| As the penetration ratio of renewable energy generation in modern power systems increases recently,the problems caused by the intermittency and randomness of renewable energy resources become prominent.Battery Energy Storage System(BESS),as an effective means to improve the utilization rate of renewable energy generation systems,will play an increasingly important role in the power system.Power Conversion System(PCS)is the core component of BESS,interlinking the energy storage elements and power systems.In recent years,the Modular Multilevel Converter with integrated BESS(MMC-BESS)has attracted widespread attention.MMC-BESS has the advantages of MMC itself,such as modular structure,flexible configuration,low output harmonics,etc.Furthermore,it can operate as a flexible interface between AC and DC systems to improve the flexibility of system cooperative control.Therefore,MMC-BESS has great potential in the grid-connected application of high-voltage and high-power renewable energy generation systems.MMC itself is a multi-frequency,non-linear,strong-coupling system.After integrating BESS,its operating characteristics are more complicated.Generally,MMC-BESS is used both as an interface converter between AC / DC networks and a PCS for BESS.Consequently,it should not only meet the requirement of AC and DC power transmission according to upper-layer instructions,but also be able to achieve the State of Charge(SOC)balance of the energy storage unit quickly and accurately as the basic function of a PCS.Although the operation characteristics of MMC and the realization of the basic functions of MMC-BESS have been researched at home and abroad,as a new topology,its corresponding theoretical research results are not deep enough,there are certain limitations and optimization space.Unbalanced battery power is a new factor that affects the voltage balance of submodule(SM)capacitors,causing frequency components in the circulating current that are significantly different from MMC;and the choice of different degrees of freedom for capacitor voltage control corresponds to different system control strategies,and multi-loop multi-controller coordination makes the control strategy more complicated.The applicability and optimization method under different working conditions need to be further studied.Based on the above problems,this paper focuses on the new operating characteristics and optimized control strategies of MMC-BESS after integrating BESS.Firstly,based on the power balance relationship of MMC-BESS,the power coupling modes of AC side,DC side and batteries are quantitatively analyzed,the energy conversion mechanism of MMC-BESS is explained,and then multiple operation modes of MMC-BESS are obtained.To investigate the emerging problems of MMC-BESS caused by unbalanced battery power among SMs which is significantly different from conventional MMC,the time domain numerical calculation method is proposed.The quantitative relationship among modulation function and external voltages and currents,internal circulating currents and capacitor voltages are analyzed,and a set of nonlinear equations is constructed to calculate the precise modulation function after considering closed-loop control.Consequently,all electrical quantities of MMC-BESS are decoupled and the physical meaning is clear,and the efficiency is higher than the simulation.Based on the steady-state analysis method,the influence of the capacitor voltage ripple on the output characteristics of the AC side is analyzed.It is pointed out that the capacitor voltage ripple is equivalent to adding a capacitive impedance in series with the grid impedance,making the original AC inductor design conservative,and the safe operating range of the system at different steady-state operating points is checked to ensure the safe operation of the actual engineering design system.The quantitative relationship between volatility of capacitor voltages and unbalanced battery power is established,and the variation of the maximum capacitor voltage volatility is obtained as the basis of SM capacitance design.When MMC Side Capacitor Voltage Balancing(MSCVB)control is implemented,according to the basic requirements of MMC-BESS as a PCS,an SOC equalization control strategy that takes into account the difference in capacity between different battery packs is proposed.The conditions of SOC equalization controller parameters between phases and arms are analyzed.Since the capacitor voltage balance between SMs depends on the MMC side terminal voltage adjustment,in order to meet the requirements of the MMC-BESS extreme operation modes,a capacitor voltage balancing method that changes the AC and DC modulation indexes of the SM terminal voltage at the same time is proposed.By analyzing the AC and DC modulation coefficients and the average power of SM,the ration between the AC and DC modulation coefficients is optimized to improve the SOC equalization rate of batteries.An analytical design method of capacitor voltage balance controller based on small signal model is given.When DC/DC Side Capacitor Voltage Balancing(DSCVB)control is implemented,the task of capacitor voltage balance is distributed to each SM,simplifying the original capacitor voltage balancing control structure at MMC side.Because the battery power is indirectly controlled at this time,the control on the AC and DC sides of the system will affect the battery side,so additional measures need to be taken to ensure the dynamic response of the battery and the effectiveness of the SOC equalization.On the DC / DC side,,the low-frequency components in the capacitor voltage ripples must be prevented from flowing into batteries.In order to suppress the fluctuation of the capacitor voltage in the dynamic process,a capacitor voltage filtering scheme using a common moving average filter for each arm is proposed,and the feedforward component is utilized to reconstruct the battery power.On the MMC side,based on the equivalent circuit of MMC-BESS,the problems of operation in the rectifier mode and under unbalanced AC voltages are analyzed,and corresponding control strategies are proposed.In the rectifier mode,the closed-loop output component of the DC bus voltage is utilized as the DC circulating current references,meanwhile it is allocated according to the SOCs among phases.Under unbalanced AC voltages,the degrees of unbalanced power between phases with different control targets are analyzed,then this unbalanced power is compensated at DC side to prevent it from deteriorate the SOC equalization among phases.Finally,a seamless transfer method between MSCVB and DSCVB controls based on control variables hot backup is proposed,which guarantees the normal operation of MMC-BESS,improving the reliability of the whole system.The experimental prototype is an important means to verify the power transmission mechanism,circuit design,and control strategy implementation of MMC-BESS.Due to the significant difference of capacitor voltage ripples between upper and lower arms caused by unbalanced battery powers,the capacitance parameter designed according to the conventional MMCs cannot meet the constraint of the maximum capacitor voltage volatility.Consequently,according to analysis by the time-domain numerical calculation method,the capacitance parameter is quantitatively designed to meet the constraint.The structure of the prototype is designed hierarchically as four layers,including the top layer(upper monitor),main controller,phase auxiliary controllers,and the bottom layer(main circuits,sampling sensors,drivers,etc.).Finally,based on the designed MMC-BESS experimental prototype,the improved control strategies of both MSCVB and DSCVB under the rectifier and inverter modes proposed in this paper are implemented.The operations under steady-state,dynamic state and extreme conditions are also verified.The experimental results validate the effectiveness and correctness of the proposed control strategy in this paper. |
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