Grid-connected Control Strategy Of Photovoltaic And Energy Storage Generation System Based On Hybrid Energy Storage

With the increasingly severe global energy and environmental situation,the installed capacity of photovoltaics in power systems continues to increase,and photovoltaic and energy storage generation systems based on hybrid energy storage can not only overcome the problem of photovoltaic power generation fluctuations,but also take into account the advantages of high energy density and large power density of energy storage devices,and it has gradually become a hot topic in academic research.The grid-connected inverter is the core equipment of photovoltaic and energy storage generation systems,and its topology and control algorithm directly affect the grid-connected power quality.Compared with the traditional two-level inverter,the diode neutral point clamped three-level inverter has the advantages of small power device pressure,low grid-current harmonic distortion rate and low switching loss.However,power switch device is still the weakest part of the system.Once the fault occurs,it will threaten the safe operation of the whole system.Therefore,this dissertation takes photovoltaic and energy storage generation systems with hybrid energy storage as the research object,focusing on grid control of fault-tolerant inverter in normal operation and single-phase bridge leg fault,in order to achieve high-performance grid-connected operation before inverter fault,the stable and reliable fault-tolerant grid-connected operation after inverter fault.Dissertations specific contents are as follows:Firstly,study the overall structure of the photovoltaic and energy storage systems with hybrid energy storage and analyze the operating characteristics of each unit.In this section,each unit mathematical model of photovoltaic and energy storage generation systems is established,including photovoltaic array,battery and supercapacitor,and their output characteristics are analyzed and studied.Moreover,the converter structure and control algorithm of each part of the system are studied,the working mode and control method of photovoltaic boost converter and the model predictive control method of energy storage bidirectional DC/DC converter are discussed respectively.Among them,the disturbance observation method is selected for the maximum power tracking control of photovoltaic power system.Secondly,according to the charge and discharge characteristics of energy storage components,energy management strategy of hybrid energy storage system composed of battery and supercapacitor is studied.The power absorbed and released by energy storage system is filtered by the low-pass filter,and low-frequency power is absorbed or released by battery,while supercapacitor bears high-frequency power generated by sudden change of light or load.At the same time,in order to avoid overcharge or overdischarge of energy storage components and frequent switching in different working areas of supercapacitor,traditional limited value management scheme is adopted for battery,and a kind of limited value management scheme based on supercapacitor SOC(stage of charge)is adopted for supercapacitor,which introduces supercapacitor SOC feedback to adjust filter time constant,improves the problem that supercapacitor is easy to overcharge or overdischarge,and maintains normal operation of the system.The simulation verifies that energy management strategy of hybrid energy storage system is effective.Thirdly,the structure and operation principle of NPC inverter are analyzed,and mathematical model of NPC inverter is established for NPC three-level grid-connected inverter under normal operation mode.Due to large amount of calculation and other problems when using traditional finite control set model predictive control in a three-phase NPC inverter,a fast vector selection finite control set model predictive control method is proposed.By judging the spatial position of the reference voltage vector,the voltage vector far away from the reference vector is not involved in the online calculation of the prediction model and the cost function,so that the number of voltage vectors involved in the calculation is reduced from 27 to 12,which reduces the complexity and calculation amount of the algorithm.Simulation results show that the proposed fast vector selection FCS-MPC algorithm has good steady-state and dynamic performance.Then,for fault-tolerant NPC three-level grid-connected inverter in photovoltaic and energy storage generation systems under ideal grid,in order to improve fault-tolerant operation capacity after inverter fault,firstly,the working mechanism of fault-tolerant operation is analyzed,and its mathematical model is established.Secondly,the research on grid-connected control strategy of photovoltaic and energy storage inverter under ideal grid is conducted by mathematical model.Then,based on SOC of energy storage device and system power balance theory,the coordination control strategy of photovoltaic and energy storage generation system under ideal grid is studied.Finally,a simulation model is built on the Matlab/Simulink platform to verify the fault-tolerant operation capability of grid-connected inverter after fault and the correctness and feasibility of the proposed coordination control strategy.Finally,the negative sequence component of grid voltage under unbalanced grid will cause output power fluctuation and three-phase current distortion of grid-connected inverter.At the same time,the overcurrent phenomenon in the case of voltage drop will seriously affect reliability of the system operation.Therefore,a model predictive control method for fault ride-through current limiting is proposed.This method calculates the reference current value and power compensation amount under the two-phase static coordinate system with the elimination of negative sequence current,suppression of active and reactive power double frequency ripple as the control target.then the over-current mechanism is derived and analyzed,and a scale factor is introduced to modify the reference current value and power compensation amount under each control target.On this basis,a cost function is constructed to select the optimal switching state and apply it to the inverter to achieve high-performance grid connection of inverter.Compared with traditional methods,no phase-locked loop,pulse width modulation and complex grid voltage positive/negative sequence extraction are required,which overcomes the problem of grid-connected inverter overcurrent during fault ride-through and can guarantee continuous and reliable operation of grid-connected inverters during single-phase bridge leg fault under unbalanced grid voltage.The simulation results verify feasibility and effectiveness of the proposed strategy.