Research On Islanding Operation And Smooth Grid-connected Control Strategy Of Grid-forming Converters

The high proportion of power electronic equipment connected to the power grid has become the trend of modern power system development,and the great difference in physical structure from traditional synchronous generators also affects the operation stability of the power system to a certain extent.In order to solve the lack of inertia caused by the connection of power electronic converters to the grid,the research on converter control has gradually shifted from grid-following control to grid-forming control.The grid-forming control can actively establish the voltage and frequency of the system,and can flexibly operate in grid-connected mode or island mode.When the grid-forming converter operates in the island mode,it must ensure the stability of frequency and voltage,as well as good dynamic response characteristics.In addition,when switching from island mode to grid-connected mode,it is necessary to achieve a smooth transition as much as possible,reduce the impulse response of voltage and current during the switching process,and provide good power quality for the system.To this end,the research contents of this paper are as follows:(1)This paper firstly analyzes the AC grid topology including photovoltaic and hybrid energy storage,establishes mathematical models for photovoltaic arrays,batteries and super-capacitors,and analyzes their working principles and characteristics.Secondly,a mathematical model is established for the grid-forming converter,the voltage and current control equations of the converter are deduced in detail,and the control principle of the grid-forming converter based on virtual synchronous generator(VSG)is analyzed from the two aspects of frequency and voltage control.A(2)A hybrid energy storage island operation control based on grid-forming converters is proposed,which can always ensure the stability of frequency and voltage,and can use the working characteristics of batteries and super capacitors to synergize output during load switching.First of all,the control of the grid-forming converter at the battery end is improved.The integral term is added on the basis of the primary frequency modulation,and the frequency offset is introduced to adjust the integral coefficient adaptively.Combined with the automatic excitation and voltage regulation strategy,the frequency and voltage of the islanded system can be adjusted without difference,and it has better frequency response characteristics.Secondly,flexibly adjust the moment of inertia of the converter at the super-capacitor end,increase the moment of inertia at the moment of load switching to increase the output,and gradually reduce the output after the load change becomes stable.This takes full advantage of the fast response characteristics of super-capacitors to provide short-term power to the system.Finally,the effectiveness of the proposed control strategy is verified by Matlab/Simulink simulation.(3)A smooth grid-connection strategy based on adaptive model predictive control is proposed,which can have better frequency response in the pre-synchronization process,and effectively reduce the voltage and current impact at the moment of islanding transfer to the grid.The pre-synchronization control of frequency and phase angle is carried out by using the characteristics of faster response and higher precision of model predictive control,and the weight coefficient of frequency in the objective function is adaptively optimized by introducing frequency offset.When the frequency offset is large,the weight coefficient also increases,which speeds up the frequency recovery speed.When the frequency offset is small,the weight coefficient decreases accordingly,which speeds up the recovery speed of the phase angle.This makes it have better dynamic characteristics and improves the power quality of the system.Finally,the effectiveness and superiority of the proposed smooth grid-connected strategy are verified through hardware-in-the-loop experiments.