NV Energy builds big in a confined space to support the new MGM Mirage project.
Las Vegas Started as a Dot on the Map in 1911. The first resort casinos were built in the early 1930s and the city's growth took off. As new hotels, casinos, commercial developments and suburbs have been built, NV Energy (NVE; Las Vegas, Nevada, U.S.) has been there to build the substations and transmission and distribution infrastructure needed to keep everything connected.
During the late 20th century, Las Vegas grew at a breakneck pace as new casinos and resorts sprang up along the east side of Interstate Highway 15, transforming plain-old Las Vegas Boulevard into “the Strip.”
NVE has worked tirelessly to keep pace with the city's incredible growth through mid-2009. A new Sinatra 230/138/12-kV substation demonstrates how NVE is meeting new load challenges on the highly congested Las Vegas Strip while thinking ahead toward system reliability and flexibility. Permitted in 2006, the substation was designed and built from 2006 through 2009 with not one but two separate gas-insulated substations (GIS) on a 1.6-acre (0.6-hectare) site.
SINATRA SUBSTATION PARAMETERS
In 2005, MGM Mirage launched plans to build its CityCenter project, a showcase resort and entertainment complex. At US$9 billion, the mammoth 16.8-million-sq-ft (1.5-million-sq-m) mixed-use complex on 67 acres (27 hectares) is the largest privately funded construction project in the U.S. For NVE, the scale of the project would require a massive construction effort to reconfigure the electrical infrastructure. This meant the utility would have to assess the transmission infrastructure serving the Las Vegas Strip, and determine the least intrusive and most economic method for delivering power to the new energy-intensive development. How would NVE serve the new load in a highly congested area? At the same time, how would it construct a facility to be flexible enough to meet future transmission challenges? Those were the essential questions.
The transmission grid in Las Vegas is composed of 500-, 230-, 138- and 69-kV transmission lines. MGM approached NVE in late 2005 about the new 100+ MVA project, expected to open in November 2009. NVE realized the project could be leveraged to do more than just satisfy MGM's needs. The 230-kV backbone could be extended into a congested portion of the city to supply additional power to support a portion of the overloaded 138-kV system serving the Strip. Additionally, this could serve as a critical first step to realize long-term plans to construct a north-south 230-kV backbone across the valley.
System planners determined that by tapping into an existing 230-kV line west of the Strip, the 138-kV system in the Strip corridor could be reinforced with a 230/138-kV autotransformer connection. The 138-kV system, in turn, would serve the four 138/12-kV distribution transformers, providing service to the project, along with four 12-kV capacitor banks. NVE's transmission planners also determined the need for a 138-kV transmission capacitor bank and line positions for future 138-kV and 230-kV circuits. That's a lot of substation equipment.
NVE and MGM worked out a cost-sharing agreement to fund the construction of the new 230/138/12-kV substation, along with 1 mile (1.6 km) of underground 138-kV transmission, a 2.4-mile (3.9-km) 230-kV loop (1.4 miles [2.3 km] of double-circuit overhead line and 1 mile of double-circuit underground line in parallel duct banks). Major modifications were made to Arden substation, as well as four other substations, putting the project total at $96 million. The new substation would have a 336-MVA transmission capacity via the autotransformer, of which 224 MVA of distribution capacity would be available to serve the new loads of the CityCenter project and other proposed projects in the vicinity. MGM would pay for only its pro rata share, which was about half of a single dedicated station for the CityCenter project alone. It was a win-win situation for both the customer and the utility.
The biggest problem, however, was that available land around the Strip for a substation with that capacity was extremely valuable. At its height, prime land near the Strip exceeded $30 million per acre. A 10-acre to 15-acre (4-hectare to 6-hectare) parcel size would be used for a traditional open-air substation. Even if that was cost effective, there were no parcels of that size even available. The only possible wedge of land nearby was a tiny 1.6-acre pie-shaped parcel between Sinatra Boulevard and Interstate 15, right behind the New York-New York Hotel & Casino.
Available routes for the new transmission lines were limited and required underground line construction for final entry into the substation. The 138-kV underground transmission route had to pass behind the CityCenter project and the New York-New York, Monte Carlo and Bellagio hotels and casinos, as well. It required extensive coordination with existing utilities and proposed new infrastructure.
