power conversion system plays a vital role in the multi-sources grid-connected system. Traditionally, the power conversion system mainly includes two-level inverter, NPC and so on. Due to the low voltage of these converters, they always coupled with power grid through step-up transformer, which will induce power losses. Compared with two-level inverter and NPC, cascaded H-bridge inverter has many advantages, which can increase output voltage and improve power quality. Due to the high level of its output voltage, the power loses induced from transformer can be reduced. Taking modular multilevel converter as power conversion system of multi-sources grid connected system has several merits. The dc source can be distributed into each half bridge converter, which can control the output power of each dc source flexibly; the system capacity can be enlarged; the circulating current can be used to distribute power among different phases and arms without influencing injected grid current. Thus, it is proper to take modular multilevel converters as power conversion system of multi-sources grid-connected system.Above all, a multi-sources grid connected system based on modular multilevel converters has been proposed in this thesis, based on summary of previous topologies. Firstly, the basic principle has been analyzed in detail. Then the basic control strategy has been introduced in following parts and applied into battery energy storage system and photovoltaic system integrated with battery energy storage components. The photovoltaic system integrated with battery energy storage components is flexible in operation, which can be operated as photovoltaic system, battery energy storage system and hybrid photovoltaic and energy storage system. The control strategies have been analyzed in detail in corresponding chapters. Finally, the proposed control strategy has been verified by simulation and experiment.The main contents of this thesis are as follows:(1) The first chapter introduced the background and significance of multi-source grid-connected system. The power conversion system topology and significance of multi-source grid-connected system based on MMC has been overview in this chapter.(2) The basic principles and control strategy of multi-source grid-tied system based on MMC have been analyzed in detail in the second chapter.(3) The control strategy of battery energy storage system based on MMC has been introduced in this chapter, which consists of SOC balancing control strategy and circulating current control strategy except aforementioned basic control strategy. The proposed control strategy has been verified through simulation and experiment.(4) The operation mode of photovoltaic system integrated with battery energy storage components based on MMC has been introduced in this chapter, which consists of PV operation mode, hybrid PV and battery energy storage operation mode and battery energy storage operation mode. The control strategies of operation modes have been proposed in this chapter with simulation verification.