On the analysis of faults along HVDC land electrode lines

(marine or ground). The electrodes have traditionally been installed in HVDC transmission systems to provide a low resistance return path during monopolar operation and to drain small unbalanced currents during bipolar operation.

Worldwide, different types of electrode lines exist, reflecting different design philosophies and system constraints. These lines may be overhead lines (OHLs), sharing towers with pole conductors or using dedicated structures. Alternatively, they may consist entirely of cables

(terrestrial and/or submarine), with the land section installed in the same trench as pole cables or routed along completely independent paths. In some cases, mixed OHL and cable lines are adopted.

Since electrode lines are shorter than the alternative solution adopting metallic return lines, they are generally preferred because they are cheaper, with lower power losses and higher reliability.

For such reasons, they are widely used in many HVDC schemes. In normal operating conditions, the voltage level of electrode lines is in the order of hundreds volt, that is significantly lower than that of the pole cables. The steady-state voltage on the electrode line primarily results from the voltage drop across its longitudinal resistance, caused by the direct current flowing through the conductor. In the case in which a fault occurs on the electrode line with its interruption, a non-negligible overvoltage may appear in the neutral area of the HVDC converter station. This overvoltage is detected by the converter station’s control and protection system, causing the pole to trip.

When the fault does not completely interrupt the electrode line, fault detection becomes considerably more complicated. Traditional protection methods used for pole lines—such as undervoltage thresholds or voltage derivative measurements—are ineffective to identify such faults in electrode circuits, due to their low voltage. Today, different methods of monitoring and protection of the electrode lines are suggested by CIGRE TB 675 but either they are not used or their application is limited.

This study aims to investigate the main potential causes of faults in HVDC electrode lines. The analysis covers one of the electrode circuit topologies currently in service, considering different fault types and locations, based on reasonable assumptions about fault resistance values. The study will be performed considering land cable faults only.

Both external faults—such as mechanical damage from construction or maintenance equipment—and internal faults caused by insulation defects will be analyzed. These defects can lead to cable faults when an overvoltage occurs in the HVDC converter station. The overall goal is to evaluate the amount of current that will flow to earth in the faulty points, as well as to provide useful insights for improving fault detection and protection strategies in HVDC electrode cable systems.