Stability Control Strategy Research And Equipment Development Of Photovoltaic Grid-connected System With Energy Storage Device

Photovoltaic power generation is an effective way for large-scale distributed photovoltaic utilization and development.It has become an important guarantee for the implementation of China's energy sustainable development strategy.The optical storage system using energy storage device and photovoltaic power generation can stabilize the fluctuation of photovoltaic output.It implements grid-friendly power generation technology.The optical storage grid-connected system works in the energy storage dominant mode to realize real-time control of the system transmission power.However,the change in power direction and size changes the DC side impedance of the system.It may cause DC bus voltage oscillation,which affects the stability of the optical storage grid-connected system.To solve the potential stability problem of the optical storage grid-connected system,this paper studies the stability of the system from the aspects of system operation mode and system control strategy.The research focus and achievements are as follows:(1)The research background and significance of this paper are introduced.The two modes of operation,the grid-based and energy storage leading modes of the optical storage grid-connected system are discussed.Three different types of DC bus voltage fluctuation problems of disturbance type,oscillation type and converter interaction type are described.And the corresponding optical storage grid-connected system stability control method is described.(2)The operation mode of the optical storage grid-connected system under the energy storage dominant mode is analyzed.According to the power flow direction of the grid-connected point,the operation mode is divided into two modes: the optical storage system outputs power to the power grid and the power grid charges the energy storage module.The output of each module in the two modes and modal switching process is analyzed.The two-stage converter structure of the system is established.The control strategy of controlling the DC bus voltage of the pre-stage energy storage bidirectional DC/DC converter and controlling the output power of the AC side of the rear-stage bidirectional DC/AC grid-connected converter is proposed.The simulation verifies the effectiveness of the proposed control strategy.(3)The DC side impedance model of the optical storage grid-connected system under the energy storage dominant mode is established.The effects of power direction and size changes on system impedance characteristics are investigated.The stability of the system in the bidirectional flow of power is analyzed by the stability criterion,which reveals the reason why the bus voltage oscillation of the optical storage system does not converge under the output mode of the grid.An improved virtual impedance type DC bus voltage stability control method is proposed,which enables the system to operate stably when the power changes.The simulation verifies the effectiveness of the proposed control strategy.(4)The engineering design of the 50 kW optical storage grid-connected system is carried out.The device selection and parameter selection of the main circuit and the hardware and software design of the control system are elaborated.According to the designed main circuit topology and component parameters,an experimental platform for optical storage and grid-connected systems in which bidirectional DC/DC converters and bidirectional DC/AC grid-connected converters are cascaded is built.The experimental method is verified by experiments.The experimental results show the feasibility of the proposed method.