Sometimes Utilities Get to Design on a Flat Blank Slate. Sometimes they have to work with what they are given. And that can make for interesting work. The traditional substation is located where the terrain is flat and ideal for placement of electrical transmission and distribution equipment. But, it is possible to successfully adapt substation design to nontraditional terrain.

THE NEED AT SHERWOOD

Dominion Virginia Power (DVP; Richmond, Virginia, U.S.) needed a 115-kV capacitor bank and line breakers to split the 91 line at Sherwood Substation to address reliability concerns. The region is experiencing significant growth and development, and the utility was experiencing voltage issues during the summer peak. DVP's planning department determined the Sherwood Substation was ideally situated, geographically and electrically, to solve both reliability and capacity issues in the middle of the Charlottesville, Virginia, area.

The site had two 115/34.5-kV transformers (50 MVA and 56 MVA) tapping off a 115-kV loop bus. The existing 91 line had no breakers at the Sherwood Substation and was serving multiple load centers from two distant sources. The only existing protective devices on the 115-kV side at Sherwood were the circuit switchers for the two transformers. The scope of the project was to install two line breakers on the 115-kV bus for added line-reliability protection and a capacitor bank equipped with a breaker for protection.

SITE-SPECIFIC CHALLENGES

Located in wooded hills, the initial site inspection quickly differentiated this project from a typical installation of line breakers and capacitor banks. The path toward the site, nothing more than a gravel access road for local residents, is nestled between large trees. It is easy to miss.

The substation and equipment were installed on the side of a large, rocky hill in 1972. Equipment was installed to adapt to the terrain. The general layout is a three-tier substation with distribution on the lowest level, the transformers and control house on the middle level and the 115-kV loop bus on the top level. The elevation dropped 25 ft (8 m) from the top tier to the middle, and another 25 ft to the lowest level. With the site being only a bit more than 200 ft (61 m) wide and having a 50-ft (15-m) elevation change, designers faced an 18% slope. The proposed location for the capacitor bank was on the top level, sprawling down the steepest part of the substation. The overall footprint required for the capacitor bank, 31 ft by 10 ft (10 m by 3 m), was sufficient, but the extreme elevation change of the ground created a difficult design task.

The surrounding terrain prevented creation of a flat terrace for the substation. The site is bordered on the lower side by a small stream and a railroad line, and on another side by a residential zone. Public roads create the other two boundaries. The site's tight physical constraints inside and outside the substation's decorative fence made constructability a major concern. In addition to the challenges presented by the steep slope and narrow access road, the site is half as wide as a new site would be for an ideal substation containing the existing and new equipment that was planned for Sherwood.

These constraints required strategic placement of equipment to both enable construction and allow for future maintenance. Little room was available for construction equipment, especially when the 12-ft (3.7-m) setback from the fence and tree line for live equipment was factored. To accommodate this, some equipment spacing was reduced slightly, while still maintaining safe electrical clearances. In particular, the three capacitor bank phases were reduced from 14-ft (4-m) spacing to 12 ft. Construction access required a clearing and installation of an additional entrance point at the back of the site.

TECHNICAL CREATIVITY

The existing bus work was a testament to the skill of the previous builders. Almost every bus run and connection had to be custom bent and calculated. There are multiple horizontal and vertical angles and bends. The engineering teams arranged for an extensive on-site construction meeting to discuss the complex foundation and bus arrangement. The easiest portion of the new design work fell along the back of the bus loop where one of the two line breakers was placed. This area had been intended for a future breaker position and corresponding switch.

The second line breaker created the first hurdle. It would have to be installed beneath a large concrete dead-end structure. Ideally, the breaker would be centered under the dead end, creating symmetry between the taps from the line side and bus side. After multiple preliminary efforts and shifting of equipment positions, the breaker had to be installed 6 ft (2 m) off center, toward the line side of the structure. This allowed installation of the corresponding breaker switch on the bus side of the terminal without disturbing the existing bus run.

