Mode Analysis Arnd Coherency Identification For Low Inertia Power System

With the integration of more and more inverter-based renewable energy sources and HVDC,inertia of today’s power system decreases and varies as the operation and control changed.This leads to a low and time-varied inertia power system which is more sensitive to the disturbance.The oscillation frequency and modality are changing,which impact generator coherence.In order to research the stability of power system,this paper aims at discussing the oscillation mode and generator coherence as the inertia of generators varying.The main works is organized as follows.Firstly,this paper reviews the research status and significance of generator coherence,especially the slow coherence in details.Based on traditional generator coherency identification method,a flexible identification method achieving optimal grouping is proposed,which provides an important analysis tools for slow coherence study of generator.The key to analyzing oscillation mode with variation of inertia is the re-calculation of eigenvalues and eigenvectors of state matrix,which is complicated and expensive.To simplify the calculation,this paper introduces the theory of matrix perturbation to estimate the oscillation frequency and modality.The matrix perturbation is classified into two section to resolve distinct mode and resonance mode respectively.For distinct mode,the paper deduces the first order distinct eigenvalue perturbation and second order distinct eigenvalue perturbation,and discusses the accuracy problems.For resonance mode,the paper discusses the resonance and the reasons distinct eigenvalue perturbation is not applied to analyzing resonance mode.Then,the first order close eigenvalue perturbation is introduced to estimate the eigenvalues and eigenvectors of resonance mode with variation of inertia.Based on mode analysis for low inertia power system,this paper discusses the impact of variation of inertia on generator coherence.Due to the limitation of validity of matrix perturbation,the paper refers to sectionalized linearization and proposes a method to identify coherency generators with large scale perturbation of the inertias through subsection of perturbation.Finally,to verify the validity of this method,three cases including off-resonance,resonance and modality transition in IEEE-118 bus system are analyzed.In summary,this paper researches the oscillation mode and generator coherence in low inertia power system in-depth.According to traditional generator coherence identification method,the coherency degree of generators and evaluation of grouping are considered.To analyze oscillation mode with variation of inertia,this paper introduces the theory of matrix perturbation including distinct eigenvalue perturbation and close eigenvalue perturbation,formulating a method of mode analysis and generator grouping.The works in this paper provide basement for stability study for low inertia power system,and offers reference for parameter perturbation problem.