Research On Operational Control Of DC Micro-grid Based On Photovoltaic Generation And Hybrid Energy Storage

Photovoltaic (PV) generation with its clean, flexible installation and so on has becoming a hot research topic at home and abroad. If photovoltaic generation is direct incorporated in the grid, its randomness and intermittent will impact the stable operation and control of utility grid to a certain extent. However, in order to solve the above problems, it is better to make them connect to the distribution network in the form of micro-grid. However, PV array and energy storage device of the micro-grid supply direct current (DC), which need to access the micro-grid through the inverters, the conversion process will generating a lot of power loss and affecting power quality. At the same time owing to the increasing use of DC equipment brings about the decreasing demand of direct current. Therefore, researching on DC micro-grid will has higher theoretical and practical value.This paper focuses on a study of the photovoltaic-energy storage DC micro-grid applied to residential or office buildings, the main research contents are as follows:First of all, according to the study of DC micro-grid topology, a topology of DC micro-grid based on photovoltaic generation and hybrid energy storage which applies to residential or office buildings is presented in this paper, including photovoltaic generation unit, energy storage unit, load unit and grid unit, simultaneously demonstrating its characteristics. The paper conducts modeling and simulation analysis of photovoltaic cells and selects a maximum power tracking method on the basis of variable step disturbance, through simulation proving that it has a good optimization precision and response speed.Then by comparing various energy storage medium main performance, considering the energy density, power density, service life and environmental protection, this paper selected the lithium ion battery and super capacitor as a hybrid energy storage system, connected to DC micro-grid by an active structure. The small signal modeling analysis of energy storage controller is carried on, and the capacity configuration of energy storage system is also illustrated in detail, for regardless of DC micro-grid in off-grid or on-grid operation.This paper puts forward the operational control strategy of DC micro-grid in the light of no backward flow connected-grid. The four operating modes nearly covers all possible operation conditions, which including general discharge mode, general charge mode, extreme over-discharge mode and extreme over-charge mode. In accordance with the voltage fluctuation of DC bus and the capacity change of storage battery, the smooth conversion principle of each working mode is discussed. Then in the light of coordination control of the respective power converter, a DC bus voltage control strategy based on energy balance is proposed, in which the dominant work is done by energy storage unit and supplemented by other unit, ultimately make the total energy reach equilibrium, maintaining a stable and secure operation of the entire system.Finally, modeling and simulation analysis of the operational control of the DC micro-grid is conducted in Matlab. The paper mainly takes into consideration two power imbalance which consists of PV output power and load consumption power changing respectively, then simulation analysis of micro-grid operational control is carried on ordinary charge and discharge mode, extreme over-discharge mode and extreme over-charge mode. The simulation results demonstrate the correctness of the operational control strategy for DC micro-grid under the proposed working mode.