When a fault occurs in a DC system

the fault current rises rapidly with no zero-crossing point which makes fault-line selection and fault-type identification difficult. In this paper

an online detection and protection method based on graph theory

namely the “double <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$D$ method”

is proposed for fault-line selection and fault-type identification in DC systems. In the proposed method

the entire distribution network is visualized as a “map” with vertices representing the line convergence points and edges representing the connection lines. A network topology matrix “<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$D$” is formed by detecting the current directions as the current directions are altered following a fault

whereas the current directions at the ends of non-fault lines remain the same. In order to prevent misjudgment problems arising from power flow reversal

the rates of change of the fault currents are used to further determine whether a fault has occurred and the “double <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$D$ method” is introduced to identify the fault type. Simulations results with different fault types verify the effectiveness and reliability of the proposed method.