The Red Eléctrica de España (REE) high-voltage transmission system includes existing circuits that comprise mainly overhead lines with short sections of underground cable. To comply with environmental and visual amenity considerations, the number of existing and proposed high-voltage transmission lines mixing overhead and underground cable is on the rise.

In the event of a fault on high-voltage transmission lines, auto-reclosing is one of the main protection functions. The majority of the faults on overhead lines are temporary, so auto-reclosing normally will clear the fault and restore the availability of the system. However, faults on underground cables are mainly permanent and an auto-reclosing sequence could potentially worsen the damage.

Therefore, the protection scheme challenge is more demanding with high-voltage mixed lines because they include underground cable sections. It is important to be able to determine if the fault occurred in the underground cable section to block the reclosing function. The most reliable way to achieve this is by implementing a differential protection scheme on the cable section, but this can be costly and oftentimes is not practically feasible.

Red Eléctrica de España, mixed-line protection
The 220-kV Saladas–Novelda transmission line was used for the field trial.

Selected Technology

As a result of considering the limited technology possibilities commercially available, REE, the Spanish transmission system operator (TSO), sought a technology mature enough for field deployment that could substantially improve fault identification on underground cable sections in existing and future overhead transmission lines.

To address this problem, REE selected technology developed by ARTECHE that consisted of a solution based on passive optical sensors for measuring currents to implement a differential fault detection scheme between both ends of the cable section. All the sensors installed are controlled by a single device, a signal processing and cable fault detection (CFD) unit, located in the closest available substation. The connection of the current sensors to the CFD is made by means of standard single-mode fiber-optic cables.

One of the main targets of the project was to provide CFD functionality without altering the utility’s existing infrastructure and interfering with the protection and control functions already in place. As a result, the system had to be designed to maximize the level of integration and reduce the impact on existing equipment installed on the transmission line.

Red Eléctrica de España, mixed-line protection
An optical sensor is installed on the underground cable below the cable termination.