Research On Dynamic Interaction Mechanism And Suppression Of High-proportion Power Electronics Grid Connected Interface

Most of the volatile renewable energy sources transmit energy to the AC grid through the power electronics grid-connected interface?PEGCI?.With the increasing scale of new energy grid-connected power generation,the PEGCI system is becoming high proportion.However,a high proportion system exhibits large dynamic interactions,both internal interaction to the control/component within the device itself and external interaction to the device and the grid.At the same time,the extrinsic nature of interaction is often expressed in the dynamic process of oscillation,which profoundly changes the dynamic behavior of the power system and constrains the development of renewable energy generation technologies.For the oscillation problem of high proportion PEGCI system,in this paper,the dynamic interaction mechanism and its characteristics that may cause oscillations are investigated from two perspectives:eigenvalue analysis and transfer function analysis,respectively,and corresponding negative damping oscillation and forced oscillation suppression strategies are targeted proposed.Firstly,a state space model without adding a virtual resistance at the point of common coupling?PCC?is proposed.The accuracy and applicability of the model are verified by comparing the state space model with a detailed time-domain one.In addition,the construction of the Norton equivalence circuit yields a centralized equivalence model describing the normalization of multiple PEGCIs to a single state space model,which simplifies the modeling of multi-machine systems.Secondly,based on the centralized equivalence model,the dominant oscillation mode of the system was determined using eigenvalue analysis.It was found that the negative damping oscillation of the system is closely related to the dynamic interaction of the grid impedance,LCL circuit,current loop and phase-locked loop?PLL?.The asymmetric control of the system dq-axis caused by the PLL has a strong effect on the damping of the dominant oscillation mode and gradually increases with increasing bandwidth of the PLL or with increasing number of PEGCIs.Furthermore,for the effect of continuous periodic small perturbations,the mechanism and general nature of forced oscillation are analyzed in combination with the dominant oscillation mode.It is pointed out that the possibility of large resonant type forced oscillations occurring even if the damping is positive.Analysis shows that excising or avoiding the disturbance source can eliminate or suppress forced oscillations.Then,for the negative damping oscillations,the effect of the control delay on the high-frequency resonance characteristics of the system was further analyzed in conjunction with the system's transfer function.The stable region of the LCL resonance frequencies for the converter-side and grid-side current feedback control were obtained respectively.In the stable region,the positive damping characteristics are guaranteed only by the inherent damping of the system,without requiring additional external damping.Based on the damping characteristics of the stable region,a robust negative damping oscillation suppression strategy with LCL resonance frequency design as the core is proposed using weighted geometric averaging method,considering the effects of grid impedance,control delay and filter inductance varies.For the forced oscillations,the significant effects of two harmonic interaction phenomena,asymmetric dq control type and multi-frequency modulation type,on the grid current spectrum are further analyzed,and sub-stable region criterion for suppressing forced oscillations are proposed.On this basis,the hardware parameter design flow for each element of the LCL filter was established based on the two resonance frequencies of LCL and L₁C for oscillation suppression.Finally,the RT-LAB-based hardware in the loop?HIL?semi-physical platform was constructed and used to experimentally verify the mechanism,characteristics and suppression strategies of negative damping and forced oscillation,respectively.