Research On DC Microgrid Stability With Mixed Energy Storage

With the rapid development of the world economy,the demand for energy is increasing day by day.The fossil energy crisis and environmental pollution caused by traditional power generation make the power industry all over the world face enormous challenges.These problems make the new energy distributed generation gets the attention,so the micro-grid has emerged.Because the output form of wind distributed power?light distributed power and the energy storage device of battery and super capacitor for DC and DC micro-grid has the advantages of simple structure,less energy conversion,output power and easy to control etc.,combined with the current DC equipment demand rising trend,this paper studies the stable operation of the DC micro-grid system.In this paper,the DC micro-grid composed of photovoltaic cells as distributed generation units,lithium-ion batteries and supercapacitors as energy storage units and analog loads is studied.First,this paper introduces the working principle of photovoltaic and builds a mathematical model to study the output characteristics.Through the comparative analysis of the advantages and disadvantages of different maximum power tracking control method,the variable step conductance increment method is used to track the output power of the photovoltaic battery.And the simulation is carried out.Followed by an analysis of the control structure of DC micro-grid hybrid energy storage.Through the research of hybrid energy storage in lithium-ion battery and super capacitor working principle,derived mathematical model and the analysis of both charge and discharge characteristics.Proposes a hybrid control strategy of energy storage,the lithium-ion battery energy storage using the current closed loop control,super capacitor with outer voltage,current double closed-loop control.And this paper also puts forward a method which wavelet decomposition on the photovoltaic cell output random and fluctuat currents are treated as the control signal of hybrid energy storage.And the simulation of stability is carried out.Again,this paper analysis the energy storage principle of the bidirectional DC/DC converter,and establishes a DC/DC bidirectional converter small signal model.Also,this paper designs the energy storage controller of lithium-ion battery and super capacitor,and the stability analysis of different charging and discharging modes is carried out.Finally,a DC micro-grid simulation model has been established in MATLAB/Simulink,and the two major unstable factors of example that change of output power of photovoltaic cell and load consumption power in DC micro-grid which lead to system imbalance has been analyzed under the normal working,over discharge and overcharge model condition.The simulation results show that the lithium-ion battery can be a timely response in the system of photovoltaic cell and load power demand for a reasonable charge and discharge,and the discharge current is stable,the super capacitor can quickly absorb high-frequency power fluctuations in the system and maintain the stability of the DC bus voltage in the situation of output power of photovoltaic battery and load changes,making the DC micro-grid system stable operation.