| With the increasing energy crisis, a new technology is in urgent need to improve the energy situation and realize sustainable development in current society.As a single controlled power supply system, micro-grid can reduce the loss of feeder,increase the reliability of local power supply and improve energy efficiency. At the same time, the energy management system is an important part of the micro-grid which can guarantee the safe operation of system and improve the efficiency of the system. Aiming at photovoltaic micro-grid, energy management strategy is studied in this paper according to the characteristics of different energy storage devices.Firstly, brief introduction is made for photovoltaic micro-grid. Its operation state can be divided into two types: grid-connected type and off-grid type. The structure of photovoltaic micro-grid system is identified in this paper. In order to obtain simulation results accurately, the mathematical models and working characteristics of photovoltaic cells, lead-acid battery and ultracapacitor of hybrid energy storage system are analyzed carefully.Secondly, the advantages and disadvantages of several parallel modes of two kinds of energy storage components are discussed. An energy management strategy is analyzed emphatically based on the capacity of energy storage device. In this paper, the Low Pass Filter(LPF) is used to distinguish the distributed power by each energy storage component. At the same time, the control strategies of photovoltaic cells, battery and ultracapacitor are analyzed, and each converter parameters is designed. In this paper, the output of maximum power and variable power are realized for photovoltaic cells when maximum power point tracker(MPPT)- constant voltage control method is used. For battery, complementary PWM control strategy is used to achieve the constant current- constant voltage charge and variable power discharge; for super capacitor, independent PWM control strategy is used to achieve variable power charge and discharge.Then, an energy management strategy which takes power balance and the SOCof energy storage elements into account is proposed, and this energy management strategy is applied in photovoltaic micro-grid which contains the hybrid energy storage system. According to the different conditions of the system, suitable operating modes are divided which can determine the working states of each converter. At the same time, based on different characteristics of two energy storage elements, power frequency division method is adopted to achieve complementary characteristics between different components. The designed system is simulated in Matlab/Simulink environment. The simulation results show that the proposed energy management strategy can control each converter effectively in each working state.Hybrid energy storage system for micro-grid has played a important role in power leveling, and the characteristics of each energy storage component are fully represented. The designed system can switch rapidly and smoothly between various operating modes.Finally, a hierarchical control strategy of dc micro-grid based on the voltage signal of dc bus is proposed which can be divided into MMS(micro-grid management system) and LC(local control). The system is divided into four kinds of operation modes. The MMS has two signals: the main signal is dc bus voltage and the assistant signal is the SOC of energy storage element. In order to achieve the hierarchical control of system, the MMS sends predefined value of power to LC so that LC can control each unit directly. The proposed hierarchical control strategy is verified in Matlab/Simulink environment. The simulation results validate the effectiveness in each mode of the proposed control strategy. |
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