Between 2002 and 2003, Southern California Edison experienced several high-voltage transformer fires in three of its 500-kV substations. Without a fire mitigation system in place, the fires spread rapidly and resulted in tens of millions of dollars of damage. The destruction extended to neighboring transformers and other adjacent electrical equipment, conductor in cable trenches, structural steel frames and so forth.
Typically, transformers are the most important piece of equipment in a substation; therefore, the loss of SCE's transformers was catastrophic with regard to substation operation and, thus, the cause of service disruptions. SCE came to the realization that a transformer fire-mitigation system needed to be developed and implemented to reduce potential future losses, minimize service disruption and reduce environmental damage.
Design Strategy
After site visits to other California utility substations that use fire-mitigation systems and discussions with the design engineers of those systems, it was decided SCE's transformer fire-mitigation system should focus on minimizing the impact to the substation rather than developing a fire prevention system.
The SCE task team designed the transformer fire-mitigation system based on two fundamental design strategies. The first tactic was to prevent collateral damage by containing both the shrapnel (typically associated with the transformer bushing failure) and any released transformer oil within a cell. The second strategy was to minimize the heat from the fire by reducing the amount of burning oil.
System Components
Based on SCE's design objectives, a transformer fire-mitigation system was developed with four components: barrier walls, collector basins, an underground reinforced-concrete pipe system and detention basin.
Barrier walls are located on either side of a transformer as a way to divide the different phases to prevent the spread of fire between the ignited transformer and the adjacent transformers and electrical equipment. The walls also prevent any debris from a transformer or bushing explosion from impacting adjacent transformers and electrical equipment. The walls are removable to provide space for transformer maintenance and replacement.
The collector basin is an engineered drainage pan to collect and guide the flow of oil and firefighting water away from the power block area by funneling it into an underground reinforced-concrete pipe system. The basin is equipped with drain rocks to suffocate the fire as the burning oil flows toward the underground reinforced-concrete pipe system.
The underground reinforced-concrete pipe system, or subterranean pipe system, drains the oil-water mixture to a remotely located detention basin. This extinguishes the fire by limiting oxygen supply in the pipe and cooling the oil-water mixture as it flows along the length of the pipe. This system employs a gravity flow design to eliminate the need for any pump or valve operation during an emergency situation. To prevent oil from leaking into the soil, all of the pipe joints are watertight, and the gasket material is oil resistant and temperature resistant up to 150°C (302°F). Pipe diameters are sized to accommodate either expected flows for transformer cells during a 50-year rainstorm or for the firefighting water and oil from one transformer fire.
The detention basin holds the oil-water mixture at a remote location, minimizing the risk of fire damage in case of reignition. The basin's capacity was designed to contain a 50-year rainstorm or the volume of transformer oil and firefighting water until it could be disposed of per U.S. Environmental Protection Agency (EPA) requirements. Polyvinyl chloride (PVC) water stops at all construction joints and keyways prevent any liquid from seeping into the soil.
Fire Incident
Shortly after midnight on Feb. 18, 2012, the 500-kV bushing on an SCE single-phase 500/220-kV transformer failed. The resulting explosion sent oil and fractured pieces of porcelain up to 120 ft (37 m) from the transformer. Exposed to oxygen in the air and the arc from the electrical malfunction, the oil burst into flames. The single-phase transformer was immediately consumed by fire and continued to burn until firefighters could extinguish it more than six hours later. The substation had been retrofitted with a newly installed transformer fire-mitigation system that allowed SCE, for the first time, to evaluate the performance of the system in an actual event.
The barrier walls performed as intended by controlling the spread of fire as well as providing a blast barrier to protect the adjacent transformers and other electrical equipment from debris. The wind was blowing northward, so the wall on the north side of the failed transformer bore the brunt of the fire. Coincidentally, the north wall also sustained the most damage from the bushing explosion.
