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Vantage to Pomona Heights Chapter 2 230 kV Transmission Line Project FEIS Proposed Action and Alternatives PAGE 2-38 operable pipe-type system, cathodic protection must be applied to the cable pipes to mitigate corrosion. This in turn helps prevent fluid leaks which pose both an operational and an environmental concern. If a loss of coolant fluids were to occur it would result in environmentally hazardous coolant materials contaminating the surrounding soil. A coolant fluid leak can be caused by several means including thermal expansion and contraction of the cable due to power cycling, ground movement, splice breakage, termination movement, improper installation, and a cable fault. The fluid is under pressure, so if a leak occurs, it can spread. Using an HPFF system does provide high reliability, but requires additional equipment, resulting in additional opportunity for component failure, and specially trained personnel are required to maintain these systems. Self Contained Fluid Filled Cable SCFF cable systems are very similar to the HPFF systems. The cable is typically constructed around a hollow tube, used for fluid circulation, and uses Kraft paper or the same LPP insulation materials. Because the fluid system is self-contained the volume of fluid required is significantly less; however, the same distribution of pumping plants would be required. While SCFF cable systems have the longest running history at the extra high voltage levels, their use is typically limited to long submarine cable installations. Superconducting Cables Research is currently underway in the advancement of high temperature superconductors (HTS). Utilizing a unique cable design where all three phases are centered concentrically on a single core, the cables are capable of displaying low electric losses with the same power transfer capabilities as compared with a standard non-superconducting cable. The core, filled with a cryogenic fluid, super cools the conducting material resulting in extremely low losses and high electrical power transfer capacities. Most HTS systems are located adjacent to large metropolitan areas, where they are capable of transferring large quantities of power a few thousand feet at the distribution line level (12 to 34.5 kV). However, technological advances in the last few years have seen the first 138 kV HTS system installed in Long Island, New York in early 2008. Because HTS systems have not been established at the 230 kV or 500 kV voltage levels, superconducting cable would not be a technology option for this Project. Reactive Power Compensation-Maintaining Stable Power Flow The characteristics of the underground cable insulating material and the close proximity of the cables to one another results in the cable system introducing high reactive loads (voltage rise) onto the electrical system that affect safe and reliable power flow. These reactive loads (voltage rise) would have to be offset with above ground compensation stations located every 7 to 20 miles to maintain stable power flow along the transmission line route (Xcel Energy, Inc. 2011). A further consideration is that the electrical system as a whole may or may not be capable of reliably accommodating these very significant reactive power loads, making the integration of long underground alternating current power lines into the overall power grid questionable or infeasible. Design Considerations The following are key considerations for underground transmission line design of a 230 kV cable system: • A 230 kV cable system would consist of multiple cables per phase to achieve the target power transfer requirements and to provide redundancy in the case of a cable failure. • Concrete encased duct banks would be installed at a minimum cover depth of three feet or as required by routing design and would be backfilled with specially engineered thermally favorable backfill to assist in heat dissipation, if necessary. • To obtain further redundancy, multiple duct banks per circuit are required to minimize same mode failures of the systems.