Bus Voltage Stability Control Of Photovoltaic DC Microgrid With Constant Power Load

With the development of distributed energy and energy storage technology and the wide application of DC loads,the research on DC microgrid has attracted wide attention in academic circles.Because the requirement of phase and frequency synchronization only exists in AC system,there is only one requirement for stable operation of DC microgrid,that is to keep the bus voltage stable.However,in practice,the direct current generated by the distributed power supply can meet the use requirements only when it is connected to the DC bus through the converter,and some DC loads can be connected to the bus through the converter.In the case of steady-state,the distributed power or load and its connected closed-loop converter can be regarded as constant power load.Constant power load has negative impedance characteristic,which will lead to significant oscillation of DC bus voltage when the constant power load changes greatly.This will reduce the power quality and bring security risks.Therefore,it is of great significance for the design and operation of the DC microgrid system to study the stability control of the bus voltage when the constant power load changes.In this paper,the bus voltage stability control of independent DC microgrid with resistive load and constant power load is studied.The main work of this paper is as follows:Firstly,the renewable power supply and the constant power load are regarded as the equivalent constant power load combination in parallel.The mathematical model of the system is established by taking the converter which connects the battery and the bus as the control object.On this basis,the mechanism of DC bus voltage instability caused by large disturbance of constant power load due to the negative impedance of constant power load is analyzed.In order to ensure that the bus voltage remains at the reference voltage when the load and the source change randomly,a conventional sliding mode controller is introduced.The stability of the system under the conventional sliding mode control is analyzed theoretically,and the stability criterion of the system is derived.The influence of the change of equivalent constant power load and control parameters on the stability of bus voltage is studied by the stability region diagram.The simulation results show that the conventional sliding mode controller can stabilize the bus voltage on the reference value when the equivalent constant power load changes randomly,but this method can not meet the good performance and the large stability range of the system at the same time.In order to solve the problem that the conventional sliding mode controller can not satisfy the good performance and expand the stability range of the system at the same time,a sliding mode controller based on a washout filter is established according to the strong robustness of the washout filter.The stability conditions of the system are analyzed theoretically.According to the stability conditions,the stability region diagram of the system is obtained,and the system simulation is carried out.The results show that the proposed control strategy can not only reduces the steady-state error of the system and the fluctuation range of the transient response,but also broaden the stability range of the system.