Analysis Of Coupling Oscillation Mechanism Between SVG And Doubly Fed Machine In Offshore Wind Farm

With the maturity of new energy power generation technology,the grid-connected capacity of new energy represented by wind power and photovoltaics is increasing year by year;under the requirements of "carbon peaking and carbon neutrality" and "focusing on green and low-carbon development of energy",it is urgent to build a new power system with new energy as the main body.The large-scale grid connection of wind turbines reduces the damping characteristics of the system,showing weak damping or negative damping;when there are disturbances such as sudden changes in voltage,the system may oscillate and become unstable due to insufficient damping.The purpose of this paper is to study the oscillation mechanism and stability analysis of wind turbines and SVG in offshore wind power systems with long-distance grid-connected DFIGs.Firstly,the mathematical model of the doubly-fed wind turbine in the synchronous rotating coordinate system is established,and the control model based on the SVO converter is established.A linearized small-signal model of the doubly-fed wind turbine is established to analyze the influence of the converter parameters on the distribution of the characteristic root,and the change of the GSC parameter has an obvious effect on the distribution of the characteristic root of the system.Secondly,based on the linearized small signal model,the equivalent impedance model of the offshore doubly-fed wind power system connected to the grid via long-distance transmission lines is established;the stability criterion based on the system equivalent impedance-frequency characteristic curve is studied,and the virtual impedance-frequency characteristic curve is calculated.And the system aggregated RLC series equivalent circuit at the zero-crossing frequency of the imaginary part of the impedance-frequency characteristic is calculated to quantify the system oscillation mode.The research shows that the longer the transmission line,the smaller the system equivalent resistance,the smaller the oscillation damping,and the lower the system stability.Finally,the working principle of SVG is analyzed,its equivalent admittance model is established,and the accuracy of the model is verified by the frequency sweep method.Establish an equivalent impedance model of offshore doubly-fed wind power system based on static var generators,and study the influence of the coupling of static var generators and DFIG generator-side and grid-side power electronic converters on system oscillation modes;study offshore wind power under the parallel operation of doubly-fed wind turbines with different numbers of fields,the oscillation characteristics of the system change after external disturbance.A method of adding a voltage damping controller to the machine-side converter is proposed.By limiting the rise of the stator and rotor currents and indirectly suppressing the rise of the torque,the system oscillation can be suppressed,and the voltage support capability of the DFIG unit to the system is improved.And build a time domain model on the simulation platform for simulation verification.