CityCenter hired a design firm to master plan the layout for all the new and existing utilities that planned to use the street, including one double-circuit transmission duct bank, five distribution duct banks, manholes, water, sewer, storm drain, high-pressure natural gas, telephone and cable — while maintaining their respective installation and maintenance separation distances. Some of the distribution duct banks were 19 ft (5.8 m) deep to cross under other utilities. Another challenge was that NVE would have to bring the 230-kV lines underneath the eight-lane Interstate 15 to connect to the existing transmission 2.4 miles (3.9 km) to the west.
PLANNING AND DESIGN CHALLENGES
When utilities are given the opportunity to build a centrally located substation like Sinatra, it's important to think long term. In other words, it's important to have an ultimate build-out plan that is flexible, so when electric needs change (a given in Las Vegas), utilities are able to accommodate the construction of new infrastructure. NVE wanted to build a substation that would serve its immediate customer needs but also potential future loads.
NVE has 217 substations in its service territory. Of those, only six have 138-kV GIS equipment. Because of the site-specific characteristics of the pie-shaped piece of land available, it became clear from the start the only solution would involve GIS equipment. There was simply no other way to fit that much high-voltage equipment in such a confined space.
Even with tightly packed GIS equipment, it was not easy to design the substation. Engineering consultant POWER Engineers (Hailey, Idaho, U.S.) worked with NVE to develop 11 different substation arrangements and detailed 3-D photo-simulations to identify the optimal arrangement from both operability and constructability standpoints, as well as expansion in the future. The photo-simulations also helped to gain MGM acceptance and obtain the requisite permits.
For a substation that can provide transformation from 230 kV down to 138 kV, and from 138 kV to 12 kV on a 1.6-acre site, one would expect to find a nightmarish cluster of equipment. Although the Sinatra substation is essentially three substations on one small site, it has a surprisingly clean layout. The substation is even designed to accommodate a 138/12-kV mobile transformer, with consideration for the mobile unit's turning radius.
The 230-kV GIS from ABB (Zurich, Switzerland) at the northern end of the site features three 230-kV line terminals in a four-breaker ring bus. The two initial underground 230-kV lines use two cross-linked polyethylene (XLPE)-insulated 2500-kcmil copper cables per phase supplied by Prysmian Power Cables & Systems (Lexington, South Carolina, U.S.). South of the 230-kV GIS is a 180/240/300//336-MVA, 230/138-kV autotransformer manufactured by Siemens (Munich, Germany).
The low side of the autotransformer is attached to an ABB 138-kV GIS, vertically stacked six-breaker ring bus. In addition to being connected to the autotransformer, the 138-kV GIS is connected to two underground 138-kV circuits from the Suzanne and Bellagio substations, plus a future 138-kV position. The two initial underground 138-kV lines use Nexans (Paris, France) XLPE-insulated 2000-kcmil copper cable. The 138-kV GIS also features two air-insulated terminals for mobile transformer connection.
Since the separation distance between the autotransformer and both the 138-kV and 230-kV GIS is less than required by the National Electrical Safety Code, 28-ft (8.5-m) firewalls were installed on both sides of the autotransformer. But these were not ordinary concrete-block firewalls. The ceramic-panel firewalls — a first for NVE — provided by Composite Support & Solutions (Rancho Palos Verdes, California, U.S.) were designed by a former NASA engineer. They not only protect the expensive GIS equipment in the event of a transformer fire but enable the firewalls to be easily removed to allow access for autotransformer replacement.
The 138-kV GIS feeds power via underground cable beneath the substation site to four 138-kV/13.09-kV, 30/40/50//56-MVA transformer banks manufactured by EFACEC in Portugal. Each transformer bank is arranged in a redundant configuration, connecting to a set of metal-clad 15-kV class switchgear units from Powell (Bradford, U.K.), for a total capacity of 224 MVA through 24 12-kV feeders (six on each bank). Finally, at the southern end of the site is space for a future 138-kV capacitor bank that can be used for power-factor correction.
