Research On Direct-drive Wind Power Converter Which Connected To The Weak Grid Through An LCL Filter

Wind energy is clean, non-polluting and can be recycled, it is the ideal alternative totraditional fossil energy. Wind farms and wind turbines which almost located in the remoteareas and connected to the grid through long transmission lines, work under weak gridcondition. The grid current control will be affected by the variation of the weak gridequivalent impedance; the study of wind power connecting to weak grid stably hasimportant practical significance. The LCL filter instead of single-L filter is used in directdrive wind turbine, and the LCL control strategy under weak grid condition is morecomplex and difficult.The active damping of the LCL filter under weak grid condition is studied firstly.Stability is the most basic requirement of the control system, but the control system iseasily led to instability by the LCL filter resonance peak，therefore a certain dampingprocedure is needed.The LCL active damping strategies include the filter-based activedamping and the multi-loop active damping. As for the filter-based active damping, thedamping characteristic will change as grid equivalent impedance variates, it cannot beapplied to the weak grid condition. Discrete state space active damping belongs to themulti-loop active damping, and it can be applied to the weak grid condition. The systempoles and zeros can be configured flexibly by the discrete state-space feedback activedamping concerning the variation of the grid equivalent impedance. The target of the poleplacement is configured according to the system open-loop characteristic, three constraintsare derived inequality, and the system poles-zeros configuration method is given out.The grid background harmonic suppression strategy under weak grid condition isresearched secondly. Based on stable control system, the grid background harmonicsuppression strategy is studied further to upgrade the grid current power quality. The gridbackground harmonic suppression strategies of the LCL filter include theproportion-resonant (PR) controller based harmonic suppression strategy and the gridvoltage feedforward harmonic suppression strategy. As for the PR controller based harmonic suppression strategy, the system is easily led to instability by the PR controllerconsidering the system bandwidth limitation, it cannot be applied to the weak gridcondition. As for the grid voltage feedforward harmonic suppression strategy, it does notaffect the forward channel, and it does not affect the stability of the system. The samplingdelay and control delay are considered in this paper, the grid voltage full feedforwardexpression in discrete domain is derived and the admittance characteristics are analyzed.The grid voltage estimator and the system parameter robustness are investigatedthirdly. Grid voltage is estimated by the grid voltage estimator, substituting theundetectable grid voltage, providing input to the grid voltage feedforward channel. Theprinciple of grid voltage estimator is illustrated, its design is given out, and itsapplicability to weak grid condition is observed. The damping characteristic parameterrobustness, the grid voltage feedforward parameter robustness, and the overall controlsystem parameter robustness are analyzed.Finally, the simulation analysis and experiments are conducted according to thetheoretical analysis results of the overall control system.