Research On Low Voltage Ride-through Technology Of Grid-connected Photovoltaic Based On Virtual Synchronous Generator

With the continuous development of new energy power generation technology,grid-connected photovoltaic(PV) has become one of the most promising new energypower generation. Its development speed is faster and faster; the grid capacity isbecoming bigger and bigger; and it has deeper and deeper influence upon power gird.Outstanding grid control technology is important measures to improve thegrid-connected PV permeability. This paper adopts the advanced control technology,namely virtual synchronous generator(VSG), which makes grid-connected PVconverter have all the external properties of synchronous generator(SG), with havingfrequency and voltage regulating function and improving the grid-connected PVpermeability. At the same time, with the increasing of PV grid capacity, when gridfault and voltage sags happen, that PV system could disconnect brings unsteadiness tothe grid, and what’s more seriously is that it may paralyze power system. It is verynecessary that when grid voltage dips, it can provide a certain amount of reactivepower compensation in order to support the recovery of grid voltage. This articlestudies grid-connected PV system based on virtual synchronous generator. When thegrid voltage drop occurs, the increase of output current can be limited and couldensure the safe and stable operation of PV grid inverter through improving the controlstrategy and controlling the output power cooperating with energy storage devices,thus achieves Low Voltage Ride-through(LVRT).First of all, this paper introduces the overall trend of PV power generation,domestic and foreign present research situation, and the development ofgrid-connected PV control technology, at the same time briefly analyzes the standardsand requirements of LVRT in some other countries. Grid-connected PV system’sstructures are introduced. This text establishes PV simulation models, and introducesseveral MPPT control method, finally expounds commonly used structure ofgrid-connected PV.Secondly, this paper is on the basis of LC three-phase PV systems, as well asconnects in parallel energy storage on DC bus. Grid-connected PV system’s structuresbased on virtual synchronous generator are presented. This text illustrates theprinciple of VSG and its mathematical model is established and sets up simulationmodule in Matlab/Simulink in order to verify the effectiveness of the control technique.At last, this paper studies LVRT technology of grid-connected PV based on VSGand introduces the methods of symmetrical voltage dropping test and asymmetricalvoltage dropping test. The control strategy is improved when the grid voltagesymmetrical and asymmetrically sags are put forward. For the grid voltage unbalancedfault, improved instantaneous symmetrical components method is proposed to extractpositive sequence components. This technology could balance power on both sides ofthe inverter, cooperating with energy storage devices, ensuring that the output currentflow doesn’t overcurrent. At the same time, it produces reactive power and helprecover the grid voltage with the reactive power compensation.