Analysis Of VSC Impedance Characteristics And Research On Grid Connected Stability

The new round of world energy revolution is in progress,China follows the path of energy transformation closely,in the new situation,to develop renewable energy.Especially solar and wind energy gradually applied to the power generation industry these years,and in recent years has been rapid development.These new energy sources are connected to the grid,accompanied by a large number of power electronic devices access.A large number of power and electronic devices will also be introduced in these transmission methods,such as static var generator(SVG)、unified power flow controller(UPFC).These devices are equipped with a voltage source converter(VSC).The integration of a large number of VSC into the power grid will change the dynamic characteristics of the power grid and affect the stability of the system.Especially in the case of relatively weak AC power grid,the dynamic of the power electronic equipment and its interaction with the AC and DC power grid are obvious,which is likely to cause complex oscillation problems,such as the subsynchronous oscillation of the system.Therefore,it is of great significance to study the influence of VSC interconnection on system stability.First of all,this paper,taking SVG as an example,sets up a simulation model of wind power grid connection in DIgSILENT software.The small signal stability analysis is used to explore the influence of SVG capacity and number and the number of wind farms on the stability of the system under constant voltage control mode and constant reactive power control mode.The results show that the constant reactive power control mode has better effect than the constant voltage control mode in maintaining the stability of the wind power system voltage,and is more conducive to the suppression of wind subsynchronous oscillation.Next,the main factors affecting the stability of the system are analyzed by the participation factor method.For multiple SVG systems,the oscillation of the system under constant reactive power control is mainly caused by controlling my proportional integral parameters.Secondly,three impedance analysis methods used to determine the stability of the system are introduced in this paper,which correspond to three different coordinate systems: positive and negative order coordinate system,dq coordinate system and rotating polar coordinate system.Then a VSC grid connected model is built in the RTDS software.the constant reactive power control mode was selected.According to the model,the impedance matrix of the three coordinate systems is theoretically linearized,and the consistency of the three methods in analyzing the stability of the system is verified.Thirdly,this paper is different from the way to measure the stability of the system by measuring positive sequence impedance,but rather to measure the form of impedance matrix with positive and negative sequence coupling.This impedance matrix is more accurate than the descending positive sequence impedance.The impedance matrix obtained by the measurement is compared with the derived impedance matrix,and the result is very close in the low frequency section.Finally,a closed loop system is constructed by measuring the impedance matrix of VSC connected to the grid and the impedance matrix of the system side.Then we use the generalized Nyquist stability criterion to determine the stability of the system.The influence of the change of the proportional integral parameters of the inner and outer ring of the VSC controller on the stability of the system is explored.And compared with the results of time domain simulation,the results of the generalized Nyquist criterion and the time domain simulation are also consistent.