| Large-scale photovoltaic power plants are connected to the grid by DC convergence mode,which can fully and efficiently utilize solar energy,reduce the cost of power generation,and have obvious advantages,so they have been widely studied.Large-scale photovoltaic DC convergence system mainly contains DC/DC and MMC power electronic devices,in which several DC/DC converters are connected in series for DC boost convergence,and MMC transmits the collected energy to the power grid.The coordination of DC/DC and MMC is the key to the safe and stable operation of the system,in the start-up and shutdown process of the system,as well as the normal operation and failure state of the system.In this paper,the coordinated control strategy of PV DC boost collector system is studied based on hybrid MMC.The main research contents are as follows:(1)The topology and working principle of DC/DC and hybrid MMC are analyzed,and the mathematical models of each converter are established.The advantages,disadvantages and application scope of different modulation modes of MMC are compared and analyzed.At the meantime,the modulation strategy of MMC in PV DC convergence system is selected.(2)The process and characteristics of hybrid MMC and DC/DC charging through AC network during system start-up are analyzed.In the uncontrolled charging stage,the capacitor voltage of different sub-modules in hybrid MMC rises at different rates and the DC bus voltage is too low.By controlling the input and removal of full bridge sub-modules,the capacitance voltage of all sub-modules is ensured to be consistent at the end of this stage.The soft start process of the system can be realized smoothly through the uncontrolled charging stage,the controllable charging stage and the power lifting stage.(3)The control strategy of mixed MMC and DC/DC simultaneous discharge during system shutdown was studied.The system adopts traditional MMC grouping discharge strategy,which will cause the excessive discharge current at the initial discharge time and the switching time of the discharge group.Therefore,the energy allocation stage and the balanced discharge control strategy are added,which cooperates with power reduction and energy feedback stages can reduce the impact of outage on power grid and power equipment,save energy and reduce the cost of discharge resistance.(4)The coordinated control strategy of the system in normal operation mode and fault traversing mode is studied.The control strategies of DC/DC and MMC are different when the system runs normally or fails.This paper studies the coordinated control strategy of the converters when the power dispatch of the receiving power grid and different types of faults occur in the system,which ensures the safe and stable operation of the system under multiple operating conditions.(5)In order to verify the effectiveness of the coordinated control strategy when the system starts and stops,normal operation and AC faults occur,a simulation model of the PV DC boost convergence system is built by using MATLAB/Simulink,and the simulation results under various operating conditions are verified.The results show that with the coordinated control strategy,the system can achieve soft start-stop,accept active and reactive power dispatch of the power grid,realize low voltage crossing when voltage sag fault occurs in the AC system,and the system can operate safely and steadily under multiple operating conditions... |
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