Research On Energy Coordination Control Strategy Of Island DC Microgrid

With the rapid development of power industry and the extensive use of renewable energy,the application of DC microgrid has become more and more significant in mitigating the energy crisis and diversifying the power grid.In this paper,the island-based DC microgrid with multiple micro-sources/multi-energy storage is taken as the research object.The photovoltaic power generation units and the operational control strategies of energy storage units as well as the overall energy coordination of the system are thoroughly studied.First of all,the structure of the DC microgrid is analyzed,the mathematic models of the photovoltaic power generation system and the energy storage system are built.Then the output characteristics of the PV array and the charge and discharge characteristics of the battery are simulated and analyzed.Finally,the working principle of boost Boost converter and bidirectional DC/DC converter are studied in depth.To solve the problem of high control complexity and large amount of computation for most of the maximum power point tracking(MPPT)algorithms,an MPPT algorithm based on variable step conductance increment method and pseudo MPP correction is proposed in this paper.Through the two stages of pseudo MPP tracking and pseudo MPP modification,this algorithm can maximize the proximity to the real MPP,which can significantly improve the transient steady-state performance of the tracking system.In addition,the mutation detection of the external environment is applied in this algorithm so that the robustness of the algorithm can be improved.By comparing with the perturbation observation method and the conductance increment method under different parameters,the superiority of the proposed algorithm is verified.Moreover,considering the stability of bus voltage under extreme conditions,a constant voltage control based on gradual approximation is designed to achieve smooth switching from MPPT control to constant voltage control,which can reduce the equipment costs.Because of the fixed droop coefficient,the traditional droop control can't effectively balance the power distribution among the energy storage units.In order to solve this problem,an adaptive droop control based on the SOC is designed in this paper.The algorithm adjusts the droop coefficient in real time according to the current SOC condition of the energy storage units.Under the premise of maintaining the bus voltage stability,the output power of different energy storage units are reasonably adjusted to achieve the SOC consistency of each energy storage unit.The performance of the algorithm under various operating conditions is simulated and verified.On the basis of the above-mentioned control strategies,the energy coordination control strategy based on DC bus voltage layering is designed based on the bus voltage value.The strategy is divided into four levels of control,and the system is divided into seven operation modes.Under different control levels,each component unit coordinates the energy distribution within the control system to ensure the safe and stable operation of the system.The different operation modes of the system are simulated under different levels of control.The experimental results show that the system can operate stably in different modes,and the transient handover between modes is very smooth.