Research On Grid Connection Control Strategy Of Virtual Synchronous Generator With Low Voltage Ride Through Capability

In recent years,the scale of grid connection of new energy power generation has been increasing yearly.Due to the lack of inertia and damping of the distributed generation unit with a power electronic converter,the power system gradually develops into a low inertia and underdamping network,which brings insufficient stability problems to the power system.Virtual Synchronous Generator(VSG)control technology introduces a mathematical model of the generator to make the inverter gain inertia and damping,which has a positive affect on the power system of high permeability distributed power sources.However,the improper selection of inertia and damp parameters of VSG will cause power oscillation in parallel operation.At the same time,if the voltage drops due to the faults in power network,traditional VSG may experience off-line operation because it does not have the ability of low voltage traversal.And the off-line operation of VSG will make the problem of insufficient stability of the system more serious.Therefore,this paper takes the grid-connected inverters under VSG control strategy as the research object,and designs a reasonable grid-connected control strategy to suppress the oscillation of grid-connected power and achieve the low voltage ride through capability of VSG.Firstly,a mathematical model of the inverter grid-connected system under the VSG strategy was established,and a voltage and current dual loop controller was designed.Based on the basic principles of VSG control,we analyzed the impact of critical parameters damping and inertia on system performance,and explored the reasons for power and frequency oscillations during grid connection.The angular frequency oscillation of VSG at grid-connected time is divided into four sections.Based on the influence of damping and inertia parameters on frequency characteristics,a parameter self-adjustment method that introduces negative inertia was designed.This method can select appropriate parameter values in different intervals to suppress the oscillation of gridconnected power and frequency.Secondly,in order to solve the problem that VSG lacks the low voltage ride through capability when connected to the grid,the output characteristics of VSG during a grid voltage drop and the reasons affecting its regular operation were analyzed.On this basis,a method of generating instantaneous voltage based on dynamic compensation combined with instantaneous virtual resistance and damping was designed.This method can generate VSG voltage reference value and power reference value in real-time based on the grid voltage amplitude without the need for control strategy switching,which makes the system have the suitable low voltage ride through capability.Finally,a VSG simulation model with low voltage ride through capability and suppression of grid-connected power oscillation based on PSIM was built,as well as an experimental platform for a three-phase full-bridge grid-connected inverter controlled by an STM32F407 ZG chip.And the output performance of VSG control and droop control inverters under off-grid conditions was compared.The simulation and experimental results showed that the system control scheme we designed can effectively suppress grid-connected power oscillations and have suitable lowvoltage ride-through performance.