| Building a new power system represented by photovoltaic and wind power is an important means to achieve the "double carbon" energy strategy,and energy storage is the key technical support to solve the fluctuation of new energy power,energy consumption and ensure the stable operation of the power grid.Energy storage bidirectional DC converter and related control technologies continue to receive extensive attention.In the energy storage system,batteries and other energy storage devices are connected to the power grid through a bidirectional DC converter to dynamically absorb and release energy to the power grid to keep the voltage of the DC network constant.When a single-phase inverter load or a three-phase unbalanced load is connected to the DC network,low frequency fluctuating power will be injected into the DC network.If it is not suppressed,the low frequency fluctuating power will be transmitted to the energy storage battery side through the energy storage bidirectional DC converter,which will not only increase the current stress of the device and reduce the energy conversion efficiency,but also affect the service life of the energy storage battery and other equipment.Flying capacitor bidirectional DC converter is widely used in energy storage system because it can realize both step-up conversion and step-down conversion when transmitting energy in any direction,which solves the bidirectional power control problem of energy storage system under the operation condition of double-terminal wide voltage conversion range.In this paper,the flying capacitor bidirectional DC converter is taken as the research object,and the low frequency fluctuation power suppression strategy is studied.This paper analyzes the topology and working principle,determines the optimal operation mode of the converter,and gives the design rules of its hardware circuit.Based on the working mode of the converter,analyzes the low frequency fluctuation power suppression path of the converter in detail,which is more advantageous than the traditional converter.In this paper,based on the working mode of the converter,the mathematical model of the converter is constructed,and the Model Predictive Control,MPC)strategy based on singleobjective fixed frequency model is proposed.This control strategy only needs to establish a single-objective constraint function for the inductor current,and does not need to optimize the flying capacitor voltage separately.At the same time,the traditional PI control is used in the voltage outer loop,which can realize the unified control of inductor current,output voltage and flying capacitor voltage at both ends,greatly reducing the calculation of the system and having good control performance.It solves the problems that the improper selection of weight coefficient will affect the performance and control effect of the converter when the traditional MPC algorithm controls multiple objectives.Although the single-target fixed-frequency MPC strategy has good control performance,it does not have the ability to suppress low frequency fluctuating power,and it has no suppression effect when the energy storage system contains low frequency fluctuating power.In order to achieve this goal,this paper proposes a low frequency fluctuation power suppression algorithm,which is an efficient ripple suppression algorithm with strong frequency adaptability and good effect on specific secondary ripple suppression.The introduction of low frequency fluctuation power suppression algorithm and the proposed MPC strategy make the final control scheme have the ability to suppress low frequency fluctuation power and maintain good dynamic performance on the premise of achieving the energy storage control goal.Finally,it is verified by building a low-power experimental platform.In order to reflect the superiority of the low-frequency fluctuation power suppression strategy of the target fixedfrequency MPC of the Bill of Lading,the traditional low frequency fluctuation power suppression algorithm is selected for comparative experiments.The experimental results show that the proposed strategy can effectively suppress the low frequency fluctuation power and greatly improve the dynamic response speed of the system. |
