| In recent years,the huge environmental impact of the massive use of fossil fuels has prompted people to turn their attention to new energy sources.Solar energy is one of the largest sources of renewable energy,and the rapid development of photovoltaic power generation technology has made it one of the main micro sources of DC microgrids.However,solar energy has the disadvantage of being random and unstable,which leads to fluctuations in the output power of photovoltaic power generation systems,making the quality of power seriously affected.The hybrid energy storage system(HESS)smooths out unbalanced power in the system and increases the efficiency of solar energy use.This thesis analyses a variety of energy storage systems,chooses a battery and superconducting energy storage to composing of the BESSSMES hybrid energy storage system.This hybrid energy storage system offers the advantages of both the durability of energy-based energy storage and the speed of power-based energy storage and can cope with the different power response requirements in DC microgrids,improving the utilisation of renewable energy and the stability of the microgrid.This thesis investigates the hybrid energy storage system composed of SMES and applies it to the PV DC microgrid.The main work carried out in the thesis for the study of the control strategy of the BESS-SMES hybrid energy storage system in PV DC microgrid is as follows.Firstly,establishing a mathematical model of the solar cell,analysing the operating characteristics of the PV array and studying the control strategy of the PV array.Carrying out the simulation tests of maximum power tracking control and constant voltage control of the PV power generation unit when the temperature varied and the light intensity varied,respectively,with other conditions remaining unchanged.Investigating the hybrid energy storage system,which made by the lead-acid battery and superconducting magnetic energy storage,and analysing their operating principles,mathematical models and operating characteristics separately.Analysing the results of the DC-DC converter and its control strategy.Establishing the small-signal mathematical model and transfer function of the bidirectional DC-DC converter in different modes,and building the topology of the hybrid energy storage system.Secondly,decomposing the unbalanced power of the system of the PV DC microgrid by using the empirical modal decomposition method,and smoothing out the high and low frequency components of the unbalanced power by using different types of devices in the HESS respectively.Proposing a hierarchical control strategy based on the DC bus voltage,which divides the DC bus voltage fluctuation range into five hierarchical zones and uses the increment conductance method and the constant voltage method to control the PV power generation unit.Smoothing out the bus fluctuations by switching the PV generation unit between the above two control strategies and by the operation of the hybrid energy storage system.In order to prevent overcharging and over-discharging of the hybrid storage system,limiting the real-time charge state of the hybrid storage device is read and its range,which effectively extends the life of the battery.Validating the control strategy by building a model in MATLAB/Simulink,which allows the system to switch between different operating conditions and adaptively adjust the bus voltage to achieve stable operation of the DC microgrid under different operating conditions.Then,investigating the life cost cycle of the hybrid energy storage system.Establishing the objective function of the minimum cost of the hybrid energy storage system and its constraints,using a particle swarm algorithm with the introduction of an acceleration factor,and verifying the degree of impact on the life cost cycle of the system by varying the acceleration factor.The strategy not only speeds up the optimisation,but also provides good results in terms of economics.Finally,simulating the proposed hierarchical control strategy and capacity allocation strategy by building a PV DC microgrid model in MATLAB/Simulink,and the simulation results verify the effectiveness of the above strategies. |
PDF Full Text Request