| The environmental problems caused by the consumption of conventional energy sources such as coal and oil have become more and more serious,and the demand for energy has been increasing.In this context,distributed energy sources such as wind energy and solar energy consume more and more,and they are characterized by clean and sustainable,little harm to the environment,the distributed energy and energy storage and control equipment combined to form a small capacity Micro-grid,to meet the local load at the same time,And exchange power with the main grid,the application of more and more widely.Grid connection,the micro-grid control for the current source and the main power grid for the exchange of power,if the unplanned island of the main power failure occurs,this time in the micro-grid of the sensitive loads may achieve poor power quality with the main grid fails.Therefore,it is necessary to realize the seamless switching of PCC point to complete the smooth transition of the PCC point voltage during the process of grid-to-net switching to ensure the sensitive loads have good power quality.Firstly,the control strategy of microgrid grid-connected,isolated-net and switching between the two is studied in this paper.Secondly,based on the study of the available literature,this paper proposes a seamless switching control strategy based on PCC point voltage.The PI controller parameters of voltage limiter and frequency limiter and inductor current loop,outer voltage loop and feedforward voltage loop are designed,and the effect of parameter selection is illustrated by open loop Bode diagram.In the PISM,the loads with different resistance values are switched from grid-connected to isolated-net and simulated by isolated-net-to-grid reconnection.According to the simulation analysis,the simulation and experiment results verify the correctness of the seamless switching control strategy based on the PCC point voltage.Then,a seamless switching control strategy based on inductor design is proposed.Depending on the angle of the output current to the grid voltage when the grid is connected,the power and direction of flow of the microgrid converter to the mains are also different.Based on the standard of the PCC voltage of the international association and each country,the range of allowable fluctuation of the PCC point voltage is given in this chapter.Considering the practical engineering inductor ripple and fundamental wave According to the parameters of the microgrid active capacity and other parameters,the inductor parameters of the converter are achieved.It can complete the seamless switching process without changing the control strategy under the inductor parameter.Finally,when the microgrid contains multiple converters,the sag control ca n be used to seamlessly switch to the isolated network.When the grid is connected,the microgrid runs in the PQ control,the limiter does not work;when the isolated network occurs,because the given power may not match the load power,the limiter will work,and the voltage and frequency limit value will depend on the droop Modify the power reference command value to complete the PCC voltage seamless switching.When the isolated island detects an isolated network,the state operation switch is switched to the droop control,and the power is optimally allocated according to the relationship between the capacity of the microgrid and the load.The seamless switching control strategy based on droop control is verified by simulation in PSIM. |
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