From a construction and electrical outage standpoint, it was beneficial to modify the existing bus work as little as possible. The unique bends required to accommodate the elevation changes would have been expensive and time consuming to modify. The chosen location of the line breaker still allowed for the taps off the existing line switch on the dead end to connect to the new breaker without creating electrical clearance issues. The new breaker switch on the bus side of the dead end was placed so the center phase of the existing bus would connect directly with the center phase of the switch. The outer phases of bus terminate on the switch with strain bus jumpers. This design allowed placement of the new switch and line breaker without affecting the existing bus run or equipment.

CAPACITOR BANK

The final stage was to install the capacitor bank on the side of a sloping hill. The first challenge was to efficiently tap from the existing bus to the new capacitor bank system. This was done through a series of strain and rigid bus connections.

First, the height of the new bus work was established at a safe elevation from finished grade. The new bus work tapped off the existing bus using a strain conductor and dropped below the existing run. That strain bus was connected to the new rigid bus running perpendicular to the current design. To eliminate the need for new bus supports, horizontal standoff insulators were placed on present bus supports. This created a minimal impact on the existing work; however, one support and foundation had to be replaced to safely sustain the new loading. Strain bus was again used to tap off the new rigid bus to terminate on the new capacitor bank's disconnect switch. From the switch, the strain conductor connected to the capacitor bank breaker.

Surge arresters and reactors, extending from the capacitor bank breaker, were placed on a combination steel structure. The arrester/reactor stand was a single unit holding all three phases on one structure.

The last challenge was to place the three phases of the capacitor bank. Each phase of the capacitor bank system stood on individual foundations. The terrain beneath the bank made it impossible to use similar foundation elevations for all three phases. Ideally, a capacitor bank is positioned on flat, level ground. This allows the three banks to be connected by a rigid neutral bar passing through each unit.

The capacitor system had to be modified to accommodate this unique situation. Cooper Power Systems (Waukesha, Wisconsin, U.S.), the manufacturer of the capacitor bank, addressed this issue by combining the rigid design with flexible braids. Each bank had a rigid neutral bus extending toward the adjacent bank but stopping half the distance to the next bank. The two neutral buses were connected by dropping a flexible braid from the end of the higher neutral bus to that of the lower one. This enabled installation of all three phases at unique elevations.

This project was a testament that a substation can successfully be designed on imperfect terrain. To paraphrase an old song: If you can build it here, you can build it anywhere. The successful upgrade of the Sherwood Substation — which went on-line May 14, two weeks ahead of schedule — demonstrates that nontraditional substation projects can be a viable option as the transmission line industry improves reliability and capacity. The project was completed within budget and without injuries. It was randomly selected for state environmental testing and it passed.

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Dominick Piccolomini is supervisor of the substation department for Dominion Virginia Power. He has more than 20 years of electrical utility experience with DVP in the distribution, transmission, metering and substation departments. He received his BSEE degree from The Pennsylvania State University and is a registered professional engineer in several states, including Virginia. dominick.piccolomini@dom.com

Jason Meidinger is a substation engineer/project leader for Dominion Virginia Power. He has been with the company for more than five years, and has experience in both physical design and system protection for transmission substations. He received his BSEE degree from Virginia Commonwealth University. jason.meidinger@dom.com

Keegan P. Odle is a design engineer in the substation department of the Burns & McDonnell Transmission & Distribution Group. He has been involved in the design of substations ranging from 12.47 kV to 500 kV. His responsibilities include both physical substation design and protection and control design. He has a BSEE degree from Kansas State University. kodle@burnsmcd.com

Equipment Selections
Capacitor bank	Cooper Power Systems
145-kV synch-close breaker	Mitsubishi
Two 145-kV line breakers	Siemens
One 245-kV center break switch	USCO
Two 115-kV center break switches	USCO
Three 115-kV, 250A, 750uH, 15KA reactors	Phoenix Electric Corp.
Two 1200A line wave traps	Trench
Five 115-kV CCVTs	Trench
Three 90-kV/74kV MCOV station arresters	Cooper Power Systems