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Owing to more fault types in the of two bipole lines (TBL) rather than single circuit line, and there is mutual influence between the two DC circuits on the same tower. Furthermore, being susceptible to the grounded fault loop impedance, the traditional traveling wave protection cannot satisfy the demand of reliability and sensitivity on the dc line protection of fault pole selection for two ±500kV HVDC systems on the same tower. So, the fault analysis with the phase-mode transformation (PMT) method for double-circuit transmission lines on the same tower is discussed. Considering a more practical case, the mutual influence of electromagnetic coupling between the double HVDC circuits is discussed firstly. A PMT method of TBL on the same tower is proposed to decouple the phase (or named pole) network equation and build the four fault modal (one ground-mode and three line-modes) networks. After that, the theoretic analysis is discussed on different dc line faults, especially electromagnetic decoupling of double circuits and bipolar dc lines, and propagation characteristic fault waves. The simulation model of the two ±500kV HVDC systems erected on the same tower is established by the PSCAD/EMTDC software and extensive simulations are performed to analysis fault signature of transmission lines.