| With the change of the global energy structure,the penetration of renewable power generation is increasing year by year.The power converter is playing a more and more critical role in the power system.Virtual Synchronous Machine(VSM)is the grid-friendly power converter that can realize grid-connected and stand-alone operation,which is a significant technology for renewable power generation and distributed generation.VSM inherits the transient electromechanical characteristics of the traditional synchronous machine as well as its oscillating transient characteristics.Improper parameter selection can easily cause power oscillations,so it is necessary to study and improve the transient characteristics of VSM.At present,the research of transient characteristics of VSM faces with many key issues.On the one hand,the VSM has complex and multiple parameters,and the mechanism of the transient process has not been standardized and quantified.On the other hand,as a power electronic converter,the capacity of the VSM to withstand overload is limited.The power oscillation makes it unable to operate safely,stably and efficiently,which is easy to cause substantial economic losses.This disertation focuses on the key transient characteristics of VSM in the future power system application,starts from the modeling and analysis of the transient characteristics,reveals the functional mechanism of the quantitative specification,and lays a theoretical foundation.Then,according to the three dimensions of VSM,which are robustness,inertia and damping,modification methods are proposed to improve transient characteristics and improve the adaptability of parameters,frequency stability and control flexibility of VSM.The main research work is as follows:(1)Focusing on the quantitative description issue of the transient response in VSM,the concept of power loop time constant is proposed,on this bases,a VSM parameter system oriented to transient characteristics is established to reveal the transient process mechanism.The transient response of VSM is determined by several parameters.In order to improve the transient characteristics,the modeling and analysis of VSM are needed to understand its working mechanism.The concept of power loop time constant is proposed based on the synchronous reactance of VSM,and then a VSM parameter system oriented to transient characteristics is developed.The linear model of VSM is used to reveal the effect of synchronous reactance on the response speed of power loops.The concept of power loop time constant is proposed,and the complex transient response mechanism of VSM is summarized into the interaction between synchronous reactance and control parameters.Based on the concept of power loop time constant,a VSM parameter system oriented to transient characteristics is established.Finally,the relationship between synchronous reactance,power loop time constant and transient characteristics of VSM is revealed by using this parameter system.Time domain simulation verifies the correctness of the modeling and analysis results.(2)The VSM robustness improvement method based on feedforward branch is proposed,to solve the issue that the perturbation of dominant parameters in transient response can easily cause the transient impact.Although the control strategy of VSM is superior to the traditional droop control in frequency stability,while it is more likely to produce transient oscillations.The average power calculator(APC)and synchronous reactance are dominant in transient response.This dissertation proposes a VSM robustness improvement method based on feedforward branch.Firstly,the function of APC is revealed,and the influence of synchronous reactance on the transient response is described.Secondly,the controller improvement method of VSM based on feedforward branch is proposed and researched,which reveals the essence of its improved robustness by reducing the order of control system,and the comparison of transient performance between VSM with and without this method investigated.Finally,the simulation and experiment show the effectiveness of the VSM robustness improvement method based on feedforward branch.Using this method,the VSM still maintain high relative stability under the perturbation of APC bandwidth and synchronous reactance,and the transient process is smooth without overshoot.(3)The VSM inertia improvement method based on hold filter is proposed,to solve the issue of frequency stability in the case of load mutation or power failure.When load mutation and fault occurs,VSM needs to provide inertial support for the frequency,thus improve the frequency stability of the power system.However,most of the existing methods simply equivalent the inertia of VSM with the inertia of VSM in math,i.e.,through the swing equation.Hence,the characteristics of the first-order inertia element limit the frequency response in VSM,leading a relatively high rate of change of frequency in the beginning part of the transient process as well as vast deviation range.This dissertation proposes a VSM inertia improvement method based on hold filter.Firstly,the definition and characterization of the inertia in VSM are investigated.The inertia of hold filter is revealed and used to improve the inertial frequency response in the VSM.After the improvement,the VSM would have a linear frequency response curve when the load power changes,leading to a reduced rate of change of frequency,reduced the frequency deviation range,and a much stable inertial frequency support.Finally,simulation and experiment prove the effectiveness of the VSM inertia improvement method based on hold filter.(4)The VSM damping improvement method based on virtual synchronous reactance is proposed,to solve the issue that inertia should be sacrificed inertia to damp the power oscillation.At present,the improvement of VSM damping is carried out at the expense of inertia.VSMs cannot flexibly damp power oscillations while considering the inertial support and lack the degree of freedom for independent damping configuration.This dissertation proposes a VSM damping improvement method based on virtual synchronous reactance.Firstly,the source of damping in VSM is studied,and the dynamics definition of damping of VSM is revealed.Secondly,the internal relations of steady-state droop,inertia and damping configuration of the VSM are explored.Thirdly,a VSM damping improvement method based on virtual synchronous reactance is proposed and researched,revealing the influence of synchronous reactance on the damping of VSM.The virtual damping element is introduced to change the synchronous reactance and reconstruct the damping.Finally,simulation and experiment show that the VSM damping improvement method based on virtual synchronous reactance is effective.To sum up,this dissertation is devoted to the comprehensive analysis and flexible improvement of the transient characteristics in the VSM.Establishing a VSM modeling and analysis method oriented to its transient characteristics,and on this basis,the adaptability,frequency stability and control flexibility of VSM are improved.The experimental prototype of VSM is used to test the proposed improvement methods in order to provide comprehensive theoretical guidance and technical support for the broader application of VSM technology. |
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