Research On Operational Characteristics Of Modular Multilevel Converter And Its Application In Battery Energy Storage System

Modular multilevel converter(MMC)is one of the most potential topologies in medium and high-voltage applications due to its modular structure,low output harmonic and high fault tolerance,and is gradually applied to renewable energy generation system.Submodules of MMC also can be used as interfaces of the battery energy storage system(BESS),which is named as the modular multilevel converter with battery energy storage system(MMC-BESS),providing higher reliability and flexibility than traditional 2-level or 3-level voltage source converter based BESS.However,due to the special structure of MMC,there is complex multifrequency cross-coupling,which makes the modeling and analyzing of MMC very complex.In addition,the integration of BESS changes the power flow of MMC,affecting submodule capacitor voltage balance,and leading to increased capacitor voltage ripple.Thus,focusing on the modeling,operation characteristics analysis,capacitor voltage balancing control,and capacitor voltage ripple suppression,the following researches are carried out.The ac terminal model of MMC is important for designing control strategy and improving control performance.However,the existing terminal models cannot fully reflect the influence of internal coupling,thus,they can not accurately describe the operational characteristics of MMC.Based on the analysis of internal coupling,the ac terminal of MMC is equivalent to the two-level voltage source converter that comprises series equivalent impedance.Then,according to the harmonic linearization theory,the equivalent impedance is calculated and one more accurate ac terminal model of MMC is established.Furthermore,the simplified analytical expression of the equivalent impedance is obtained,which is the first-order link of RC parallel connection,so as to make the ac terminal model more intuitive and simple.The steady-state and dynamic simulation results show that the proposed model is more accurate than the existing models.Finally,based on the proposed model,this dissertation proposes an improved power decoupling control strategy and reveals the mechanism of low-frequency oscillation.The steady-state and dynamic analysis of MMC is significant for designing main circuit parameters,revealing the coupling between control loops,designing controller parameters,and analyzing system stability.However,due to the complex internal coupling,the steady-state and dynamic analysis of MMC is always a great challenge.Harmonic state-space(HSS)theory has great advantages in modeling multi-frequency systems and is applied to MMC in this dissertation.Firstly,the steady-state model based on HSS theory is built and accurate steadystate values of internal variables are obtained.On this basis,an improved circulating current control method is proposed,which can decrease the requirements of submodule capacitance and power devices capacity.Then,the small-signal model that includes internal variables and control loops is established.The stability boundaries of different control loops are obtained by eigenvalue analysis and the participation factor is introduced to analyze the coupling among different control loops.Finally,a simple controller parameters design method is proposed by comparing the HSS model and a simplified model.After integrating batteries into MMC,the state-of-charge(SOC)balancing control is limited by the capacitor voltage balancing control.To ensure the balance of capacitor voltage and the fast equalization of SOC,it is necessary to assess the submodule balancing capability.Firstly,the submodule power unbalance degree is defined and the limit of which is utilized to assess the submodule balancing capability and designing the SOC controller.Then,the submodule balancing capability of sorting algorithm based and closed-loop based capacitor voltage balancing control is obtained,and the factors that affect submodule balancing capability are studied,such as modulation strategy,modulation ratio and battery power.On this basis,an optimized closed-loop based balancing control strategy is proposed to improve the submodule balancing capacity in high modulation applications.Finally,the detailed simulation and experimental results verify the effectiveness of the proposed control strategy and the evaluation method of the submodule balancing capacity.The integration batteries changes the power flow of MMC,resulting in increased capacitor voltage ripple.To address this problem,this dissertation proposes a novel battery integration method to reduced capacitor voltage ripple.Two submodules that belong to the upper and lower arm,respectively,share one battery unit,and they are connected by an isolated three-port DC/DC converter.By regulating the battery power distribution between the upper and lower arm,the fundamental circulating current is suppressed,and the capacitor voltage ripple is decreased.By transferring fundamental power ripple between upper and lower arm,the fundamental capacitor voltage ripple is completely suppressed.Compared with traditional battery integration methods,the proposed method can greatly suppress capacitor voltage ripple,thus reducing the requirement for submodule capacitance.