Research On Stability Technology Of Self-synchronous Voltage Source Power Electronic Equipment Connected To Weakly Synchronous Voltage Supported Grid

With the large-scale development of new energy sources in the new power system and the centralized grid-connection,the support and regulation ability of the traditional generator set to the new power system has been greatly weakened,and the power electronic equipment represented by the converter is gradually dominating the network characteristics of the new power system.The grid-connection of power electronic equipment with low inertia and low disturbance immunity not only reduces the frequency and voltage support capacity of the new power system,but also makes the stability of the ground become more and more prominent.In addition,with the continuous expansion of power electronic equipment,the occurrence of off-grid accidents will seriously affect the normal operation of the power grid and even lead to system disconnection.Therefore,this thesis takes the grid-connected converter of new energy units as the research object,mainly analyzes the control structure and function of the grid-connected converter based on the virtual synchronous generator(VSG)control strategy,and on this basis,studies the fault crossing strategy and circulation suppression method of the grid-connected converter.In order to improve the stability performance of the grid with weakly synchronous voltage support.Specific research contents and achievements are as follows:Firstly,based on the second-order mathematical model of the synchronous machine,the control structure and function of the virtual synchronous generator are introduced in detail.Considering the voltage support capability and frequency regulation response,the control structure of the self-synchronous voltage source converter is designed in this thesis and the control block diagram is given.Then,based on the analysis results of VSG transient characteristics during the failure of power grid voltage sag,the advantages of easy adjustment of virtual synchronous control strategy parameters are taken into account.The two requirements of power Angle stability and fault current limit are taken into account,and the two objectives of restraining transient impulse current and optimizing power overshoot are taken into account.A VSG fault crossing method based on virtual impedance value is proposed.This method solves the problem that VSG cannot realize fault crossing under the condition of large voltage drop,gives full play to the supporting capability of VSG to the grid voltage during the fault,and optimizes the transient response at the moment of fault occurrence and removal,improving the fault crossing capability and effect of VSG under the condition of grid voltage drop.Meanwhile,in order to simplify the fault crossing control measures and take advantage of the static difference of voltage control loop,a VSG fault crossing method based on dynamic power Angle compensation and reactive power feedback coefficient optimization is proposed.Based on the traditional energy balance principle,the active power compensation quantity is constructed to compensate the active power balance during the fault period,so as to achieve the active power balance and ensure the stability of the power Angle.At the same time,the reactive power feedback regulation ability is utilized,the current threshold and voltage drop amplitude of the inverter are considered comprehensively,and the reactive power feedback coefficient is reset to achieve the maximum voltage support effect during the fault period.In addition,the method can also be applied to the grid voltage rise scenario.Finally,a circulation suppression method based on reactive power feedback coefficient optimization is proposed to solve the circulation problem of multiple inverters in parallel system.The voltage control loop with reactive power droop control is adopted,and the reactive power feedback regulation ability and reactive power feedback coefficient can be adjusted flexibly.By adjusting the reactive power feedback coefficient according to the capacity,the inverter output power can be redistributed to achieve the effect of circulation suppression.