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Research On Control Strategy Optimization And Parallel Power Sharing Of Virtual Synchronous Generator

Posted on:2020-08-26Degree:MasterType:Thesis
Country:ChinaCandidate:S Y LuoFull Text:PDF
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With more and more distributed generation(DG)systems connecting into the power grid,the stability of power grid is facing challenges when the grid-connected inverters are still using the traditional control strategies.The technology of virtual synchronous generator(VSG)helps inverters mimic the operation of synchronous generators(SG),which can enhance the compatibility between DG systems and power gird and thus improve the permeability of renewable energy in power grid.Meanwhile,since the capacity of electrical appliances is increasing,one single inverter can hardly meet the demand of the power system.Therefore,it is vital and meaningful to research on the control strategy of VSG and the process as well as the results of power sharing when VSGs in parallel operation mode.This paper focuses on the active power loop of VSG.In order to hence the disturbance rejection performance of the system and also obtain the rules to choose key parameters,an optimal control strategy of VSG is proposed.Based on this optimal strategy,the parallel operation of VSG has been researched further.The method about how to set key parameters of VSGs is presented.With this method,the power sharing process is quick without any overshoot and the result of the power sharing is matched with the capacity of VSG.The experimental platform has been built.The results of experiments of both stand-alone(SA)mode and grid-connected(GC)mode proved the effectiveness of the proposed optimal strategy.The paper analyzed three traditional control strategies of inverters and pointed out the disadvantages of them.From the aspects of control structure and the design bases of key parameters,the development history,the classical control strategies and research hotpots of VSG are summarized.The design bases of key parameters and how to set them when VSG with different capacities operate in parallel deserve further researches.To further investigate these two matters,the paper firstly constructed the classical second-order model of SG and analyzed the working principles of SG.By pointing out the similarities of SG and inverters,the model of VSG is established.The control principle of power loop is analyzed in detail and the design process of inner loop is given.The VSG model is built in MATLAB/Simulink to simulate VSG working in both SA mode and GC mode.Secondly,by using bode diagram,two different damping expressions are compared and the one has better disturbance rejection performance is selected.Based on the selected damping expression,the equivalence of damping coefficient and frequency modulation coefficient are pointed out.Therefore,the control strategy of active power loop is optimized and the number of key parameters has been reduced from three to two.Based on the optimal strategy,the small signal model of active power loop is built,with which the influences of key parameters on the frequency response in SA mode and the power response in GC mode are analyzed.Moreover,combining with the standards of power grid and the analyses,the design bases of moment of inertia and frequency modulation coefficient are obtained.Also,the small signal of droop control is built to compare with the optimal strategy of VSG.Thirdly,based on the proposed control strategy,aiming at the parallel operation of multiple VSGs with different capacities,the influences of key parameters and line impedance on the allocation of active power and reactive power are analyzed.Especially,the dynamic process of the active power allocation is taken into consideration.In order to allocate the power quickly and properly,the matching principles of the key parameters and line impedance for VSGs with different capacities are concluded.By building the simulation models and discussing the simulation results,the rationality of the proposed matching principles is proved.Finally,a three-phase inverter experimental platform has been established in order to carry out the experiments.Different operating conditions such as GC mode,switching from GC mode to SA mode,and SA mode with resistive load are all tested.The results show that with optimal strategy the VSG can work well in all situations above.In terms of the frequency responses caused by a sudden increase load in SA mode,the results of droop control and VSG are compared.The test results verify the correctness of analyses and simulation.
Keywords/Search Tags:Virtual Synchronous Generator, Active Power Loop, Optimal Control, Small Signal Model, Parallel Power Sharing
PDF Full Text Request
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