Research And Study On Cascaded H-bridge Energy Storage System

The ever-increasing global energy demand has put pressure on energy supply and environmental protection.The development and utilization of renewable energy sources have important strategic significance for improving the energy structure and ensuring the sustainable development of the national economy.Energy storage systems play a major role in smoothing the volatility and interstitiality of new energy generation.Cascaded H-bridge energy storage systems have attracted wide attention in high-voltage and large-capacity energy storage system because of its modular structure and easy extensibility.This dissertation takes the cascaded H-bridge energy storage system as its research object,and studies its key technologies and simplified modulation strategies.The topology structure and working principle of the cascaded H-bridge energy storage system are introduced in detail,and the mathematical model of the converter is given.Based on the mathematical model of the system,a multi-objective modulation strategy for cascaded H-bridge topology based on a simplified modulation strategy is proposed.Multi-objective controls,such as modulation degree extension,power distribution and so on are achieved by injecting corrections to the duty cycle of each phase.The hierarchical control of the energy in energy storage system is divided into the total power control of the system,the inter phase power control,and the inner phase power control.In order to achieve accurate power control,this paper proposes a DC-side power calculation method based on carrier cycle.Based on the relationship between the AC side and DC side power as well as the simplified modulation strategy,the basic principle of the inter phase power control strategy is analyzed.Based on this,three different inter phase power control strategies are proposed.Combining the advantages and disadvantages of the three interphase power control strategies,an optimized power control strategy is proposed,which can achieve fast and stable power control.By analyzing the relationship among the current direction,output state and power of the cascaded H-bridge power unit,an inner phase power control strategy based on duty cycle redistribution is proposed,which can distribute the inner phase power within a relatively large range,and achieve accurate inner phase power control.The effectiveness of the proposed method was verified by simulation and experiment.Aiming at the limit range of the proposed power control strategy,the relationship between the interphase power control limit range and the modulation degree is analyzed by simulation,and it is verified that the interphase power control range increases first and then decreases as the modulation degree m increases.In addition,combining with mathematical derivation and graphical analysis,the numerical relationship is derived,that is the inner phase power coefficient and the product of the inter phase power coefficient and the modulation degree is inversely proportional.A cascaded H-bridge energy storage system with DSP as the control core is designed.The hardware principle and software flow are analyzed in detail.Finally,the three inter phase power control strategies,the optimization control strategy and the inner phase power control strategy proposed in this paper are experimentally tested.