Reseach On The Dynamic Behaviors Of Dfig Wind Turbines In Weak Grids With Synchronized Control And Its Coordinated Control Strategy

In order to replace the exhausting fossil energy,more and more renewable energy resources such as wind and solar energy are increasingly integrated to the grids.The wind power is one of the most widely used renewable energy and has the third largest capacity in the grid.The wind power generator is connected to the grid through power electronic converter,and uses the traditional vector control based on phase-lock-loop(PLL).So its dynamic behavior is not friendly to the grid,and its anti-disturbance capability is worsen under weak grid.Its bad behavior seriously handicaps the safety and stability of the power system and prevents the further development of wind power.The optimization of the wind turbine behavior to emulate the synchronized generator is a good solution.There are lots of researches on the synchronized control for the inverters,but there are few researches on the synchronized control applied to the doubly-fed induction generator(DFIG)wind turbine.Therefore,the DFIG wind turbines with traditional vector control and synchronized control are analyzed and compared from several aspects,and related solutions are also proposed.The main work is as follows:1)The sensitivity and complementary sensitivity curves under various short-circuit ratios(SCRs)are derived to compare the robustness and control performance of DFIG wind turbine with both control methods.It can be proved that the synchronized control is more suitable for weak grids.The critical SCR of the DFIG wind turbine with traditional vector control can be obtained through the extended constrained condition for maximum singular value of the sensitivity matrix.The frequency domain analysis results are validated through time domain simulations.2)The modal analysis method is used to fulfill the subsynchronous resonance(SSR)analysis for the DFIG wind turbine with both control methods.It is discovered that two pairs of inherent eigenvalues related to SSR move towards contrary directions according to the changing proportionality coefficients of rotor current loop and stator power loop.So the improper regulation of RSC control parameters may activate the potential SSR event and is not a good method to damp SSR.Meanwhile,it is proved that the synchronized control can provide good SSR damp.Therefore,a coordinated control methods of connecting the wind turbines with both control methods in parallel is proposed to mitigate SSR.The minimum ratio of the DIFG wind turbine with synchronized control is obtained under various line inductances and series compensate degrees.3)The traditional Fault Ride Through(FRT)control method for DFIG wind turbine is suitable for weak grid.Therefore,a novel FRT control method for DFIG wind turbine is proposed which can be applied to weak and islanded grids.The novel control method based on synchronized control makes the wind turbine operate as controllable voltage source via active current and reactive current droop control.It not only enables the DFIG wind turbine to provide active and reactive currents under symmetric and asymmetric grid faults,but also enhances the voltage stability during the transient re-exciting process after grid fault clearance.The effectiveness and advantages of the novel control method are validated by the simulation cases of DFIG wind turbine connected to an infinite bus,practical regional grid and islanded micro-grid.4)A novel coordinated control strategy for DFIG wind turbines integrated into VSC-HVDC transmission system is proposed:traditional control method is applied to the sending converter station,which regulates the active power according to the reference from the wind farm;meanwhile,the droop between the active power and the squared DC voltage is added to the active power control loop of the sending converter station to enhance the transient DC voltage stability;the synchronized control is applied to the offshore DFIG wind turbines,which controls the amplitude and angle of the offshore AC grid voltage.This novel control strategy can fulfill the soft-start function and the low voltage ride through(LVRT)in the case of the grid faults at the sending converter station.Compared to the classical FRT coordinated control strategy,this novel control strategy can enhance the transient DC voltage stability in case of the grid fault at the receiving converter,and is insensitive to communication delay.So that it is a cheaper solution.The simulation cases of wind farm integrated into a two-terminal VSC-HVDC transmission system is used to validate the effectiveness and advantages of the novel coordinated control strategy.