Study On Large Signal Stability Of VSC-HVDC System Supplying Passive Networks

Under the situation of global energy resources shortage and ecological environment deterioration,the development and utilization of renewable energy has attracted much attention.In China,the renewable energy represented by hydropower,wind power and solar energy and the actual load demand show the characteristics of reverse distribution,which constitutes the basic pattern of "West-East Electricity Transmission" and "North-South Electricity Transmission ".In recent years,with the advantages of no commutation failure,low output harmonic and being able to supply power to passive network,VSC-HVDC has been widely used in the centralized development of renewable energy and trans regional large capacity transmission projects,and developed rapidly in China.With the operation of several VSC-HVDC projects,the stability problems related to them have become increasingly prominent,which has become one of the hotspots of academic research.However,up to now,the research on the stability of VSC-HVDC System is mainly based on the small signal linear model and linear theory,while the large-signal stability analysis of VSC-HVDC system is lack of suitable methods and systematic analysis.In response to above issues,in this thesis,the large-signal stability analysis method based on Brayton–Mosers mixed potential theorem and the large-signal stability analysis method based on TS fuzzy model to estimate the domain of attraction are studied;in order to enhance the large-signal stability of VSC-HVDC system,the method of optimizing system parameters and adding virtual resistance / capacitance based on the large-signal stability criterion are studied;a method of guiding VSC-HVDC system to operate stably based on the domain of attraction is proposed.The main research contents and achievements are as follows:(1)Based on the eigenvalues,the small-signal stability of VSC-HVDC system is analyzed;the effective range of small signal stability analysis is defined;and the instability problems and phenomena of VSC-HVDC system which is small sigal stable under large signal disturbance are explored.The results show that small signal stability analysis alone can not ensure the stability of the system,but also needs large signal stability analysis.(2)Based on Brayton–Mosers mixed potential theorem,a large signal stability analysis method for VSC-HVDC system is proposed.On the basis of the average model,through the establishment of mixed potential function and using the theory of mixed potential theorem,a large signal stability criterion of VSC-HVDC system is derived.Based on this criterion,the large-signal stability working area of the system is pointed out,and the influence of system parameters on large signal stability is studied.It lays a theoretical foundation for the method of enhancing the large signal stability of VSC-HVDC system.Through the theoretical analysis and simulation verification of some cases,the results show that this method can effectively analyze the large signal stability of VSC-HVDC system.(3)In order to show the tolerance of VSC-HVDC system to the magnitude of disturbance signal in a more direct and intuitive way,a large signal stability analysis method of VSC-HVDC system based on TS fuzzy model to estimate the domain of attraction is proposed.Based on the state space average model of VSC-HVDC system,the TS fuzzy model is constructed,and the attraction domain is solved by linear matrix inequality.The result shows the tolerance degree of VSC-HVDC system to disturbance signal in the form of state space convex polyhedron.Through the theoretical analysis and simulation verification of the case,the results show that when the VSC-HVDC system suffers from large signal disturbance and the working point position changes,as long as the initial point of the system is in the domain of attraction,the system can safely go through the transient process and finally run stably at the new working point.The conclusion is the theoretical basis for guiding VSC-HVDC operation to maintain stability based on the domain of attraction.(4)In order to enhance the large signal stability of VSC-HVDC system,a method of setting system parameters based on the derived stability criterion is proposed to make the system meet the criterion conditions and realize the large signal stability.In view of the problem that the VSC-HVDC system which has been put into operation is difficult to realize the system main circuit parameter setting by replacing the hardware equipment,a method of adding a control loop to the controller to realize the virtual resistance / capacitance which can change the main circuit parameters of the system equivalently,and can also achieve the effect of enhancing the stability;for the case that the system parameters can not be changed,a method of guiding the operation of VSC-HVDC based on the domain of attraction is proposed,which ensures that the system state track is always in the domain of attraction,and achieves the goal of maintaining the stable operation of the system under the large signal disturbance.Through the theoretical analysis and simulation of the case,the results show that the three methods are effective and can achieve stable operation of the original large signal unstable system.