When the Jackson Energy Authority (JEA) needed to expand the number of circuits exiting from a substation, it found its options constrained by an extremely narrow right of way (ROW). Using a spacer cable system, the utility was able to more than double its capacity in the same space as the conventionally wired system. Especially for substation exits, spacer cable systems offer a variety of benefits, including fewer and shorter poles; less real estate, which can be difficult and costly to obtain; no underground residential distribution (URD) risers to get outside the substation's fence; greater reliability than bare exits; and reduced cost, often less than bare wire when taking total circumstances into account.
Growing Municipal Utility Sought Options for Expanding Substation
JEA is a municipal utility located in Jackson, Tenn., between Nashville and Memphis. One of the few public utilities in the U.S. offering customers all major utility services from one company, JEA provides electric, gas, propane, water, wastewater and even broadband services to some 40,000 residences, businesses and industrial facilities in Jackson and parts of Madison County.
In response to community growth, JEA needed to expand its single bank Moize Creek substation with an additional transformer bank. The utility faced some significant restrictions regarding available easements for placement of additional circuits and wires exiting the substation. It conducted a detailed cost estimate of a range of possible options for converting a double-circuit, bare-wire route to a configuration, allowing five circuits using the same easement.
One option considered for the Moize Creek feeder project was placing five circuits on two different routes, which would require relocating the existing circuits to accommodate an additional triple-circuit, bare-wire route with the same easement. This option would reuse existing wire, crossarms and pins. Another option was to place the five circuits on one route, which would entail replacing poles with much taller ones and converting to a five-circuit, bare-wire configuration. This option also called for reuse of wire, crossarms and pins. These options were ultimately deemed infeasible, given the limited ROW available and their lack of adherence with utility best practices. A third option was placing the five circuits using a spacer cable system, which would mean removing the old pole line and constructing new spacer circuits, with no material reuse. Finally, JEA evaluated an underground cable option, in which the two existing circuits would be left in place and three new underground circuits installed.
"We were in a tight spot and essentially landlocked because the route out to the roadway runs along a narrow sliver of land between an apartment building and Moize Creek," said William Gordon, JEA's electric project engineer. "The easement just barely accommodated the existing circuits, but the other options we looked at using conventional circuits were otherwise deemed not feasible or too costly."
Spacer Cable Selected as Best Option for Narrow Corridor
After conducting the analysis of options, JEA decided the most cost-effective choice would be to install spacer cable systems, supplied by Hendrix Aerial Cable Systems. The spacer cable's compact nature allowed JEA to use its existing corridor while adding protection from nearby trees and animal damage.
"Bare wire requires considerable space between wires for personnel safety and protection against varmints. With spacer cable, lines are insulated and can be placed in closer proximity. We decided that spacer cable seemed the most efficient and practical solution," Gordon said.
Spacer Cable System Basics
Spacer cable systems have been used by utilities for many years to improve the reliability and power quality of primary distribution systems while making them more resistant to storm damage. The compact configuration and reduced clearance requirements allow for multiple circuits to be installed on a single pole, thus combating ROW problems and saving money by reducing the number of poles required. This is the feature that most appealed to JEA. Over- or under-building spacer cable systems in substation exits can greatly reduce the cost of adding capacity.
A spacer cable system consists of a messenger cable that supports the structure, polyethylene spacers to hold the cable across spans and covered conductor cable.
The messenger cable is the support member for the structure and serves as system neutral and lightning shield. Clipped to the top of the messenger wire is a series of spacers, which support, separate and clamp the phase conductors in a triangular, diamond-like configuration.
The spacers are molded using a proprietary, gray, track resistant, high-density polyethylene and can be quickly and easily installed and removed. The spacers have very good weather washing characteristics, and their long leakage distance resists flashovers. They are highly resistant to shock, impact or rifle fire, and can be installed with hot line tools.
The individual cables used in the spacer cable system contain a three-layer covering, which can withstand temporary contact with tree branches and other vegetation, therefore reducing outages and improving power quality. The covering greatly reduces the need for vegetation removal during circuit installation and protects wildlife from exposure to lethal currents.
Advantages of Spacer Cable at Substations
"Spacer cable has some clear advantages over bare wire circuits at substations, which made it an ideal solution for JEA," said Bob Biddle, Hendrix's national sales manager. "The spacer cable systems need fewer and shorter poles for multiple circuits, whether building new or adding circuits and can under build the transmission lines that come into the substation. The reduced number of poles uses less real estate, which is a real plus," Biddle said. "Real estate for this purpose can be difficult to come by and utilities have to spend large sums for sites or additional real estate at existing sites."
No underground residential distribution (URD) risers are needed to get outside the substation's fence. If exits are bare wire, many times the section from the bus work to the fence must be URD because of bare wire reliability issues associated with close spacing at the bus. Spacer cable systems are also much more reliable than bare exits; many outages occur at substations and affect a larger number of customers.
"Many utilities are forced to spend a great deal of money to animal-proof bushings, bus work and even substation fences to keep animals out," Biddle said.
Last but not least, spacer cable systems can save significantly on costs. They often cost less than bare wire, when taking total circumstances into account, and offer far lower costs than URD.
Implementation Overcomes Construction Concerns
After JEA selected the spacer cable system as the preferred option, Hendrix was brought in to help design the system, which consists of 1,600 feet of cable for five circuits, equaling 8,000 circuit feet. The wire used was 336 kcmil (1,000 circular mils), 15 kilovolt conductor. The project also entailed the removal of two existing bare wire circuits, the addition or changing of poles and anchors, and installation of five spacer circuits for the new expansion.
Project installation went very well, though there was some initial concern from JEA construction crews that the new system would prove more difficult to install than conventional bare wire circuits. A single JEA construction crew performed the installation of the spacer cable system, while at some point all members of the entire electric department came to observe the installation or lend a helping hand to the crew.
"I give a lot of credit to Hendrix for how well the installation went," Gordon said. "They were there with us every step of the way to ease the crew's concerns. They answered all our questions prior to installation, provided assistance with a pole analysis to make sure the poles met loading requirements, sent installation videos and manuals, and came to the site to give the crew guidance the day of the installations. Actually, the word from the crews was that the installation of the unfamiliar product was not at all difficult, and we owe that to Hendrix's diligence. With their assistance we were able to transform the two-circuit, conventionally constructed route to a five-circuit one, more than doubling our capacity."
"JEA had contemplated spacer cable for some time and thought this project was a good location to install its first project, mainly because there are no customers along the express route. We knew there would be additional training necessary to be able to splice into the spacer wire for taps and transformers, and these express runs allowed a nice, easy installation without the complication of keeping customers online," Biddle said. "JEA reports that it is spending this year watching and evaluating the spacer cable system through all the seasonal changes it will encounter before feeling comfortable with expanding to other locations."
"Right now we are reviewing the spacer system to see how it holds up to trees, varmints, weather and lighting. So far, it is going great," Gordon said. Gordon admitted to some nervousness immediately after the installation when western Tennessee endured record-setting cold temperatures. The system performed extremely well during the entire three weeks when temperatures did not rise above freezing.
Other utilities have expressed an interest in going to the site, looking at the system and considering its use for substation exits where space is tight.