Enhancing safety and reliability in the Portuguese subtransmission network: E-REDES cost-effective strategies for managing increased short circuit levels

Ensuring line thermal limits of lines are not exceeded during short circuits is paramount. This paper explores the challenges and solutions for maintaining these limits in the Portuguese subtransmission network, which faces increased short circuit levels due to transmission grid changes. The primary protection system clears faults within 0.4 seconds, but if it fails, backup protection, coordinated with the Transmission System Operator (TSO), trips within 2 seconds. This duration is insufficient in some cases risking infrastructure integrity. Two solutions are evaluated: increasing the line sizing and adapting the protection system. Increasing the line rating is costly, potentially up to 34 million euros. Adapting the protection system is more cost-effective, involving additional settings in multifunction protection relays using Inverse Definite Minimum Time (IDMT) curves as per IEC 60255 standards. The extremely inverse curve effectively manages thermal stresses during short circuits, depending on precise current and time measurements. The paper discusses technical and economic aspects of adapting the protection system, costing about 20% of line uprating expenses. It concludes with cost-effective strategies for enhancing system reliability and safety by focusing on protection system adjustments rather than extensive physical upgrades.