Stability Criterion And Analysis For Three-phase LCL-type Grid-connected Converter

In recent years,along with the maturing of new energy technology,distributed power system has been widely used in renewable energy power generation systems.Since the distributed power systems are usually located at the end of the grid,the power grid generally appears as typical weak grid characteristics.Grid-connected converter is the critical equipment which is used in distributed power system.As the growing use of power electronic devices,the grid equivalent capacity at the grid connection point of the converters is declining,and the weak grid characteristics is becoming more obvious.The interaction between the high grid impedance and the grid-connected converter may cause the operation performance of grid-connected converter to deviate from the design value,which can even threaten to operation safety of power system.Since three-phase grid-connected converter system is a multiple-input-multiple-output system,the general Nyquist stability criterion is ofter applied to assess system stability,but this method has the shortcoming of high computational complexity.Further improvement of system stability criterion is needed.Thus,there are important theoretical significance and practical engineering value to conduct a comprehensive study of stability criterion and analysis for three-phase grid-connected converter.In this thesis,the stability criterion and analysis for three-phase LCL-type grid-connected converter is studied deeply.Firstly,the control strategy of grid-connected converter is studied.With the simulation experiments,the control circuit for grid-connected converter is designed,which includes active damping method and grid voltage feedforward strategy.The high frequency resonance problem caused by LCL filter is solved and the noise immunity of the system is improved by the control strategy.Then,the dq small-signal impedance model of grid-connected converter is derived by using the small-signal linearization method.The model takes into consideration the influence from the dynamic characteristics of phase locked loop.Based on general Nyquist stability criterion,a simplified stability criterion for grid-connected converter system is proposed innovatively.By adopting Gershgorin circle theorem to estimate the characteristic value locus of system return ratio matrix,the computational complexity of traditional general Nyquist criterion is reduced.The proposed simplified stability criterions provides a accurate and simple method to analyze the stability of three-phase grid-connected converter system.Finally,an integrated simulation model of three-phase LCL-type grid-connected converter is built in MATLAB/Simulink simulation software.The correctness of the proposed stability criterions is validated by simulation results.