Research On Fault Modeling Of Inverter-Interfaced Renewable Energy Sources In Transient Phases

Under the development philosophy of carbon peaking and carbon neutrality,the renewable energy generation increases rapidly,resulting in the performance deterioration,or even the mal-operation of line protections in localized areas.This threatens the safe and stable operation of power systems.The crux of the aforementioned issues is attributed to the differentiated control strategies and diversified control loops of the grid-connected converters.In this case,fault responses of renewable energy sources(RESs)are inconspicuous and show poor regularity,and thus,line protections can hardly distinguish fault characteristics of RESs.Given this,this thesis focuses on fault characteristics of inverter-interfaced RESs(IIRESs)within time scales of line protection operation,and proposes the analytical fault modeling method of IIRESs covering typical control strategies.The outcome of this thesis can provide a theoretical foundation for the design of protection principle.Main contributions are enumerated as follows.To begin with,an analytical model of IIRESs with coupled sequence control is proposed in time scales of line protection operation.On this basis,impacts of nonlinear control responses such as phase-locked loop dynamics and controller saturation on fault characteristics of IIRESs can be comprehensively investigated.Since the reported fault modeling procedure is based on linear approximation and fault stage division,mathematical regularities underlying fault currents of IIRESs considering multiple nonlinear control responses can be elucidated.Simulations and fault recording data are used to validate the proposed model.Then,an analytical model of IIRESs with decoupled sequence control is developed in the considered time scales.The inevitable time delay introduced by the positive and negative sequence components calculation(PNSCC)modes is studied,and the high-order and coupling features of fault responses of IIRESs ascribed to the PNSCC modes are analyzed.Based on the reduced-order approximation,a practical fault modeling method of IIRESs with decoupled sequence control is proposed.The effectiveness of the proposed model is verified by simulation and hardware-in-the-loop experiments.In accordance with the established fault models of IIRESs,root causes for the performance degradation of traditional power-frequency line protections in the presence of IIRESs are revealed.First,the delay of transient responses due to the phase-locked loop dynamics and controller saturation of IIRESs with coupled sequence control.Second,inrush fault currents caused by the PNSCC modes of IIRESs with decoupled sequence control.Hence,an improved transient control strategy is proposed to eliminate impacts of these control links,which can effectively enhance the performance of traditional line protections.