On Thursday, November 13, 2025, PRECorp experienced a power blackout and voltage instability beginning approximately 12:45 p.m.
The resulting outage cut power to every PRECorp service, more than 27,000 meters in Campbell, Crook, Johnson, Sheridan and Weston counties in Wyoming and parts of southern Montana.  
Regional grid regulators conducted a multi-level investigation into the blackout’s cause and lessons learned. The blackout resulted from a combination of bad data, poor communications, and improper response, according to a June report published jointly by Western Electricity Coordinating Council (WECC) and North American Electric Reliability Corporation (NERC). Power tripped during a planned outage on a 500 kV transmission line near Medicine Bow. 

• PacifiCorp (Rocky Mountain Power) had planned an outage on a sensitive transmission line at 8 a.m. 
• That outage was delayed until midday for personnel availability. 
• During the morning, conditions changed and there was more power flowing than earlier in the day. 
• Protective settings and monitoring tools did not reflect the higher power flows that had developed by midday.

When crews took the outage later that afternoon, the additional generation online and pushing power to those lines resulted in a voltage collapse of the 500 kV circuitry, caused protective relays to correctly trip 14 lines regionally, cutting off northeast Wyoming and southwestern South Dakota from the grid. 
The event progressed so fast that no manual corrective actions were able to be taken.
The cascading events impacted the PRECorp system and also affected electric providers at Montana-Dakota Utilities, Rocky Mountain Power, Black Hills Energy, Basin Electric, and Gillette City services. At its peak, more than 155,000 services from Casper to Rapid City to Sheridan and the northern edge of Cheyenne were out.
In all, approximately 30,000 square miles of the area covering western South Dakota to the Big Horn Mountains were in the dark for 3 to 8 hours.
In response, PRECorp deployed line crews to nearly all of its 50-plus substations to prepare to safely restore power to circuits across the system. Due to the volatility of the incoming voltage and changes in the grid around PRECorp, it was determined to bring the system online starting from the south and working north, with eyes on each substation as it was brought online. The first services to be re-energized were around the town of Wright and completing west of Hulett.      
The restoration process began approximately 3 p.m. as incoming voltage stabilized and PRECorp crews prepared equipment and settings to be re-energized within safe operating limits. Crews successfully restored power to communities through individual substations beginning approximately 4:30 p.m. and final services were restored approximately 10 p.m.
The PRECorp team operated nearly every substation switch and breaker in one afternoon and evening with very few issues and with good assurance that the voltage and capacity delivered was not going to negatively impact the membership.

• NERC and WECC regulators have recommended wider real-time data sharing between power generators and distributors; 
• Increased trainings between utilities and grid systems; and, 
• Improved transmission outage coordination and communication.