Research On Sub Synchronous Oscillation Of Grid-connected Photovoltaic And Permanent Magnet Direct Drive Wind Power Systems

Sub Synchronous Oscillation(SSO)is one of the classical problems in power system stability research.With the steady increase of renewable energy generation proportion,the traditional power system SSO problem presents a new trend.Among them,the SSO problem caused by large-scale access of renewable energy represented by solar energy and wind energy cannot be ignored.The application of series compensation technology and the fast direct current control of many complex power electronic devices increase the risk of subsynchronous oscillation,which endangers the stable operation of power system.In this paper,the eigenvalue analysis method and time-domain simulation method are applied to carry out the following research work on the mechanism and characteristics of photovoltaic and direct-drive wind power SSO:(1)A small signal state space model of PV grid connected system with series compensation is established.Based on the eigenvalue method,the strongly related state variables that affect the subsynchronous oscillation mode are explored,and the influence of control parameters on the subsynchronous oscillation mode and damping characteristics of PV grid connected system is analyzed.Based on psdcad / EMTDC,a time domain simulation model corresponding to the small signal model is built to verify the small signal analysis results.The results show that under certain parameters,there is a risk of SSO induced by series compensation grid connected photovoltaic system,and there is a 24 Hz divergent oscillation mode.The current inner loop controller,voltage outer loop controller and PLL are the strong correlation factors of the subsynchronous oscillation mode.Increasing the integral parameter,the proportional parameter or the current inner loop parameter of the controller within a certain parameter range will help to improve the system damping,reduce the oscillation frequency and improve the system stability.(2)Based on the graphical modeling method,the whole system state space model of PMSG(permanent magnet synchronous generator)incorporated into series compensation grid is established,and the dynamic interaction between PMSG and series compensation grid is studied by eigenvalue analysis;With the help of participation factors,participation degree of each state variable is analyzed,and strong participation factors are screened out.Time domain simulation based on PSCAD / EMTDC is carried out to verify eigenvalue analysis results.Results show that,under certain conditions,the incorporation of PMSG into series compensation grid may induce sso,which is caused by the interaction between converters and series compensation grid,and dominated by converters.The results of this paper reveal that the existing view that PMSG is immune to SSCI through series compensation is not completely compatible with the dynamic evolution of SSO form,PMSG grid connection will trigger new SSCI under certain conditions,and the oscillation frequency is strongly related to the series resonance frequency composed of the combined capacitance and inductance of power grid.(3)Innovatively analyze the differences and similarities of SSOs of photovoltaic and wind power.The grid connection interface of photovoltaic and permanent magnet direct driven wind turbine has high consistency,and SSO presents similar grid related characteristics,that is,the stronger the AC system,the lower the SSO oscillation frequency,the greater the damping ratio,and the more stable the system.In addition,the influence factors of photovoltaic and wind power SSOS are also consistent.Power electronic devices such as controllers and phase-locked loops,as well as the connection and removal of fixed series compensation,have strong influence on the oscillation.The difference is mainly reflected in the frequency of oscillation.The oscillation frequency of PMSG grid connected SSO is significantly higher than that of photovoltaic grid connected subsynchronous oscillation.