Fault Analysis Of Grid-connected Voltage Source Converter Under Grid Short Circuit Fault

With the development of power electronics dominated large power systems,power electronics devices have been displacing more traditional electromagnetic devices in power systems.The grid-connected voltage source converter(VSC)is used as the energy exchange interface between the various power electronics devices and power grid.The fault characteristics analysis of VSC is of great significance for clarifying the dynamic behavior of modern power systems and guaranteeing its safety,stability and high quality operation.In this thesis,the influence of multi-scale high-order coupling and transient switching characteristics,nonlinear elements and different transient events on the fault characteristics of VSC are investigated.Considering the change of VSC's control structure resulted by transient switching,the transient model of VSC under severe grid fault and non-severe grid fault are established respectively.Based on the models,the fault characteristics of fault current and VSC internal voltage under different transient events and the crucial factors related to the fault characteristic are explored in the single VSC infinite bus system.The details are as follows:(1)The multi-time scale control and transient switch of VSC are introduced.Besides,the original control structure of VSC under different fault depths are summarized.And then the overall framework of the fault characteristics analysis of VSC is defined and the main works and challenges of VSC fault characteristics analysis are elaborated and analyzed.(2)The transient models for analyzing fault characteristics of VSC under severe grid fault and non-severe grid fault are established respectively.Under severe grid fault,the influence of phase-locked loop(PLL)and polar coordinate transformation between dq rotating reference frame and amplitude/frequency frame on the fault characteristics of VSC are the main concerns.The VSC transient model under severe grid fault is composed of a nonlinear subsystem and linear subsystem.Under non-severe grid fault,the fault characteristics of VSC are mainly determined by the cascade control of the outer voltage loop and inner current loop.Learning from multi-time scale theory,the transient model of VSC superimposed by a voltage outer loop subsystem and a current inner loop subsystem is established.The transient models are validated in Matlab/Simulink.(3)Based on the VSC transient model under severe grid fault,the analytic formula of VSC fault current with the consideration of PLL dynamics is derived.According to the analytical formula,the factors affecting the transient characteristics of fault current under short-term fault and long-term fault are analyzed.What's more,the influence of polar coordinate transformation on the fault characteristics of VSC internal voltage's amplitude and frequency are considered in detail.Based on Hilbret-Huang transform time-frequency analysis method,the aperiodic time-varying characteristics of fault current and amplitude/frequency of VSC internal voltage are founded and summarized.(4)Based on the VSC transient model under non-severe grid fault,the analytic formula of fault current and internal voltage's amplitude/frequency are derived in a single VSC infinite bus system.Then the effects of voltage and current controller parameters,current filter and depth of grid drops on the oscillation frequency and attenuation factor of fault current are discussed in details.Finally,the periodic time-varying and multi-scale characteristics of VSC fault current,internal voltage in dq rotating reference frame and amplitude/frequency frame are summarized.