The Virtual Capacitor Control For Dc Microgrids

The traditional AC power grid has a large number of synchronous rotating generators,and the system has large inertia.The rate of change of system frequency and voltage are small under sudden power variations,and the AC power grid is stable.The power electronic converter in the AC and DC microgrid replaces the synchronous generator in the traditional AC grid,but the inertia of the power electronic converter is small.When the distributed power source and load fluctuate,the frequency and voltage of the microgrid change greatly,threatening Stable operation of the microgrid.Simulating the synchronous generator through the inverter can enhance the virtual inertia of the AC microgrid,reduce the rate of change of the AC microgrid frequency,and improve the stability of the AC microgrid.In this paper,by comparing the analogy between microgrid and DC microgrid,the droop control with virtual capacitor is studied to improve the virtual inertia of DC microgrid and reduce the rate of change of DC bus voltage.The steady-state characteristics of the DC bus voltage are changed by adjusting the droop coefficient;the dynamic characteristics of the DC bus voltage are adjusted by changing the size of the virtual capacitor,and the dynamic characteristics and steady-state characteristics of the DC bus voltage are separately adjusted to improve the stability of the DC microgrid.The DC microgrid adopts the master-slave control strategy.When the DC microgrid is operating in grid connected mode,the PWM bidirectional grid-connected converter adopts the droop control strategy with virtual capacitor to control the DC bus voltage.The PWM bidirectional grid-connected converter uses the load current feedforward to eliminate the disturbance of the DC bus voltage when the load current changes.The virtual capacitor changes the dynamic characteristics of the droop control load current to the DC bus voltage,and reduces the DC bus voltage change rate.Through theoretical analysis,mathematical modeling,simulation,PWM converter whether it is two-way grid rectifier mode or inverter mode,can improve the dynamic characteristics of the DC bus voltage and improve the stability of DC micro grid.When the DC microgrid is in the islanded mode,the energy storage battery bidirectional DC/DC converter acts as the main control unit,and uses a drooping strategy with a virtual capacitor to control the DC bus voltage.The bidirectional DC/DC converter uses power feedforward control to eliminate the disturbance of the load on the DC bus voltage,and uses the virtual capacitor control to improvethe dynamic behavior of the traditional droop control DC bus voltage.After theoretical analysis,mathematical modeling and simulation verification,the bidirectional DC/DC converter of the energy storage system works in both buck and boost modes to improve the dynamic characteristics of the DC bus voltage and improve the stability of the DC microgrid.Through system-level simulation verification,this paper adopts the droop control strategy with virtual capacitors to enhance the virtual inertia of the DC microgrid,reduce the DC bus voltage change rate,and improve the stability of the DC microgrid.