Detecting a fault in a high voltage direct current power transmission system

An apparatus for detecting a fault close to a grounding circuit in a high voltage direct current, HVDC, power transmission system, wherein the HVDC power transmission system comprises a first converter device having a first terminal and a second terminal, a second converter device having a third terminal and a fourth terminal, a transmission cable connecting the first terminal of the first converter device with the third terminal of the second converter device, and a neutral line connecting the second terminal of the first converter device with the fourth terminal of the second converter device, the apparatus comprising:

• a means for generating an alternating current signal by a source converter device, such that a major portion of the generated alternating current signal flows through a direct current circuit, DC circuit, the DC circuit comprising a stray capacitance associated with the transmission cable and at least one return path, wherein the source converter device is either of the first converter device or the second converter device, wherein the source converter device is located electrically close to the grounding circuit;
• a means for measuring current at a plurality of points in the HVDC power transmission system;
• and
• a means for detecting a fault electrically close to the grounding circuit based on the measured currents.