In order to extend the 230-kV backbone into this area, NVE had to find a route through congested residential and commercial urban development. NVE hired Burns & McDonnell (Kansas City, Missouri, U.S.) to design the 230-kV line and PAR Electrical Contractors (Kansas City) to install 1.4 miles of double-circuit overhead 230-kV transmission on Thomas & Betts (Memphis, Tennessee, U.S.) structures using compact street-side construction in which the two three-phase circuits are stacked vertically, overhanging the street. This minimized the easement requirements along high-cost properties.
From the transition structures, Wilson Construction (Canby, Oregon, U.S.) installed the underground portion of the 230-kV loop into the substation. With two cables per-phase, Wilson installed about 12 miles (19 km) of cable, weighing almost 2 million lb (0.9 million kg).
Another major hurdle was crossing under Interstate 15 with not only two 42-inch (1.1-m) casings for the new 230-kV circuits, but also three additional casings for future circuits. Sinatra substation was built like a layered cake, with the deepest installation first. Hofsommer Excavating (Las Vegas) dug five bore pits 18 ft deep (5.5 m) within the substation to install the five 42-inch steel casings under the interstate — right through the middle of an extremely hard-rock shelf known as caliché — to a receiving pit 360 ft (110 m) on the west side of Interstate 15. Caliché is a sedimentary calcium carbonate soil found in southern Nevada deserts. Typically in Las Vegas, the compressive strength of caliché averages 6000 psi to 10,000 psi (41 MPa to 69 MPa), but this caliché averaged 13,000 psi (90 MPa) — basically solid rock more than three times harder than concrete. The rock wore down the boring rig's drill teeth in no time, and it snapped the drill shaft twice during installation. This unexpected natural phenomenon made for some interesting challenges to the project schedule and budget.
Once the 230-kV line casings were installed, crews set to work building the next layer of the ultradense Sinatra site. Again, while the substation viewed from above appears surprisingly sparse and clean, what lies below boggles the mind.
Like stacking a cake, NVE's contractor Energy Erectors (Leesburg, Florida, U.S.) had to first assemble concrete vaults underneath the substation site grade, ensuring the vaults were perfectly positioned to bend the XLPE 230-kV transmission cable — 5 inches (127 mm) in diameter, with a 96-inch (2.4-m) bending radius — from the entry points at the bottom of the vault to the terminations on the GIS. They proceeded to install the maze of conduit, grounding and cable trench, while avoiding the numerous equipment foundations, manholes and transformer oil containment pits.
A NEW SUBSTATION STANDARD
The Sinatra substation set several new benchmarks for NVE. It featured the longest 230-kV underground transmission line in its transmission system, two GIS systems at one facility, the first 230-kV GIS installation, the first ceramic firewall and more distribution capacity per square foot than any other substation in its 4500-sq-mile (11,655-sq km) service territory. Most notably, the three-year project was built safely, with no recordable accidents.
Benchmarks aside, it was truly a remarkable project from both design and construction points of view. After all, it's not often that utilities install two GIS substations with the ability to terminate six transmission lines and more than 500 MVA of transformation on a site one-tenth the size normally used for such facilities. The Sinatra substation is also evidence of continued long-term thinking by NV Energy.
Jennifer Kelly (firstname.lastname@example.org) has been with NV Energy (formerly Nevada Power) for nine years. She has been manager of the major projects group since 2006, leading a team of project mangers focused on the infrastructure requirements of the Las Vegas Resort Corridor (average $70 million annual budget). Prior to this position, Kelly was a senior project manager at NVE for two years and a substation engineer at NVE and Central Hudson Gas & Electric in Poughkeepsie, New York, for nine years. She has a BSEE degree and an MSEE degree concentrating in power engineering from Clarkson University and is a registered professional engineer.
Jay Keeling (JKeeling@POWEREng.com) is a senior project manager at POWER Engineers in Billings, Montana. With more than 21 years of experience in the transmission and distribution field with emphasis on specialty projects like SVC, STATCOM, series compensation and extra high voltage, Keeling has managed dozens of substation projects ranging from 2.5 kV to 800 kV around the U.S. and overseas. He holds a BSEE degree from Montana State University and is a registered professional engineer in six states.