Research On MMC-STATCOM In Voltage Fluctuation Suppression For Wind Farm

With the rapid development and increasing scale of wind power grid integration,the point of common coupling(PCC)which is at the end of the distribution grid is easy to be affected by the fluctuation,uncertainty and intermittence of wind,the voltage stability of the grid becomes an issue.The operational level of the grid is determined by its reactive power balance,but reactive power of the grid which contains wind farm always changes because of a variety of disturbance such as the change of wind speed,load fluctuation,open circuit and close circuit of the grid.In these circumstances,reactive power compensators show their importance.Because of its small volume and fast response time,static synchronous compensator(STATCOM)is pop.u.lar among all of the compensators,and modular multilevel converter(MMC)is highly concerned in the research of STATCOM topology by its modular structure,which is easy to expand,maintain and outp.u.t low harmonic wave.This paper first illustrated the power generation principle of two types of wind turbines,summarized the common dynamic model of them.No matter it is fixed speed wind turbine which using squirrel-cage induction generator or variable speed wind turbine which using double-fed induction generator/permanent magnet synchronous generator,the low voltage ride through(LVRT)performance of wind turbine is bad.This paper then analyzed the benefit of reactive power compensators in improving static voltage stability of the grid according to P-V flow curve.Also analyzed the improvement of transient voltage stability of the grid by using reactive power compensators according to electromagnetic torque-rotor speed curve of induction generator.Analyzed the compensation principle of STATCOM and the operating principle of MMC,by learning from the modeling method of three-phase two-level VSC,we obtained the mathematical model of MMC-STATCOM under dq synchronous rotating coordinate system,also the control equations after state feedback cross decoupling is obtained.Designed the classic voltage/current dual-loop control combines carrier-phase-shifting PWM modulation(CPS-PWM)as basic control strategy.The reactive power command based on the instantaneous reactive power theory is fast and precise.Also designed arm capacitor voltage balancing control and individual capacitor voltage balancing control to help average capacitor voltage balancing control to realize the voltage balance of all sub-module.For situation that compensation object is unbalanced,resulting in AC error signal which PI controller cannot track well,designed repetitive control to improve the tracking effect of the signal.A three-phase 48 sub-module MMC-STATCOM is built in Matlab/Simulink as well as a 10 kV distribution grid contains wind farm.The simulation results shows that under the circumstances of stable grid,changing wind,load fluctuation or unbalanced load,the compensator is helpful to the voltage of the grid,especially when the grid has short circuit,MMC-STATCOM can offer massive reactive power to wind turbines,which supports wind farm to rebuild voltage rapidly after faults cleared,restrains rotor acceleration and shortens the fault limit clearing time of wind farm,improves its LVRT performance.Last,a three-phase 12 sub-module MMC-STATCOM porotype which is 66 V/2 kVar using multi-DSP hierarchical control is built to verify its reactive power compensation ability,continuous adjustment ability and unbalance compensation ability.