With gas-insulated switchgear, Georgia Power's designs are compact and pleasing.
by Holly Bounds
Just 3 percent of the world's population lived in cities at the beginning of the 19th century. Last year, for the first time in human history, more people lived in cities than in rural areas and by 2020, it's estimated that 60 percent of the population will reside in megacities.
Large cities are the growth engines of their respective economies but as these cities and economies grow, so do the challenges. "Megacities," cities with populations of more than 10 million, present the biggest challenge, especially in terms of the burden growth puts on the urban infrastructure, including reliable electricity networks needed to support a high quality of life.
Georgia Power's Virginia Ave. substation in Atlanta. The utility built four compact and aesthetically pleasing substations using medium voltage GIS.
According to the 2007 population estimates, the 28-county Atlanta metropolitan area is currently the fastest-growing metropolitan area in the U.S. in terms of numerical gains. At a population of more than 5.3 million, Atlanta isn't a megacity yet, but its urban population growth necessitates expansion in infrastructure, including transmission and distribution lines for electricity. Georgia Power is required by law to serve all customers and must build and maintain the transmission and distribution equipment and lines needed to do that.
Space for substations at a premium
Georgia Power is investing $3 billion on electrical system upgrades over the next few years to expand or build power lines, substations and other facilities in heavily developed areas. Load growth in Atlanta created the need for four new substations in the metropolitan area: Ashford (230/20 kV), Virginia Avenue (230/20 kV), Morningside (230/20 kV) and Kirkwood (115/20 kV).
But where do you erect substations in densely populated areas? How do you expand existing substations where residential and retail structures have been built almost to the fence? There was no room for expansion at existing stations and though the utility bought what it could, additional land was both scarce and expensive. Additionally, the utility discovered that neighborhood opposition was high, with a pervasive "no substation in my neighborhood" mentality among local residents to the point that one neighborhood had launched a website opposing a new substation.
Georgia Power needed a creative solution to its urbanization problem: A compact design on a smaller site with improved aesthetics to lessen the impact on the neighborhood.
Gas-insulated switchgear (GIS) was the answer. Why choose GIS? Space! Space! Space! GIS offers a much smaller footprint, by area and volume, than air-insulated switchgear (AIS). In fact, GIS requires up to 80 percent less space than conventional switchgear.
As part of its due diligence, Georgia Power sent a team to visit Anaheim Public Utilities' Park Substation, America's first underground GIS substation, which was completed in 2006. ("Hidden Substation; Anaheim Public Utilities meets rapid growth with innovative solutions," Electric Light & Power, May 2008, is available in the archives at www.elp.com.) The site visit helped provide confidence that GIS was the right answer for the utility's urban substation dilemma.
GIS has the same bus configuration as the AIS that Georgia Power presently uses, so learning the operations requirements took minimal time and effort, although there are physical and operational differences between GIS and AIS. GIS is price-competitive, with medium voltage (25 kV equipment class) metal-clad switchgear, low maintenance and high reliability.
Georgia Power had no history with metal-clad switchgear or medium voltage GIS in substations, so there were obstacles to overcome regarding the personnel responsible for the switching and grounding of the equipment for maintenance purposes. All of the safety issues were addressed head-on and upfront. In addition to significant safety gains, the switchgear maximizes revenue and profit because of its low maintenance. Georgia Power decided to build the substations with conventional AIS equipment on the high side. Since the stations are strictly load-serving facilities with minimal transmission voltage equipment required, the AIS equipment did not adversely impact the overall space requirements and overall station costs were held down.
Highly reliable, aesthetically pleasing
Georgia Power enjoys a high customer satisfaction rating, in part because it listens to its customers. The growth surge in its urban neighborhoods certainly required a great deal of listening and educating—not to mention creativity—to ensure that customers received highly reliable electricity while reducing the aesthetic impact of the necessary substations on their surroundings.
For Ashford Substation, Georgia Power bought enough land to leave a buffer zone between the substation and the surrounding residences. The utility constructed walls in a material that matched the surrounding development's retaining walls. The company kept old trees and as many of the beautiful magnolias growing there as possible. Both the 230 kV lines and 20 kV distribution lines that ingress and egress from the substation were buried underground. By buying the extra land, Georgia Power could put a significant number of circuits in the space while still accommodating the transformer capabilities, necessary for the space to add a second transformer that will double the capacity of the substation on the existing footprint.
Utilities aren't the only agencies battling aging infrastructure. The Virginia Avenue substation presented its own set of unique challenges: aging sewer lines. Georgia Power had to move antiquated sewers out of the way. The new substation will provide 120 MVA of initial capacity and is expected to provide additional capacity for growth associated with the BeltLine, a project in Atlanta that is creating greenspace, trails, transit and new development along 22 miles of historic rail segments that encircle the city's urban core.
The state of Georgia was involved in the purchase of property for the Kirkwood Substation. Georgia Power needed to buy land from the state that was adjacent to an historical structure. The state required the utility to construct walls with veneers that complemented the historical district.
The Morningside Substation site is located in a residential area, bordering the Peachtree Creek floodplain. Space is constrained and aesthetics are of great concern to the community. Having an enclosed switchgear house was a selling point to the community and the space saved allowed for a trailhead to be created through Georgia Power property for accessing the WildWood Urban Nature Preserve.
Ready for the future
For urban substation construction, medium voltage GIS has proven to be a valuable design solution for Georgia Power. The utility accelerated quickly from no experience with GIS to four substations in a single year. Georgia Power will continue to employ medium voltage GIS in future projects where space constraints and aesthetics dictate the need. GIS projects at 115kV and 230 kV are also being considered where even greater levels of space savings are required or where aesthetics are of the utmost importance.
Holly Bounds is staff marketing communications specialist at Siemens Power Transmission & Distribution Inc. Contact her at firstname.lastname@example.org.