Study On The Contribution Of Power System Stabilizer For Doubly Fed Induction Generator To Network Damping

With the development of wind power technology，more and more large scalewind farm was interconnected to the power system.There is a trend that theproportion of grid-connected doubly fed induction generator(DFIG) increases andthat of conventional synchronous machines decreases in future power system,thecontribution of the DFIG to network damping is generally not considered in itscontrol systems when the network is under oscillatory conditions,so the networkdamping tends to be reduced.In this paper, by means of leading power systemstabilizer (PSS) to the rotor-side control system of DFIG, the DFIG can offeradditional damping to suppress oscillation during the disturbance process.First,the basic principle of DFIG is demonstrated in this paper,mainly onvoltage and flux linkage equations,the power flow in DFIG,the converter controlsystem of DFIG, maximum wind energy tracking control strategy, and pitch controlscheme. The mathematical method for the study of the small signal stability anddamping characteristics of power system is also introduced. Small signal stability ofpower systems can be accurately characterized by eigenvalues of the state matrix ofthe system linear model. In addition, the right and left eigenvectors corresponding tothe eigenvalues of interest are the basis of sensitivity tools to identify therelationships between variables and oscillation modes.Second,differential equations describing the dynamic behavior of DFIG ispresented,a complete small signal mode of DFIG is given by the linearization ofdifferential equations.Take single machine infinite bus system as an example, bycalculating eigenvalues and participation matrix of state matrix of small signalmodel, the influence of DFIG on oscillation modes and damping charateristics ofpower system has been studied,the mechanism of oscillation modes has also beenanalyzed.Finally, by means of leading PSS to the rotor side control system of DFIG,theactive power control system, the DFIG can offer additional damping to suppressoscillation during the disturbance process，The effectiveness of the proposed point ofview is verified by eigenvalue analysis and mult-machine time-domain simulation.Simulation results show that the lead in PSS can enhance system damping andimprove power system stability. The contribution of PSS to netwrok damping relatesto the operation conditions of DFIG, the enhancement of network damping is more obvious because of a greater active power output when DFIG is running athypersynchronous speed, while the contribution of DFIG to netwoking damping ismuch more smaller because of a smaller active power output when DFIG is runningat subsynchronous speed.