Power Distribution And Capacity Configuration Optimization Strategy Of Hybrid Energy Storage System In Microgrid

Hybrid energy storage system(HESS)takes an important part in micro-grid in several aspects,such as peak shaving,load leveling and uninterrupted power supply.In a micro-grid consisting of renewable energy sources and loads with different characteristics,the energy storage system has to respond to power and energy demands of different time scales,so that both better power performance and adequate capacity may be required.The requirements can be achieved by taking the advantage of different types of energy storage device.In this case,an optimal energy management for hybrid energy storage system must be considered.Focus on the hybrid energy storage system in micro grid,the power distribution method and the capacity allocation optimization method of the hybrid energy storage system are researched.And according to the different control objectives of the energy storage converter,the current control methods are improved.Firstly,in order to keep energy balance for all storage devices on different time scales,the adaptive SOC power distribution method is proposed,which is composed of two steps of power distribution.The first step is to optimize all the power orders of energy storage devices,taking operating cost of the energy storage system as objective function,to assure the energy balance in long time scales.To further optimize the power distribution result of first step,adjust the SOC constraint condition of the power-type storage device from a fixed range to a dynamic rang.The second step is real-time adjustment to compensate the unscheduled power fluctuation undertook by the energy-type storage device,ensuring the power balance in short time scales.The validity of this method is verified by case studies.Secondly,considering the indirect impacts of micro-grid operation target on the life cycle cost of HESS,a two-layer composite model balancing the life cycle cost of HESS and the operating cost of micro-grid is established.To solve this model,a simplified alternate iteration method combining genetic algorithm and traversing method is used,and a sliding time window method is designed to calculate the initial value,which is also the capacity configuration result of HESS under a certain reliability requirement.The validity of this method is verified by case studies.Finally,aiming at overcoming the inherent defect of pure PI controller in slow dynamic response and poor robustness,a new MPC control strategy with feedback adjustment is proposed and it replaces PI controller using in the DC/DC converter.As for the DC/AC converter(inverter),to prevent the nonlinear loads "inject" the harmonic current into the power systems,an improved droop control strategy is proposed.Through redesigning the voltage loop,the harmonic voltage of AC-bus is compensated and the new controller possesses the function of APF,which can effectively reduce the total harmonic distortion(THD)of the grid-connected current.The effectiveness of these proposed methods are verified by simulation analysis.