Figure 2.8-1. Single-Line Diagram for proposed Seminoe Pumped Storage Project

Final License Application – Exhibit A Seminoe Pumped Storage Project

Figure 2.8-1. Single-Line Diagram for proposed Seminoe Pumped Storage Project.

January 2023 | 19

2.9. Switchyard

2.9.1. General

GSU transformers for each unit will be provided rated at 360 MVA, with 18 kV, deltaconnected, LV windings, and 500 kV, grounded wye, high-voltage windings located in the transformer gallery of the underground complex. Appropriate basic insulation level and surge-arrester ratings will be selected based on technical standards for these ratings.

Transformers will be 3-phase construction, oil-filled, water-cooled type oil directed water forced or oil forced water forced, but use natural, ester oil-type FR-3 or equal with 600°C flashpoint instead of the 300°C flashpoint of mineral type insulating oil. GSUs will be provided with oil-capture basins that guide oil to closed tanks so that any potential fire hazard is transferred away from the transformer. A low-volume, nitrogen-injected fire protection system similar to a Victaulic Vortex system is envisioned to be provided with each GSU. Each transformer will be contained in a vault with a 4-hour fire rating. A singleline diagram for the transmission system is shown in Figure 2.8-1.

2.9.2. Switchgear

The underground facilities will incorporate a 500 kV, gas-insulated switchgear arrangement in the crown of the transformer gallery above the GSU transformers.

The underground, 500 kV, gas-insulated switchgear will be configured as a breaker and a half arrangement. SF6 is currently used for such applications, but alternate gases are under development for active components and are expected to be available by the year 2024. Wherever possible, gas-insulated buses or lines will use compressed air instead of SF6 to reduce the carbon potential. This could be accomplished by selection of an 800 kV, gas-insulated bus to account for the lower dielectric strength of compressed air.

To avoid issues with transformer energization and de-energization, point-on-wave circuit breakers will be provided for all 500 kV breakers. Transmission line feeders will be supplied with high-speed ground switches. Surge arresters are provided at both ends of the underground transmission line feeders and at the terminals of each GSU.

2.10. Transmission Lines

2.10.1. General

Power input and evacuation will be by two, 500 kV circuits from the underground gasinsulated switchgear through the low-level cable tunnel, up the cable shaft to a take-off structure at the surface, and thence via two separate, 500 kV, overhead primary transmission lines extending to the 500 kV interconnection at Aeolus Substation, approximately 30 miles to the southeast of the Project (Sheets 32-40 of Exhibit F).

The proposed design for transmission lines from the transformer hall to the surface provides for the use of either solid dielectric cables (separated in the tunnels and shafts)

or a gas-insulated bus with no combustible materials. Either option provides acceptable fireproofing. The 28-foot-diameter vertical shaft, which will also function as the air and smoke evacuation conduit, will include an elevator for maintenance access to the cables or bus in the shaft as well as access to the outdoor cable terminations and disconnect switches. The vertical shaft will include steel grating platforms every 30 feet. Steel stairs will also be included for emergency egress from all landings. Stairs and platforms will use gratings to maximize airflow capability of the system.

To interconnect the Project to the existing electrical grid, approximately 30 miles of BLM and private lands will be traversed by an overhead transmission system. The power delivery will be split into two self-supporting or separate circuits, at the request of the interconnecting utility. 500 kV is the preferred transmission voltage. It should be noted that negotiations for transmission right-of-way with private landowners are ongoing, and the final routing of the Project transmission line in particular areas will depend on the results of those negotiations.

The proposed 500 kV transmission line corridor from the Seminoe gas-insulated switchgear to the Aeolus Substation follows the existing WAPA transmission line and is located based on construction, maintenance, and engineering requirements; land use; and potential impact to landowners in the area.

The initial routing of the transmission lines from the take-off structure will be on the south side of the existing WAPA 230 kV and 115 kV lines. At the bottom of the initial slope, the 500 kV lines from the Project will cross the WAPA lines and will be to the north of these lines for the remainder of the transmission route. The 500 kV lines will be on steel lattice towers approximately 100 feet tall and spaced evenly across a 450-foot-wide right-of-way.

At certain locations, to avoid stringing lines directly above buildings, a slight line diversion will be proposed away from the WAPA lines (See Exhibit G).

The 500 kV Aeolus Substation at the terminus of the primary 500 kV lines will require the addition of two feeders, assumed as one full new bay of breaker and a half. This will require the addition of civil works to expand the 500 kV substation, including the ground mat and fencing. New dead-end towers will be required to terminate the incoming transmission lines from the Project.

Transmission line protection of the 500 kV transmission circuits will be determined in later design phases and likely consist of transfer trip, carrier blocking, and/or line differential arrangements. It will incorporate high-speed differential protection of the underground circuits that will block reclosing of both ends of the line and close a high-speed ground switch. No automatic reclose of these circuits should occur until they have been proven to be unaffected and functional.

2.10.2. Towers, Foundations, and Conductors

Drilled pier foundations will be utilized as the primary foundation solution along the transmission line. If upon detailed design, drilled piers are deemed unsuitable due to