Research On Subsynchronous Oscillation Caused By Grid-integration Of Doubly Fed Wind Power Generation System Via Series Compensated Lines

Doubly Fed Induction Generators (DFIG) are being more and more widely used inwind generation area with its flexibility on control. With the expansion of the scale ofwind farms, the large-scale wind power is outgoing in series capacitor compensationcircuit (short for series compensated lines). Because the series capacitor compensationtechnique improves line transmission capacity and enhances system stability, it is aneconomically realizing long distance transmission mature technology. While doubly fedwind power generation system is connected to power grid via series compensation, theSubsynchronous Oscillation (SSO) problems may also appear similarly with whichtriggered by thermal power connecting to power grid through series compensatedtransmission lines. The SSO problems in doubly fed wind power systems are mainly twotypes of phenomena as induction generator effect (IGE) and subsynchronous controlinteraction (SSCI). This study analyzes the mechanism of SSO problems caused bydoubly fed wind power generation system when connected to power grid via seriescompensation and proposes a corresponding suppression measure.Firstly, a doubly fed wind power generator model for the analysis on SSO isestablished. The mathematical model includes wind turbines, doubly fed inductiongenerator and various parts of the transmission line. Studies are done on maximum windpower tracking algorithm and pitch angle control as well as control principles of bothgrid side converter and rotor side converter. Based on mathematical models of DFIG, asimulation model of DFIG connecting to the grid via series compensation is built underMatlab/Simulink environment.Then, the frequency-scanning method is utilized to analyze the mechanism andimpacting factors of IGE and the mechanism and impacting factors of SSCI are derivedand analyzed in the viewpoint of closed loop control of rotor side current loop, andtime-domain simulation is utilized to analyze and validate the impacts of wind speed andseries compensation degree on IGE respectively as well as the impacts of controlparameters of rotor side current loop on characteristics of SSIC. Research results showthat the increase of compensation degree and the decrease of wind speed may cause IGE;the increase of proportionality factor and integration coefficient of current loop controllermay cause SSCI. Using Prony algorithm to analyse the relationship between the systemdamping with the proportional factor and the integral integral factor respectively, theresults show that the proportional coefficient have a greater impact on the systemdamping. Thus, SSCI phenomenon is much greatly affected by the proportionalcoefficient than the integral coefficient.Lastly, reference to the suppression method of SSO in traditional thermal power system, for the double-fed wind turbine has its own control system, improve controlstrategies of DFIG rotor side controller to suppress SSO in double-fed wind powersystems. Adding damping controller in the active and reactive power outer looprespectively, using active and reactive power in the line as the input signal, through thefilter, phase, gain, limiting aspects of the deal, and getting the additional power dampingsignal, thus the sub-synchronous oscillation problems in doubly fed wind powergeneration system connected to power grid via series compensation can be effectivelysuppressed. And it is proved by simulations that the design of additional dampingcontroller is validated.