Research On Control Method Of Cascade STATCOM In Wind Farm

With the country’s emphasis on new energy and the rapid development of wind power technology,the scale of domestic wind farms is increasing year by year.However,a large number of wind farms will cause a series of problems such as system voltage and frequency instability after they are incorporated into the system.By adding a reactive power compensation device at the grid connection point,the reactive power of the system can be effectively compensated,the voltage at the grid point is stabilized,and the power oscillation of the damping system is maintained,thereby effectively improving the stability of the power system.Due to its modular structure,the H-bridge modules on the DC side of the cascade static synchronous compensator(STATCOM)is suitable for high-voltage and high-power applications and is one of the most advantageous reactive power compensation devices.However,it is easy to generate the DC side voltage imbalance of each module,and the problem is more obvious in the power unstable wind power system.Therefore,this paper studies the control method of cascaded STATCOM.Firstly,the working principle of cascaded STATCOM is discussed in the case of system with and without loss.Based on this,the steady-state mathematical model of cascaded STATCOM in dq coordinate system is established and the mathematical model is solved.Coupling analysis,the system control model of feedforward decoupling is obtained.Secondly,an improved control method is proposed based on the hierarchical control and energy exchange ideas for the STATCOM DC side voltage imbalance problem.The upper layer adopts voltage and current double loop control to realize the global voltage stability of the device and the reactive power compensation to the system.The second layer is the phase-to-phase voltage equalization control.An improved error vector superposition method is proposed.The correction of the modulation voltage command is used to fine tune the phases.For the purpose of the absorbed power,the DC side voltage of each phase is equalized.The third layer is the phase-in-phase voltage equalization control,and the error vector superposition is also adopted to realize the DC-side voltage equalization of each module in the phase.Through vector analysis,it is concluded that there is no coupling in the three-layer control,and the next two layers of control do not affect the active power of the upper layer control.It is verified by simulation that this method can effectively solve the DC side voltage imbalance problem.Finally,aiming at the system compensation and DC-side voltage equalization of cascaded STATCOM under system imbalance,the mathematical model of cascaded STATCOM under system imbalance condition is established,and an improved positive and negative sequence double loop control strategy is proposed.Through the independent control of positive and negative sequence active and reactive components,the global voltage regulation control of the device is realized while realizing reactive and negative sequence current compensation.A phase-injection voltage is applied between the phases to balance the phase-to-phase power and achieve phase-to-phase voltage equalization.The method of error vector superposition is adopted between the modules in the phase to realize the DC side voltage balance of each module.The effectiveness and feasibility of this method are verified by simulation.