Research On MMC Based Multi-port Energy Collection System And Control Strategy

The rapid development of electric vehicles and renewable energy is of great significance to solve the problem of environmental pollution.Although the use of electric vehicles reduces the emission of carbon dioxide,it also brings the problem of the decommissioning of power batteries.Because of the randomness and volatility of renewable energy,the high proportion of renewable energy grid will have an impact on the surrounding power users.After the power battery of electric vehicle is retired,although its capacity is reduced to 70% to 80%,it can still be used as the energy storage device of renewable energy,which provides a low-cost solution for solving the volatility and randomness of renewable energy.Among them,modular multilevel converter(MMC)can be used as a multi-port energy collection topology which is integrated organically by many DC modules such as electric vehicle,retired battery,and renewable energy due to its high modularity,high integration,high efficiency,and high redundancy.To form the gas isolation between equipment,the isolated bi-directional DC-DC(IBDC)is introduced as the isolation link between MMC and DC module,and the multi-port energy collection topology(IBDC-MMC)based on MMC is constructed.This paper focuses on the research of the IBDC-MMC system.Firstly,the power battery and distributed power supply integrated interface topology circuit based on IBDC are analyzed.By analyzing the topology structure and working principle,the mathematical model of the IC is established.The control strategy of battery charge and discharge and the control strategy of the interface circuit between distributed power supply and IBDC-MMC are proposed.The simulation analysis of the charging and discharging process of the power battery is carried out in PSCAD/EMTDC environment,and the effectiveness of the integrated interface circuit control strategy based on IBDC is verified.Based on the model of the IBDC integrated interface circuit,the paper analyzes the topology and working principle of the IBDC-MMC system by studying the operation principle of each sub-module after IBDC is connected to MMC.Based on the switching function and d-q coordinate,the mathematical model of the IBDC-MMC system is established,and the equivalent circuit model of the IBDC-MMC system is proposed.Considering that different DC modules are connected to IBDC-MMC system,a multiobjective hierarchical control strategy is proposed to realize the differential control of different DC modules;The proposed multi-objective hierarchical control mainly includes:DC voltage and output reactive power control,submodule voltage and power balance control,integrated interface circuit closed-loop control;Finally,considering the small and medium scale integration scenario of 150 KW,three typical conditions are simulated in PSCAD / EMTDC environment to verify the effectiveness of the proposed control strategy.