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Vantage to Pomona Heights Chapter 2 230 kV Transmission Line Project FEIS Proposed Action and Alternatives PAGE 2-39 • Depending upon installation location, a permanent access road approximately 14 feet in width would be required to perform operation and maintenance activities. • The total construction surface impact of the underground cable system would be approximately 55 to 60 wide feet at a minimum, plus any permanent access roads, or approximately 70 to 75 feet wide total surface disturbance. • Splicing of the cable would be required approximately every 1,500 to 2,000 feet. Splicing would be performed inside large underground vault structures. Vault dimensions would be approximately 9 feet wide by 28 feet long by 10 feet high, depending upon the cable manufacturer splice and cable racking requirements. • Depending on the terrain characteristics, burial depths may need to be increased to avoid heating the soil and changing the conditions of the vegetation and wildlife habitat above the duct bank or pipe type cables. • Underground to overhead transition stations would be required at each end of the underground transmission line, and at each intermediate reactive compensation stations. Each transition station would require between 1 to 2 acres, with each site consisting of pedestal type termination structures and reactors (similar to a large power transformer in appearance). In addition to these structures, A-frame dead-end structures, approximately 80 feet tall, would be required at each end of the system. Reliability and Maintenance of Underground Transmission Lines The frequency with which customers experience a power outage and the duration of the power outage are the criteria with which electric reliability is typically measured. The outage frequency of overhead systems is usually greater than that of underground systems and underground utility construction is often perceived as more reliable, primarily because lines are buried and appear to not be as susceptible to inclement weather conditions or potential vegetation-related outages. This is generally accurate for wind and vegetation-related outages; however, underground transmission facilities are not impervious to weather-related outages. Additionally, the majority of the transmission grid in the United States is constructed using overhead construction and underground facilities are connected to the overhead system since they are integrated into the grid. Weather-related outages that affect regional and local portions of the overhead system will also affect the underground facilities attached to them. Underground transmission facilities always have some overhead components such as substation terminations and transition structures and these components are subject to weather-related failures just as overhead transmission lines are. Failures in underground transmission facilities can be more difficult to troubleshoot and repair than those in overhead facilities. It often takes more time to locate and diagnose problems, as well as to perform the necessary repairs, to underground transmission lines than is typically experienced with overhead lines. As a result, the time the circuit is out of service is increased. Underground line repairs, depending on the system, can be disruptive to the environment, are time- intensive, and relatively costly. Both overhead and underground facilities become less reliable with age, making long-term reliability an issue (EPRI 2008). While underground transmission lines are relatively immune to weather conditions, they are vulnerable to washouts, seismic events, cooling system failures, and inadvertent excavation. Other possible causes for cable failure include water intrusion into the cable, overheating of the cable, high voltage transients, thermal movement during load cycling, and aging of the cable. The repairs of high-voltage underground cable systems have relatively long outage times compared to repairs of traditional overhead lines. When a fault occurs the circuit is out of service and cannot be placed back into service until repair and test of the system is completed. Because the cable contains a central hollow duct in the conductor that carries cooling dielectric fluid, outage levels can be lengthy until fluid levels are restored. Qualified cable splicing personnel may be difficult to retain on short notice. It would take at least 5 to 10 days to mobilize qualified technicians and equipment to splice a failed cable. The minimum outage duration for locating, excavating, and repairing a single cable failure is estimated to be at least 20 days.