Project implementation and field experience with a transmission cable differential protection system using non-conventional current sensors

Limited rights-of-way in populated areas may prevent delivery of electric power to substations via overhead transmission lines. This situation may require transition from overhead lines to power cables for power delivery to urban areas. Mixed overhead/cable topologies are becoming more common in both new and refurbished circuits and may require unique protection approaches. To ensure reliable power supply, protection systems must differentiate between cable and overhead transmission line faults. For example, after operating for a line fault, the main protection system may initiate auto reclosing only after positive confirmation that there is no fault in the power cable. The Portuguese Transmission System Operator, REN, owns and operates such a power system that includes transition from overhead lines to power cables for power delivery to urban areas. The system was designed for four in-service 220 kV power cables that interface a gas-insulated substation with overhead transmission lines in the city of Lisbon.