| In recent years,the challenges of power-flow management in electrical transmission grids have increased significantly due to the increasing penetration of renewable energy and the rise of electricity demand.The power flow controller is one of the most effective ways to change power flow distribution in the grid.Two-Core Extended SEN Transformer(TCEST)is a kind of novel electromagnetic power flow controller that can provide a higher control accuracy and a large operational range than the conventional SEN Transformer(ST).Compared with the Unified Power Flow Controller(UPFC),TCEST has a better economy and higher reliability with similar power flow control capabilities.Therefore,it has great significance to carry out the research on modeling and control systems for TCEST.Firstly,the development of the ST and its variants at home and abroad are reviewed,and the risks and the shortcomings of the ST for applications in the extra high voltage or ultra-high voltage(EHV/UHV)power grids are discussed briefly.Inspired by the Two-core phase shift transformer,the Two-core ST(TCST)is introduced as a potential application form of ST in the UHV/EHV grid.Then,the application of the electronic on-load tap changer(EOLTC)in the transformer and the phase shifter is sorted out,and the TCEST is proposed via the combination of the TCST and the EOLTC.In addition,the research status of research on the steady-state and transient models and control systems of ST are summarized.Secondly,the working principle basic structure of the TCEST is introduced,and the differences between the TCEST and the conventional ST in the operating domain and the control accuracy are analyzed.Further,the switching process of EOLTC in TCEST is briefly analyzed,and the safe switching method of the TCEST is explained.Then,the technical feasibility and economics of TCEST are analyzed.In addition,the basic modeling steps of TCEST are illustrated.Thirdly,based on the Unified Magnetic Equivalent Circuit(UMEC),an electromagnetic transient model of the TCEST is proposed.The influence of core structure on flux and the hysteresis and eddy current effects of the core are considered,and the reverse recovery process of the thyristor is modeled based on the turn-off model of the thyristor.The validity of the proposed electromagnetic transient model is verified in a power system containing the TCEST with the help of MATLAB,PSCAD/EMTDC,and ANSYS/Simplorer,and the excitation transient process,the steady-state power flow distribution,and the transient overvoltage due to steady-state magnetizing current switching of TCEST are simulated and analyzed.Finally,according to the working principle of TCEST,a control system of the TCEST is designed,which can effectively follow the change of the control target and change the compensating voltage quickly.The feasibility of the designed control system is verified in a power system containing TCEST with the help of MATLAB/Simulink.In addition,a performance comparison between the TCEST and the Single-core ST(SCST)is presented to further explains the difference between the TCEST and the SCST.The power flow regulation ability of the TCEST and the UPFC is also compared in a modified IEEE 2 machine 5 node system,and the differences and advantages of TCEST compared with UPFC are analyzed. |
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