Analysis And Research On Sub-synchronous Oscillation In Grid-connected System Of Permanent Magnet Direct Drive Fan

With the wide application of new energy and DC engineering,the characteristics of electric power system are becoming more and more obvious.With the rapid development of fan,large capacity and high proportion of renewable energy are connected to power grid,and a large number of direct current(DC)transmission is connected to power network,which forms a new situation of "strong and weak AC" power system.Sub-synchronous oscillations occur frequently,resulting in serious consequences.At present,the mechanism of sub-synchronous oscillation has not been thoroughly studied and a unified understanding has not been formed.Previous studies have concluded that sub-synchronous oscillation will not occur in direct-drive wind turbines.The sub-synchronous oscillation of direct-drive fan in Hami area of Xinjiang has broken the existing knowledge and belongs to a new type of sub-synchronous oscillation.It has become a research hotspot in the field of power system for a time.The research on this aspect has engineering and theoretical value.Firstly,this paper starts with the modeling and simulation of the grid-connected system of permanent magnet direct-drive fan based on RTDS(real time digital simulator,(a real-time digital simulation software),and on the basis of mastering the basic principle of permanent magnet direct-drive wind power generation system,The simulation model of fan grid-connected system is established in RTDS,and the correctness of the model is verified by steady-state time-domain simulation,which lays the foundation for the later research and provides reference for related scholars.Then the problem of sub-synchronous oscillation is studied theoretically by mathematical analytical method.Impedance method is a new research method of blower sub-synchronous oscillation in recent years.The impedance can be used to determine whether there is an unstable frequency of sub-synchronous oscillation in the system.In this paper,based on the complete nonlinear equations of the whole system,the dynamics of the phase-locked loop in the controller and the function of the complete structure of the inner and outer loop are considered.The abc three-phase coordinate system is transformed into the dq coordinate system by Park transformation,and the system is linearized at the steady operating point of the system.The time-domain linearization equation is established and the expression of port impedance is obtained by converting Laplace transform to complex frequency domain and eliminating intermediate variables.Then,the linearization method is not feasible in practice because the controller parameters are unknown in practice,and the impedance can only be obtained by measuring the method.In this paper,the basic principle of frequency sweep is described,and the impedance frequency characteristic(also known as Bode diagram)is measured in RTDS simulation.The results are in good agreement with the Bode diagram derived by linearization in the previous chapter.It not only verifies the feasibility of the method and the accuracy of the sweep results,but also verifies the accuracy of the theoretical derivation in the previous chapter.In view of the disadvantages of safety and economy in the traditional frequency sweep,a new method of adding disturbance to the controller is proposed,the principle of the second side sweep is expounded,and the linearization equation is deduced again.The results of frequency sweep are compared with the traditional method of adding disturbance to the primary side.The results show that the method of adding disturbance to the controller not only has the advantages of safety and economy,but also has higher measurement accuracy.Finally,the obtained impedance is analyzed by the generalized Nquist criterion,and the Nyquist criterion is used to obtain the conclusions of stability,small stability margin and instability for three groups of different controller parameters.The simulation waveform of RTDS in time domain can be seen.The three sets of controller parameters correspond to stability,critical stability and instability,respectively,which verify the feasibility and accuracy of the generalized Nyquist criterion used in the dq impedance analysis of sub-synchronous oscillation of fan grid-connected system.