Research On Sub-synchronous Oscillation(SSO)problem In Pmsg-based Wind Farm Considering SVG

Recently,the WECS(wind energy conversion system)has been extensively exploited.In China,the wind energy sources are mainly located in the north and northwest regions.To consume the renewable energy,large-scale,and long-distance transmission is carried out.However,sub-synchronous oscillations(SSO)phenomena have occurred in direct-drive permanent magnetic synchronous generators(D-PMSGs)based wind farms which has greatly restricted the ability of renewable energy transmission.Moreover,the most majority of wind farms have installed the static var generator(SVG)which is sensitive to the sub-synchronous signals.There probably exist interactions between direct-drive wind turbine and SVG that reduce the system’s stability.In order to well comprehend the mechanism of the problem,this paper have conducted some researches from time-domain perspective and frequency-domain perspective respectively.1)The detail mathematical model of the D-PMSG-based wind turbine and SVG have been built and the state space model of the system is established.Then eigenvalue analysis is performed to understand the mechanism and characteristics of the system’s sub-synchronous control interaction(SSCI)and the sub-synchronous interaction(SSI)between wind farm and SVG.Further,the correctness is verified via electro-magnetic simulations.The results indicate that there could occur SSO problems in D-PMSG-based wind farm system when connected to a weak grid.The characteristic of sub-synchronous oscillation is affected by many factors,where the operating state,the strenth of the grid and the control parameters of wind farm’s converters are included.Moreover,there exists obvious SSI between wind farm and SVG.The control parameters of the SVG will have great impact on the characteristic of sub-synchronous oscillation.2)The impedance model of the D-PMSG-based wind farm system integrated with SVG has been built and the correctness of the model is verified through method of frequency scanning.Then impedance analysis is performed to understand the mechanism of the system’s SSCI and the SSI between wind farm and SVG.The results indicate that SSO problem is concerned with interaction between wind farm impedance’s capacitive region and the inductive region of the system.This ineraction causes different resonance point to the system.When one of the resonance point is located in the system’s negative resistance region,the system will become unstable and start to oscillate.At the same time,when the system is integrated with SVG,frequency characteristic of the system’s impedance will be affected.The frequency characteristic will be dominated by the SVG at some frequency range which will impact the stablity of the system.3)The tranfer function models of the converters in D-PMSG-based wind farm system have been built.Based on the tranfer function model,the control parameters of the converter have been well designed and coordinated.Firstly,the control parameters’ stable and feasible region of the inner-loop current control and outer-loop voltage control are determined according to the stability criterion.Then,taking the impact of the grid strenth and SSI between the devices,the control parameters have been opmtimized and coordinated and the stablilty of the system have been ensured.