Research On Virtual Synchronous Generator Of Inverters

With the increasingly serious energy crisis and environmental problems,the distributed generation using renewable energy has been widely concerned.Due to the uncontrollability and stochastic volatility of distributed generation,the increase of its proportion has also increased the negative impact on the stability of power system.If the distributed generation can simulate the operation characteristics of synchronous generator,it will be easier to be accepted by the grid,and the utilization and application value of renewable energy will also be improved.This paper focuses on the research of virtual synchronous generator applied to the control of the microgrid inverter,which includes the following aspects.(1)The detailed theoretical analysis of the principle of virtual synchronous generator is described,and the method using voltage outer loop and current inner loop in d-q coordinate system with additional virtual impedance is introduced,which can be used for control of three-phase inverter.The power regulation principle of virtual synchronous generator in grid-connected state is analyzed.It can not only respond to the change of power instruction,but also can adjust the power according to the change of the frequency and amplitude of the grid.(2)The small signal modeling of the virtual synchronous generator is established,and the mathematical formula for the change of power instruction and the state of the power grid is derived.By drawing the diagram of eigenvalues and simplifying the formula,the effect of rotational inertia,damping and virtual impedance on the operation stability is illustrated.(3)The characteristics of virtual synchronous generator under unbalanced grid is analyzed.Firstly,problems that need improvement under unbalanced grid are demonstrated by analyzing the model of three-phase inverter,and three control targets including active power oscillation suppression,reactive power oscillation suppression and balanced three-phase current with corresponding negative sequence current instructions are illustrated.Then a control method based on positive and negative sequence d-q coordinate system is gived,which including virtual synchronous generator control in positive sequence d-q coordinate system and negative sequence current closed-loop control in negative coordinates.It ensures the steady operation of virtual synchronous generator and realizes the setting control target.(4)The principle of the first regulation of virtual synchronous generator in off-grid state is analyzed,which can ensure the stability of the system when the load changes.In order to maintain the frequency and amplitude within the allowable range,a method of second regulation by shifting adjustment characteristic curves is given.Then an adjustable active power distribution method of virtual synchronous generator in parallel-inverter system is introduced,and the feasibility and accuracy of virtual synchronous generator in parellel is illustrated.(5)The switching process between grid-connected state and off-grid state of virtual synchronous generator is analyzed,and its characteristics of the voltage source can reduce the risk of failure in switching process.A pre-synchronization method is presented.The frequency and phase angle of the virtual synchronous motor are adjusted through the active power control,and the voltage amplitude is adjusted through reactive power control.