Research On Series Compensated Muti-function Converter In Weak Grid

The proportion of new energy generation connected to power grid is increasing.However,due to its distributed and intermittent characteristics,as well as the situation of multi transformer links and long-distance transmission lines when connecting to the distribution grid,there are following key problems when connecting new energy generation to weak grid: Due to the requirements of load and grid-connection,it is difficult to control the power transmission to the grid by regulating the micro source output.Another problem is that the distributed current control grid-connected inverter and the line inductance oscillate at low frequency,which reduces the stability of grid connected system.In this paper,the series compensation converter composed of voltage source inverter and its series transformer is taken as the research object,and its control function is analyzed through modeling.On this basis,the control strategies under different application s are designed to realize multi-functional application and provide a solution of introducing additional devices for the reliable grid connection of distributed new energy in weak grid.Aiming at the problems of limited voltage amplitude and phase adjustment of power station and the resonance between grid-connected inverter and grid impedance affected by phase-locked loop,the connection mode of series compensated converter is defined,and its related parameters are analyzed and designed.The small signal model of series compensated converter is studied and the voltage control loop is determined.Based on the principle of variable voltage in series,this paper analyzes the application functions in various scenarios under different voltage in series and verifies the basic voltage amplitude and phase control functions through simulation.The power control scheme of series compensation converter is studied to improve the transmission power of new energy power station.Through the establishment of grid-connected system model,the power relationship of the line is analyzed,and the power change of each line node is studied after the compensation voltage of the series compensation converter is added.According to the demand of transmission power of the line,the series compensation voltage is determined,and then the transformer parameters can be designed.Based on the principle of power transmission,the power control scheme of the line is studied to r ealize the controllable power transmission of new energy power station under the weak grid.The correctness of the scheme is verified by the simulation.This paper studies the ability of series compensated converter to improve the grid-connected stability of current controlled inverter by reshaping the grid impedance.Based on the harmonic linearization method,the sequence impedance model of current controlled inverter is established,and the low frequency resonance between inverter and grid is studied by using the stability analysis method of impedance ratio.Based on the analysis of resonance problem,a variable impedance control strategy of series compensated converter with voltage phase-locked is proposed,which realizes grid impedance cancellation by c ompensating line-impedance voltage.The simulation results show that the strategy can improve the grid stability and transmission current capacity of the weak grid within its own capacity.Finally,based on the hardware in the loop simulation platform of RT-Box,the software scheme is designed,and the simulation experiments of voltage,power and variable impedance of the series compensation converter are carried out on the platform,which proves that the converter can achieve a variety of control functions in multiple application scenarios.