Some of the firewall panels experienced minor damage from the shrapnel, but none of the panels were ruptured through and, thus, the fire did not penetrate. Although the walls were designed and laboratory tested to sustain four hours of fire exposure at 1,200°C (2,192°F), they actually successfully protected the adjacent transformers for more than six hours. After the event, the two panels with the highest amount of exposure to the fire were sent to a laboratory to undergo strength testing for reference purposes. Initial results confirmed the panels, although weakened, had adequate remaining strength to be reused. The panels that sustained minor blast damage were replaced. The remaining panels were cleaned and are being reused.
The collector basin below the damaged transformer worked as intended by collecting the oil and firefighting water mixture, and guiding it to the underground reinforced-concrete pipe system. None of the liquids flowed to neighboring transformer cells. The drain rocks located at the entrance of the drainage pipe system within the sump area of the collector basin successfully extinguished burning oil.
The underground reinforced-concrete pipe system performed as expected by automatically draining away the burning oil before any firefighting assistance had arrived on the scene. Having a limited amount of burning oil in the power block area greatly diminished heat generated by the fire. The heat was sufficiently low enough not to induce further damage, such as causing the main transformer tank to explode or the structural steel frame to warp, as it did in the 2003 substation fire. It is strongly suspected the oil and firefighting water were further cooled as they flowed down the underground reinforced-concrete pipe system.
The extinguished oil-firefighting water mixture was collected in a remote collection basin. The mixture was both sufficiently cool and isolated from the transformer failure site to avoid reignition. Furthermore, the detention basin served as a secure containment site until an environmentally approved disposal company could remove the contaminants.
System Performance Evaluation
The failed single-phase transformer bank area was successfully isolated; consequently, it was the only area that required any refurbishment. Adjacent equipment and structure damage was limited to the removal of soot and some minor cosmetic patching of porcelain bushings. Because of the success of the fire-mitigation system, SCE saved millions of dollars in equipment replacement and repair.
The site cleanup was minimized and expedited because of the isolation and detention of all liquid contaminants. Limiting the amount of burning oil reduced the airborne contaminants to a minimum. From an environmental perspective, the transformer fire-mitigation system was a success.
Finally, and most importantly, the isolation of the failure negated any service disruption to SCE customers. System stability was not compromised because of the isolation of the damage to a single transformer. Furthermore, the containment of damage allowed SCE to return to fully operational status in record time for such an incident.
The transformer fire-mitigation system is a unique engineered solution independently developed by SCE. Since its inception in 2005, it has been implemented throughout most of the 500-kV substations within SCE's system. Initiated from need, refined through project implementation and now proven effective by a real fire incident, the SCE transformer fire-mitigation system is now considered a mature system. The SCE fire-mitigation system has been recognized by the industry as being at the forefront of transformer protection technologies. As a result, many other utilities, both foreign and domestic, have adopted SCE's concept, either partially or as a whole, and have wisely built their own fire-mitigation systems.
Acknowledgement
The authors thank SCE management for the support and encouragement the team was given to pioneer this design and achieve these goals.
Philip T. Mo ([email protected]) is a senior structural engineer at Southern California Edison. He spent five years in consulting firms designing pipe supports, mining structures and industrial precipitators before joining SCE. While in the generating department, he was responsible for projects of design, modification, maintenance and construction in all types of SCE's power plants. In substation engineering, he was in charge of the seismic qualification of substation equipment. He has since pioneered, developed and built the fire mitigation system in multiple SCE substations. Mo is a professional engineer.
Kaolyn M. Mannino ([email protected]) holds a BSCE degree from the University California State University of Los Angeles. At Southern California Edison, she has had the opportunity to work on the fire-mitigation system and on seismic testing and analysis of substation electrical equipment. She is a member of IEEE 693 Recommended Practice for Seismic Design of Substations and the secretary of IEEE 1527 Recommended Practice for the Design of Buswork Located in Seismically Active Areas.
Michael V. Cadena ([email protected]) holds a BSCE degree from the University of California, Irvine, and is pursuing a MSCE degree from California Polytechnic University of Pomona. Within his first two years with Southern California Edison, he has had the opportunity to be involved in the fire-mitigation system.
