Research On Valve Control Strategy For The Modular Multilevel Converter With Battery-based Energy Storage System

The intermittent and random renewable energy causes fluctuations of grid-connected power and brings negative impacts to the stable operation of system.Therefore,battery energy storage system(BESS)with high energy density can reduce power fluctuations for long-term operation,it has become a potential option amongst many ESSs.Battery packs can be embedded in the submodules(SMs)of the modular multilevel converter(MMC)to form the so called MMC with battery-based energy storage system(MMC-BESS)due to their excellent modularity performance.It can play a role in energy transition and renewable energy consumption,due to the superior ability to energy transmission and flexible adjustment of energy storage capacity.In this paper,the research work is carried out from the mathematical model and valve control strategy of the MMC-BESS to provide theoretical pavement and model support for the application of the MMC-BESS in renewable energy system.Firstly,in order to directly reflect the operating rules of the system and analyze the factors affecting the fluctuation of the capacitor voltage and battery current,a time-domain analytical model of the MMC-BESS based on modal division has been established.The switching function,average value method and component voltampere characteristics are combined to write and derive the analytical expression in time domain by coupling different working states and modes of SMs.By analyzing the temporal analytical expression of the explicit function,the fluctuation characteristics and influencing factors of the electric parameters in the SMs are reflected.Secondly,in order to improve the energy utilization of batteries and address the state of charge(SOC)imbalance problem amongst batteries,which caused by manufacturing process,different cycles of charging or discharging and different aging degrees,etc.To fill the research gap,this paper proposes a SOC balancing control strategy amongst batteries based on discrete time-domain predictive power model in the MMC-BESS based medium-voltage direct-current(MVDC).It is a double-loop control to accurately control the charging and discharging power.The outer power control establishes a discrete time-domain predictive power model for the SOC differences.The inner current loop designs a hybrid controller based on the continuous control set-model prediction control and the feedback control strategy to accurately track the dynamic current reference.Finally,on the basis of the above research,in order to lighten the size and weight of the MMC-BESS and reduce the construction cost of the offshore platform and the converter station itself,a coupled harmonic online injection strategy for the MMC-BESS under sudden changes in operating conditions is proposed.A mathematical model of the arm power of the MMC-BESS considering different operating conditions is established,and the constraints of coupled harmonics are proposed.On this basis,the coupling harmonics parameters are optimized online by particle swarm optimization algorithm with the arm power model as the objective function,and the near-optimal parameters of coupling harmonics under different operating conditions are obtained to achieve the mitigate arm power fluctuations,capacitor voltage fluctuations and battery current ripples.