| Under the background of "emission peak and carbon neutrality",distribution network has become the core link of "new power system" and "energy Internet" construction.With the massive development and utilization of new energy,grid-connected converters,as the hub device between new energy and p ower grid,take an increasing proportion in the distribution network year by year.The grid-connected converters have rapid response and strong controllability,which provide important technical support for t he regulation and compensation of electrical parameters of the distribution network.The development of grid-connected converters greatly promotes the high efficiency,flexibility and intelligence of distribution network operation in our country,and the development of relevant key technologies is of great research value.There are many control types of grid-connected converters,which can be mainly divided into two categories: grid-forming converters and grid-following converters.Virtual Synchronous Generator(VSG),as a typical representative of grid-forming converters,has similar damping and inertia l characteristics as traditional synchronous generator and can provide voltage and frequency support for the distribution network.The stable operation of VSG is of great significance for the high-quality,efficient,safe and reliable development of distribution network.This paper takes VSG as the object to carry out relevant research.There are many mature studies on the analysis and control o f VSG under steady conditions,but the transient characteristics and control methods of VSG under fault conditions,especially the whole fault p rocess,are rarely involved.At the same time,the distribution network is at the end of the power system,with many levels of voltage droop,many points and aspects,and many types of frequent faults.The transient operation of VSG under fault conditions cannot be ignored.With the support of "Analysis and Countermeasures of Transient Interaction Between Multiple Micro-sources in Microgrid(51907057)" and "Multi-functional multiplexing methods for grid-connected converters towards active cont rol of active distribution network faults(52077072)",this article starts from VSG single system operation mechanism,to carry out research of the transient characteristics and the active control method of VSG under the typical scenario.Based on this,a series of key technologies and active control methods for VSG parallel systems are carried out to help the secure and reliable gird connection of new energy.The main work and innovations are as follows:(1)The transient output characteristics and transient instability mechanism of VSG in the whole symmetric fault process are emphatically analyzed,and the influence laws of key parameters on the transient running trajectory of VSG are summarized.The operation performance of VSG is affected under fault conditions.Analyzing the transient characteristics and instability mechanism of VSG under fault conditions is the basis of transient regulation.Firstly,the basic control principle of VSG is introduced.Secondly,the mathematical principle of equal area criterion(EAC)is described from the perspective of energy balance.Then,the effects of key parameters such as damping coefficient,moment of inertia,Q-U droop coefficient,active power command value and line impedance on VSG transient characteristics ar e deeply analyzed,and the influence laws of different key parameters on the transient running trajectory of VSG are summarized.(2)A stability interval analysis method considering the transient characteristics of VSG in the whole fault process is proposed,which provides a theoretical basis for the active control of VSG in the whole fault process.The transient stability performance of VSG after fault clearance is directly related to whether VSG crosses the transient stability interval during fault.Therefore,the analysis of the transient stability interval of VSG can provide guidance for relay protection.Firstly,CCT and CCA are quantitatively described based on the equal area criterion and with the help of Runge-Kutta algorithm and least square method.Considering the transient characteristics of VSG in the whole fault process,the transient stability interval of VSG is analyzed.Secondly,based on clarifying the transient characteristics of VSG output current in the whole fault process,this paper fully considers the influence of fault current limitation to revise the VSG transient stability interval.The analysis results indicate that the fault current limitation link wound significantly reduce the transient stability performance of VSG,and the quantitative calculation results can provide reference for the relay protection operatio n time.(3)An active control method of VSG in the whole remote and weak impact fault process is proposed and the transient stability performance of VSG is effectively improved.Control parameters have a significant impact on the VSG transient stability interval.Considering the advantages of VSG control flexibili ty and strong controllability,VSG control parameters are regulated during the whole fault process,which provides a way to improve the performance of VSG transient stability.Firstly,based on the traditional equal area criterion,a more universal VSG equ al proportional area criterion(EPAC)is proposed to guide the flexible control of moment of inertia.Secondly,combined with the influence laws of control parameters on VSG transient stability interval,an active control strategy of VSG in the whole fault process is proposed.Finally,considering multiple constraints,a VSG control parameter setting method during the whole fault process is proposed.The proposed active control method can realize simultaneous optimization of CCA and CCT,enhance the power angle stability of VSG and obtain enough time margin for the relay protection devices,and significantly improve the transient performance of VSG.(4)An active control method of VSG in the whole proximal and strong impact fault process is proposed and the fault current is regulated while VSG power angle stability is guaranteed.Under proximal and strong impact fault,VSG faces the risk of both power angle instability and fault overcurrent.Starting from the fundamental cause of VSG power angle instability and fault overcurrent,a comprehensive transient stability enhancement(CTSE)control strategy of a VSG considering power angle stability and fault current limitation is proposed.Firstly,a transient power angle control method is proposed to ensure that VSG transient power angle does not shift during faults.Secondly,the Q-U control loop is optimized to achieve effective control of VSG fault current steady state component.Then,the current control inner loop is optimized to further suppress VSG fault current impact component.The proposed CTSE control strategy can ensure the VSG power angle stability and effectively regulate the fault current,which ensures the safe and reliable operation of VSG.(5)The active control method of VSG parallel system in the whole fault process is proposed to ensure the safe operation of different types of VSG parallel system under fault conditions.In active distribution ne twork,multiple converters often exist in the form of direct parallel connection or indirect interconnection.Compared with the single-machine infinite system,the transient characteristics of the multi-converter parallel system will be more complex,and t he difficulty of transient regulation will be higher.Firstly,the homogeneity of VSG parallel system is judged,and the equivalent modeling of the homogeneity VSG parallel system and the non-homogeneity VSG parallel system is further achieved.Secondly,the transient characteristics of the homogenous VSG parallel system and the non-homogenous VSG parallel s ystem are analyzed,which lays a solid theoretical foundation for the transient control.Then,through theoretical analysis,it is proved that the correlation transient control method of single VSG system is also suitable for the VSG parallel system.Final ly,the traditional mode switch-based fault traversal method is optimized,and a n active fault traversal method suitable for non-homologous VSG parallel systems is proposed.The proposed method fully draws on the transient control method of a single VSG grid-connected system,meets the actual demand under the background of large-scale grid-connected new energy,and can effectively assist the reliable grid-connected new energy power generation devices.According to the requirements of active power distributi on in safe,reliable,efficient,high quality,this paper takes VSG,a typical representative of grid-forming converters,as the research object,focusing on the study of transi ent characteristics and active control in the whole symmetric fault process,an d focuses on several key problems existing in the transient interaction between VSG single syste m and VSG parallel system and power grid.Systematic research is carried out in t he aspects of characteristic description,mechanism analysis,parameter design and control optimization.The research results can provide useful reference for other technology research and transient control of grid-forming current devices. |
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