| The intermittent and unstable nature of new energy power generation causes problems such as large power fluctuations in the microgrid system and low energy utilization rate.The continuous development of the economy makes the power load increase continuously,and the phenomenon of difference between peak and valley of electric load is more obvious.At the same time,the existence of a large number of power electronic devices in the power system,the access of a large number of single-phase loads and three-phase unbalanced non-linear loads in the low-voltage distribution network seriously threaten the power quality of the microgrid.As a crucial part of the microgrid,the energy storage system plays an irreplaceable role in enhancing the safe and stable economic operation of the microgrid.Aiming at the current operating conditions of low-voltage photovoltaic microgrid system with actual load,a compensation control scheme based on battery energy storage technology under unbalanced load conditions is proposed.According to the design scheme of the system,the energy storage converter connected between the grid and the battery system is taken as the research object,and the circuit structure of the three-phase four-leg energy storage converter suitable for the system is selected.Based on the analysis of its working principle and mathematical model,the corresponding control algorithms are designed.Using the energy storage technology,the function of the converter is fully utilized,and the power quality problem caused by the unbalanced load is effectively controlled while realizing the basic power regulation.On this basis,combining with the peak and valley time-of-use electricity price policy,the charge-discharge operation strategy is formulated to improve the energy efficiency and economic benefits of the entire system.Firstly,according to the operation situation of our school's photovoltaic power station,the energy storage system scheme is formulated on the AC and DC side respectively.Through comparative analysis,the technical solution of configuring the energy storage system on the AC load side is finally selected,which can deal with the power quality problem caused by the actual unbalanced load in the photovoltaic microgrid and the matter of energy storage caused by the photovoltaic output characteristics.According to the scheme of the photovoltaic storage microgrid system,the working principle and operation characteristics of each component unit are analyzed.At the same time,based on the grid-connected operation mode of the non-reverse power flow on photovoltaic microgrid,combined with the time-of-use tariff policy,the daily charging-discharging operation strategy of the energy storage system is formulated,and the capacity and power of the energy storage system are configured.Secondly,aiming at the control objectives that need to be realized in this subject,the control scheme of energy storage system is put forward.Taking the bipolar three-phase four-leg energy storage power conversion system(PCS)as the research object,the key technologies of the four-leg converter system are analyzed and designed in detail,such as instruction current detection and synthesis,software phase-locked loop and current inner loop tracking.Aiming at the detection and synthesis part of instruction current,this paper improves the conventional i_p-i_q harmonic current detection method,and synthesizes the instruction current required by the control system by separating the zero sequence components and introducing the active current compensation value in the power loop.The current inner loop control strategies of different working principles are analyzed,according to the control demands of simultaneously tracking the fundamental wave and each harmonic current,a compound current controller is designed with PI and repetitive control in parallel under the synchronous rotating coordinate system.At the same time,based on the working principle of bidirectional DC/DC circuit,a two-stage charge-discharge control strategy is designed.On the basis of realizing the control objectives of the front and rear stage converters respectively,the cooperative control between the two stages of converters is realized by introducing power feed-forward and combining the control of the middle DC side voltage.Finally,an entire hardware implementation scheme of the energy storage system in photovoltaic microgrid is designed.Moreover,a hardware experimental platform is built for the back-stage bidirectional DC/DC part of the energy storage PCS and the corresponding software program is designed.In the part of hardware design,the key component parameters of the system circuit are calculated and selected,and the detailed circuit design of each part of the control system is analyzed.For the realization of the charge-discharge control strategy,the specific software flow of the control algorithm is designed based on the DSP controller.By constructing the simulation model of the energy storage PCS in Matlab/Simulink,the key algorithm modules are simulated and analyzed to verify the feasibility of the system scheme and the effectiveness of the control strategy.Moreover,the battery charging-discharging process is experimentally verified on the built-in hardware circuit platform.The simulation analysis shows that the four-leg energy storage power converter can realize the charge-discharge control of the active energy in the photovoltaic microgrid system,and effectively countervail the harmonic and asymmetric current components caused by the load,ameliorate the power quality of the grid side.By using the energy storage system,the peak-valley power difference can be reduced and the economic stability of the system operation can be improved.The research of energy storage system in photovoltaic microgrid can effectively deal with the power quality problem caused by unbalanced load while realizing the basic regulation of power balance in microgrid.It has theoretical guiding significance and reference value for the reliability and economy research of energy storage configuration in microgrid. |
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