A DC distance protection scheme (10; 60; 80; 90; 100; 110) for protecting a DC power transmission medium within a DC electrical power network, the DC distance protection scheme (10; 60; 80; 90; 100; 110) comprising: a protection device (16; 64; 84; 94; 104; 114) coupled in use to a DC power transmission medium, the protection device being operable to protect the DC power transmission medium from an electrical fault; a measurement apparatus to selectively measure the current (i m) and voltage (u m) of the DC power transmission medium; and a controller (22; 62; 82; 92; 102; 112), the DC distance protection scheme (10; 60; 80; 90; 100; 110) being characterised in that the controller is programmed to: (i) calculate a first operating voltage (u op 1) of the DC power transmission medium using the measured current (i m) and voltage (u m); (ii) perform a transient comparison between the calculated operating voltage (u op) and the measured voltage (u m); and (iii) operate the protection device (16; 64; 84; 94; 104; 114) to protect the DC power transmission medium if the transient comparison meets a predefined criterion; wherein each of the protection device (16; 64; 84; 94; 104; 114) and the measurement apparatus is electrically coupled in use to a first terminal of a DC electrical power network in which the DC power transmission medium is located, and the controller (22; 62; 82; 92; 102; 112) is programmed to calculate the first operating voltage (u op 1) of the DC power transmission medium at a first point in the DC power transmission medium, the first point lying in a first section of the DC power transmission medium extending between the said first terminal and a second terminal of the associated DC electrical power network. a protection device (16; 64; 84; 94; 104; 114) coupled in use to a DC power transmission medium, the protection device being operable to protect the DC power transmission medium from an electrical fault; a measurement apparatus to selectively measure the current (i m) and voltage (u m) of the DC power transmission medium; and a controller (22; 62; 82; 92; 102; 112), the DC distance protection scheme (10; 60; 80; 90; 100; 110) being characterised in that the controller is programmed to: (i) calculate a first operating voltage (u op 1) of the DC power transmission medium using the measured current (i m) and voltage (u m); (ii) perform a transient comparison between the calculated operating voltage (u op) and the measured voltage (u m); and (iii) operate the protection device (16; 64; 84; 94; 104; 114) to protect the DC power transmission medium if the transient comparison meets a predefined criterion; (i) calculate a first operating voltage (u op 1) of the DC power transmission medium using the measured current (i m) and voltage (u m); (ii) perform a transient comparison between the calculated operating voltage (u op) and the measured voltage (u m); and (iii) operate the protection device (16; 64; 84; 94; 104; 114) to protect the DC power transmission medium if the transient comparison meets a predefined criterion; wherein each of the protection device (16; 64; 84; 94; 104; 114) and the measurement apparatus is electrically coupled in use to a first terminal of a DC electrical power network in which the DC power transmission medium is located, and the controller (22; 62; 82; 92; 102; 112) is programmed to calculate the first operating voltage (u op 1) of the DC power transmission medium at a first point in the DC power transmission medium, the first point lying in a first section of the DC power transmission medium extending between the said first terminal and a second terminal of the associated DC electrical power network.