UHVDC transmission project will adopt hierarchical connection mode at its receiving end with its high and low converters integrated to 500 kV and 1000 kV A C grids respectively on inverter side. The short-circu it fau lt at AC line of hierarchical connected UHVDC transmission system is a serious threat to the safe and stable operation of the system. In this paper, the equivalent model of UHVDC h ierarch ical connection to AC Grid is proposed. The dynamic phasor theory is used to analyse the

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... t characteristics of UHVDC system equivalent fundamental frequency current. When the AC fau lt occurs at the distal of the converter station, the UHVDC system equivalent power frequency current shows a decreasing trend; When the AC fau lt occurs at the pro ximal of the converter station, a high current occurs in the UHVDC system equivalent fundamental frequency current. RTDS model simu lation verified that the theoretical analysis result is correct and established foundation to the subsequent protection impact study.