Research On Protection Strategy Of SEN Transformer And Analysis Of Its Impact On Line Protectio

As the development of new energy sources such as wind and solar power accelerates,the novel power system is increasingly facing power flow control challenges.Flexible AC Transmission Systems(FACTS)provide an effective solution for optimizing power flow distribution.Among them,the SEN Transformer(ST)is an electromagnetic-based FACTS device offering similar functions of Unified Power Flow Controller(UPFC)and superior economic benefits,making it a promising technology for future applications.Due to its complex topology and the fact that its compensation units are connected in series with the transmission lines,the protection requirements for ST are particularly stringent.Like other FACTS devices,the presence of ST affects the structure and parameter distribution of transmission lines,which may negatively impact line relay protection.Thus,research on ST protection strategies and their influence on transmission line protection is of great importance.This thesis first reviews the research status of ST and its variants in transient or steady-state modeling and power flow control,as well as the research status of FACTS device protection strategies and the impact of FACTS on transmission line relay protection.It then introduces the topology and working principles of ST and develops an equivalent sequence network model based on its structure and the symmetrical component theory.The validity of the model for fault analysis in the transmission line with ST is verified using PSCAD/EMTDC software.Furthermore,the thesis investigates ST differential protection schemes based on the principles of electrical and magnetic balance,laying the groundwork for subsequent ST relay protection studies.To address the challenges posed by magnetizing inrush currents and winding saturation when conventional differential protection is applied to ST,a pilot protection algorithm for ST based on sequence fault components is proposed.This algorithm uses the ratio of sequence voltage and sequence current fault components at both ends of ST as the operating quantity,takes into account the effects of mechanical on-load tap changers and employs an adaptive braking threshold constructed by the pre-fault electrical quantities.Additionally,the thesis discusses the no-load excitation process of ST and adjusts the protection algorithm accordingly.The performance of the protection algorithm is thoroughly analyzed and validated using PSCAD/EMTDC software in a two-terminal system with ST.Lastly,to mitigate the impact of ST on conventional distance protection,the thesis derives the underreaching of conventional distance relays caused by ST voltage amplification factor,sequence equivalent impedance,and phase shift angle based on phase-to-phase and grounding distance relay algorithms and the ST sequence network model.A modified distance protection algorithm based on Phasor Measurement Units(PMUs)and ST sequence network characteristics is proposed.Simulation analysis conducted using PSCAD/EMTDC software on a three-bus system in the presence of ST verifies the correctness of the principle underlying the impact of ST on distance relays and the effectiveness of the modified distance protection algorithm.