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Vantage to Pomona Heights Chapter 2 230 kV Transmission Line Project FEIS Proposed Action and Alternatives PAGE 2-75 The separation requirement is derived from both NERC and WECC System Performance Standards. The NERC standard TPL-003-0, update October 2012, states that the network must be able to supply demand under contingency conditions as defined in Category C.5, which includes clearing of “any two circuits of a multiple circuit powerline.” The revised WECC standard (TPL-001-WECC-CRT-2) goes further by stating that Adjacent Transmission Circuits on separate towers must meet the NERC Category C.5 criteria. The reason for WECC’s adoption of the new Adjacent Transmission Circuits Standard with a separation distance between centerlines of 250 feet is to require transmission owners to place adjacent circuits on separate tower structures rather than using double-circuit towers. The justification for the change in the centerline distance is based on WECC Western Interconnection transmission reliability data for years 2008 through 2011 comparing outages of circuits on a common ROW corridor and circuits on common structures when two or more circuits went out of service. The average annual outage data showed that the number of two-circuit outages within a 10-minute period reduces from 0.288 outages per 100 miles on a common structure to 0.136 outages per 100 miles on separate structures in a common ROW corridor. The outage data also suggest the average annual outage frequency for two circuits in a common ROW corridor on separate towers is even less than the average annual outage frequency for two circuits not in a common ROW corridor. WECC concluded that the outage data suggest that requiring further separation (greater than 250 feet) would not provide a significant reduction in the outage frequency (WECC 2013). Placing the existing Pacific Power Pomona-Wanapum 230 kV Transmission Line and the proposed Vantage to Pomona HeightsTransmission Line on the same set of poles would violate NERC and WECC reliability standards and would not provide the needed reliability of physically separate transmission lines. The alternative of double circuiting the existing Pomona-Wanapum 230 kV Transmission Line was eliminated from further consideration because it would violate mandatory NERC and WECC reliability criteria regarding separation standards for multiple circuits serving the same load (i.e., Yakima Valley). 2.5.1.2 New Midway-Vantage 230 kV Transmission Line The Lower-Mid-Columbia 230 kV transmission system delivers power to the lower voltage load area systems and transfers surplus power out of the Mid-Columbia area. The major load areas receiving power from the system include: Yakima County, Grant County, and Benton County. The 230 kV transmission system is exposed to thermal violations during the summer ambient temperatures and peak conditions. Additionally, there is exposure to voltage collapse for bus contingencies at the Wanapum/Vantage Substation. The objective of the regional transmission system planning study was to determine the best reinforcements to mitigate the thermal violations and exposure to voltage collapse identified on the Mid- Columbia 230 kV system. The study focused on the Wanapum/Vantage to Midway transmission system. Power flow studies were used to analyze the system for three reinforcement plans. The performance of each plan was compared to identify the plan that provides the most benefit. Benefit was measured in terms of system loading relief and mitigation of thermal violations. Three major reinforcement options were studied and compared: 1) A new Vantage-Pomona 230 kV Transmission Line (Proposed Action) 2) A new Midway-Vantage 230 kV Transmission Line 3) Tying the Wanapum-Walla Walla, Midway-Potholes-Coulee, and Midway-Rocky Ford- Coulee 230 kV Transmission Lines together at their crossing about 12.6 miles east of Wanapum Substation along the Walla Walla Transmission Line to create a new 230 kV path