Research On Control Method Of Microgrid Frequency Stability Based On VSG

With the large number of traditional micro-grids connected to the grid,it will cause a reduction in the inertia and damping of the power system and pose a potential threat to the dynamic response and stability of the power system.To solve this problem,the virtual synchronous generator(VSG)is applied by letting the inverter simulate the external output characteristics of the synchronous generator.Frequency is an important standard for the power quality of microgrids,and it is of great significance to maintain the frequency stability of virtual synchronous generators.However,the stable decrease of the system frequency will be caused by the switching of the load and the switch from the island to the grid.And in island systems that are only powered by VSG,some VSGs that exit early due to the low state of charge of some VSGs may result in insufficient maximum output power of the VSG unit and failure of secondary frequency modulation under islands.The stability of the system frequency is also threatened by this problem.In order to maintain the high stability of the frequency of the virtual synchronous generator,the three problems mentioned above that cause the frequency to drop steadily are studied in this thesis.First,by establishing a small signal model of the VSG single-machine grid-connected grid,the effects of moment of inertia and damping coefficient on system stability and power are analyzed by using damping ratio and characteristic root locus.The effects of rotational inertia and damping coefficient on the frequency in transient state are also analyzed.According to the analysis results,a two-parameter fuzzy adaptive control method is proposed.Compared with the traditional virtual synchronous generator control,this method found that under the same load disturbance,the frequency overshoot was significantly reduced,and the system dynamic frequency response was improved.Then,by using frequent deviation-free adjustment conditions obtained by analyzing the principle of secondary frequency modulation of the synchronous generator,the island-based secondary regulation link based on VSG is established.It can maintain the stability of voltage and frequency.Then the principle of traditional power distribution is analyzed.And on the basis of traditional power distribution,an equal state-of-charge power distribution method is proposed.Compared with the traditional method of power distribution according to the rated capacity,this method can not only dynamically adjust the power distribution between the VSGs when the SOC is inconsistent,so that the SOC gradually becomes consistent,but also when the SOC is consistent,each VSG is still allocated according to the rated capacity.And it also avoids the problem that the SOC of some VSGs is too low,causing the VSG to withdraw from operation early,which may cause the system to fail the second frequency modulation.Secondly,in order to maintain the frequency stability during the off-grid process based on the VSG microgrid,this thesis analyzes the principle of the phase-locked loop(PLL)-based pre-synchronization method and finds that the method is affected by the accuracy of the PLL.To solve this problem,a pre-synchronization control method based on coordinate transformation without phase-locked loop is proposed in this thesis.Compared with the PLL-based pre-synchronization control method,this method gets rid of the dependence on the PLL,avoids the deterioration of the pre-synchronization effect caused by the low accuracy of the PLL,and reduces the frequency disturbance during the system grid connection process.However,compared with the PLL-based pre-synchronization method,this method has the problem that the secondary frequency modulation method cannot directly be switched to the pre-synchronization control method based on coordinate transformation without phase-locked loop.To this end,a set of switching methods is designed in this thesis to achieve switching between the two methods without causing frequency fluctuations.Finally,the effectiveness of the above methods was verified on MATLAB/Simulink.