Research On Low Voltage Ride Through Strategy Of Grid Connected Virtual Synchronous Generator

The adjustment of Power structure makes the permeability of new energy grid-connected inverters increase gradually.Due to the lack of inertia and damping of power electronic devices,it is difficult for to participate in the frequency and voltage regulation of power grid,which has brought a significant impact on the safe and stable operation of power grid.Therefore,virtual synchronous generator control technology is used to provide virtual inertia and damping for grid-connected inverters.However,under this control,the grid-connected inverters have limited ability to cope with low voltage,and the grid-connected operation ability is weak when a fault occurs.If there are no protective measures or appropriate control schemes,the inverters will have to be removed from the power grid,resulting in a large number of active or reactive power shortfalls,which will further reduce the voltage of the power system,and even cause the collapse of the entire power system.In order to solve the above problems,the low voltage traversing strategy of grid-connected virtual synchronous generator is deeply studied in this paper.The model prediction algorithm is improved to improve the dynamic response speed of the system for low voltage traverse control.The low voltage traverse control scheme of virtual synchronous generator is analyzed and designed,so that it can operate without disconnection for a certain period of time when the grid voltage drops.The main contents of this paper are as follows:(1)The principle of photovoltaic cell,maximum power point tracking control,energy storage supercapacitor and DC/DC converter control are studied and modeled.The principle and control strategy of virtual synchronous generator are studied to provide virtual inertia and damping for the inverters.On this basis,active/frequency controller and reactive/voltage controller are designed to enable them to participate in power grid regulation.In order to analyze the influence of inertia time constant and damping coefficient on the dynamic response and stability of virtual synchronous generator,the small signal model of the motion equation of the virtual synchronous generator rotor is established,which provides a reference for parameter selection.(2)In order to improve the current controller of virtual synchronous generator,a model current predictive controller is designed according to the principle of model predictive control.A discrete model for predicting future behavior is established by using the current output of three-phase inverters in different switching states.A suitable performance index function is constructed to track the current reference given by virtual synchronous generator control.In addition,according to the error principle of the control algorithm,an errorcompensation optimization algorithm is designed to improve the performance of the model current predictive controller.The simulation results show that the model current predictive control and error compensation optimization algorithm can improve the output characteristics of virtual synchronous generator,and the virtual synchronous generator based on model current prediction can provide virtual inertia and damping under small interference.(3)Then,the voltage sag generator,phase-locked loop technology and voltage sag detection technology are studied.The current limiting link of low voltage traversing control and the negative sequence current restraining link of asymmetric fault are designed to make the current of the inverters not exceed the limit and maintain a relatively good power quality when the grid fails.The whole scheme of low voltage traverse control is designed.The virtual synchronous generator control and low voltage traverse control are switched according to the voltage sag detector signal.The virtual synchronous generator control module tracks the state of the system during the low voltage traversing period,so that the mode transformation obtained at the instant of voltage recovery does not produce current shock.Finally,the low voltage traverse control of voltage symmetry drop and voltage asymmetry drop is simulated,and the feasibility of the scheme is verified.