Research On Control Strategy Of Grid-connected Inverter Based On Virtual Synchronous Generator Algorithms During Power Network Fault

With the continuous development of renewable energy generation and grid-connected technology,the large-scale application of distributed generation and micro-grid technology will greatly change the terminal power consumption mode of large power grid,promote the bidirectional flow of electricity between power grids and users,and put forward higher requirements for the automation of grid operation control and the intelligent construction of bidirectional interactive distribution system.As a new type of "grid-friendly" power electronic conversion device and technology,Virtual Synchronous Generator(VSG)technology is helpful to unify the behavior characteristics of power supply and load in power grid,improve the inertia compatibility between high frequency power electronic equipment and large power grid,encourage converters to participate in grid regulation spontaneously,and promote the coordinated and friendly interaction between new energy micro-grid and large power grid.However,the operation of microgrid system is vulnerable to many factors,such as energy,power grid,load and so on,and VSG algorithm must have the ability to operate under complex conditions.Based on this control requirement,this dissertation mainly focuses on several problem of VSG algorithm under grid fault,such as low voltage ride-though,islanding detection and fast frequency and voltage regulation.The main contents are as follows:Firstly,according to the basic principle and mathematical model of VSG algorithm,the essential reason why it is difficult to achieve low voltage ride-though was analyzed.Based on the principle of reactive power regulation in parallel operation between synchronous generator and power grid,a new LVRT control scheme of VSG algorithm was proposed from the perspective of excitation state.The proposed scheme does not need switching control algorithm,and can deal with LVRT problem and various asymmetric voltage drop problem at the same time.Secondly,according to the weak coupling relationship between P-f and Q-V droop characteristics in VSG algorithm,the relationship between AC bus voltage amplitude and output active power in grid-connected and off-grid operation was deduced in detail.Combing with the characteristics of high frequency oscillation of reactive power loop,a new passive islanding detection method was proposed.There are many merits such as simple parameter designing,the detection speed close to PWM switching frequency.It can accurately identify the grid-off moment under a variety of complex conditions,and has better dynamic and robustness.Then,this dissertation tried to propose a fast voltage regulation algorithm to improve the stability of output voltage waveform,including energy storage device control algorithm,power given module and output modulation module.The proposed scheme can limit the response range of droop characteristics,accurately adjust potential and frequency,maintain output voltage stability when multi-parameter changes simultaneously,and can isolate fault phase under asymmetric loads.Finally,an experimental platform was built to carry out relevant experimental to test VSG algorithm,which preliminarily verifies the feasibility of VSG algorithm.