| Virtual synchronous generator(VSG)technology has received a widespread attention for increasing the system inertia of microgrid systems with renewable energy and optimizing system frequency performance.In the traditional research on VSG control strategies,the control algorithm often focuses on the realization of the VSG control effect,ignores the characteristics of the DC side distributed power supply,and equates it to an ideal voltage source with unlimited capacity,without fully considering its power output capability during actual operation.Furthermore,the key control parameters of VSG are mainly fixed values,which cannot simultaneously simulate the required system inertia characteristics under the condition of maximum utilization of renewable energy.This paper aims at an PV/Battery microgrid unit composed of the microgrid inverter and photovoltaic array and battery energy storage devices on the DC side.The problem of power control in the case of parallel operation of multiple PV/Battery microgrid units in an islanded PV/Battery microgrid is studied.The contributions of this paper are as follows:1.The topology of the microgrid with VSG-controlled PV/Battery units in this paper is established,and the operation mode of PV/Battery microgrid according to the sourcecharge relationship is analyzed.The circuit equivalent models and control circuits of the photovoltaic system and energy storage system are modeled.The mathematical model of basic VSG control is established,and the overall inverter control strategy used in this paper is analyzed,which laid the foundation for the research of power control.2.The grid-connected small-signal model of the microgrid with one VSG-PV/Battery unit system is established,and the effects of the virtual moment of inertia,virtual damping coefficient,and equivalent impedance and other parameters on the system stability in VSG control are analyzed.Further the constraint relationship between the power change and load power change of each PV/Battery microgrid unit is analyzed when multiple PV/Battery microgrid unit of the PV/Battery microgrid are operated in parallel under the islanding mode.Combining with the operating characteristics of DC-side photovoltaic cells and energy storage devices,a method for power distribution is proposed.The power distribution coefficient dynamically adjusts the corresponding VSG control parameters of each PV/Battery unit to achieve effective power control of the microgrid,fully ensuring that the photovoltaic unit is used to supply power to the load preferentially and ensures that the inverter works within a stable range.3.The relationship between system frequency and virtual moment of inertia in VSG control process is analyzed,and an adaptive adjustment method for the virtual moment of inertia is proposed.The method uses the real-time change of system frequency as feedback to construct the adjustment function of frequency,frequency change rate and virtual moment of inertia,and uses the adaptive change of virtual moment of inertia to optimize the control of the system frequency.The real-time simulation platform is built by RT-LAB and PLC to test the proposed mentods.The experimental results show that the method increases the optimization of the system frequency under the premise of ensuring effective control of the system power,ensuring that the frequency can quickly enter the steady state and avoid problems such as overshoot and excessive frequency change rate. |
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