| With the rapid development of clean and low-carbon transformation of energy,wind turbines have developed rapidly due to their efficient use of wind energy.Permanent magnet direct-drive synchronous generators with simple structure and high operating efficiency are one of the mainstream models.The grid connection of a large number of wind turbines has also brought some challenges to the power system.my country’s power sector has issued relevant standards for wind power grid connection technology,requiring the wind power system to take corresponding reactive power compensation measures during grid failures to reduce wind turbines.network risk.In this context,this paper studies the voltage support control strategy for the point of common coupling(PCC)of permanent magnet direct-drive fans during grid voltage symmetric and asymmetric sags.First,during the symmetrical drop of grid voltage,the mathematical models of the generator-side and grid-side converters are derived,and their control strategies are analyzed respectively.On this basis,the corresponding control block diagram is obtained;when the grid voltage is asymmetric During the sag,based on the instantaneous symmetrical component method and coordinate transformation,the mathematical model of the grid-side converter is deduced,and the reason why the active power exhibits double-frequency fluctuations is analyzed.The above derivation and analysis provide theoretical support for the PCC voltage support strategy.Secondly,during the symmetrical drop of grid voltage,three traditional control strategies using grid-side converters to provide reactive power to support grid voltage are analyzed,and their shortcomings are analyzed and improved.The effective circuit diagram,phasor diagram and mathematical formula are derived,and the grid impedance characteristics and the rated capacity of the grid-side converter are comprehensively considered.On the basis of this,an active support strategy for PCC voltage of permanent magnet direct-drive fan under grid symmetrical fault is proposed.Thirdly,during the asymmetrical drop of the grid voltage,by analyzing the active output of the wind power generation system,the expression of each sequence component of the grid current that realizes the double frequency suppression of the active power is obtained.The support principle of PCC voltage,on this basis,take supporting the PCC voltage as the control target,the maximum phase amplitude of the output current of the grid-side converter is the constraint,construct the Lagrangian function,and consider the suppression of the double frequency fluctuation of the active power at the same time,the optimal distribution relationship of positive and negative sequence active and reactive currents is obtained,and the optimal PCC voltage active support control strategy is obtained.Finally,based on the Matlab/Simulink simulation platform,the effectiveness of the PCC voltage support strategy is verified during the symmetrical and asymmetrical faults of the power grid,and the low voltage ride-through capability of the permanent magnet direct-drive fan is improved. |
