Optimization Of Energy Storage Mmc Topology And Its Control Strategy Applied To Unbalanced Power Grid

Against the backdrop of global warming and the advocacy of low-carbon environmental protection by various countries,new energy generation and grid connection technologies have received widespread attention.At present,various new energy power generation forms such as wind power generation,solar energy and Tidal power power generation are developing rapidly,but they all have randomness and volatility,bringing great challenges to the safe and stable operation of the power grid.The distributed power grid based on new energy generation technology is a small power grid dispersed around users,with good environmental compatibility.However,due to its own weak power grid properties,there are inevitably power quality issues such as three-phase imbalance and large harmonics.Some scholars have found that the introduction of Energy storage in grid connected converters can overcome the power quality problems caused by new energy grid connection.Based on this,this article proposes an energy storage modular multilevel converter(MMC)for topology optimization in unbalanced power grids.The following research is conducted on its working principle,modulation and control strategies,and specific applications.Firstly,the MMC capacity selection of centralized energy storage structures has poor flexibility,high cost,and a significant impact on the service life of energy storage components.The MMC energy storage units of distributed energy storage structures are dispersed and inconvenient to assemble and maintain.In response to the above issues,this article proposes a centralized local energy storage structure MMC converter(MMC CLES),which only contains two energy storage submodules,the main components are still ordinary submodules.Then,based on the distributed energy storage structure MMC mathematical model,the mathematical model of MMC-CLES was derived,and the control system for the new topology was designed.The simulation and experimental results show that the MMC-CLES topology proposed in this paper simplifies the control system,reduces hardware costs and control complexity by optimizing the number of energy storage submodules.Then,when the three-phase voltage of the distributed power grid is unbalanced,the energy storage module of the MMC CLES system will generate large charging and discharging power.If the state of charge(SOC)of the lithium battery is unbalanced,the aging speed of the battery will be too fast and the capacity utilization rate will be too low.Under the above conditions,the traditional MMC three-level SOC balancing strategy cannot achieve superior control effects in MMC-CLES.This article proposes a single stage SOC equalization strategy,which adjusts the output power between the bridge arms by using the output energy of the bridge arm as a feedback variable,and adjusts the fundamental frequency component in the bridge arm circulation to achieve rapid SOC equalization between the bridge arms.The simulation results show that the SOC balancing strategy proposed in this paper shortens control time and greatly simplifies computational complexity while optimizing control effectiveness.Finally,in order to solve the problem that the traditional Nearest Level Modulation(NLM)modulation strategy will generate large harmonics in the distributed power grid system with too low levels,the genetic algorithm is introduced to search the optimal Floor and ceiling functions of the system THD,and the improvement of the Floor and ceiling functions realizes the increase of the level number and the reduction of the harmonic content.The simulation results show that this method can realize the automation,simplification and accuracy of Floor and ceiling functions selection.