Design and Implementation of An Undervoltage Load Shed Program To Promote System Stability

Xcel Energy’s Southwestern Public Service Company (SPS) has developed an undervoltage load shed (UVLS) scheme to be implemented at multiple substations in its service territory to enhance grid resilience and reliability. The scheme is designed to stabilize the system by responding to undervoltage conditions caused by a loss of generation, change in system flow due to loss of key transmission assets, or significant sudden increases in load. The UVLS relay scheme is a preventative, temporary solution that reduces the likelihood of widespread outages or voltage collapse, remaining in service only until additional generation is brought online and/or new transmission infrastructure is completed.

UVLS relays were installed at each substation to monitor transmission voltage and initiate staged load disconnection during undervoltage conditions. These relays trip predetermined loads in three stages based on severity, prioritizing non-critical load shedding as outlined by Transmission Planning. To streamline commissioning, existing control relays were configured to trip breakers upon receiving a UVLS signal and to block all reclosing actions until manual reset. Operators retain supervisory control to restore load once voltage conditions stabilize.

Due to the critical nature of these projects, schedules were significantly accelerated. Design efforts began in December 2024, with a target in-service date of May 2025 for many locations. The compressed timeline posed considerable challenges for all stakeholders. POWER Engineers was selected to provide System Protection Engineering (SPE) services, collaborating closely with Xcel Energy’s Substation Design Engineering (SDE), Transmission Planning, Substation Commissioning Engineering (SCE), Substation Telecommunication Engineering (STE), and Project Management teams to ensure fast turnaround times without compromising quality. This presentation examines the technical and logistical challenges of rapidly executing the design, testing, and implementation of UVLS. It includes an overview of the system design, key design considerations, coordination efforts, and the strategies employed to overcome obstacles. Finally, it outlines lessons learned and offers recommendations for future UVLS projects.