Workers drive in helical piles to augment a 69-kV tower foundation at Henry substation. Helical piles were the only way to go on this project. The work had to be completed in tight quarters, on a short deadline and while the substation was still energized.
Sometimes things do not go as planned. Northeast Missouri Electric Power Cooperative (Northeast Power) can attest to that. In 2011, it became clear the coop needed to rebuild most of its Henry substation, but it could only schedule a 30-day outage to do so.
To get the project done within the limited outage time, the coop had to do as much work as possible prior to the outage. One of the biggest challenges was finding a way to upgrade eight 69-kV foundations while the substation was still energized. Dealing with planning changes, downpours, late frosts and a transformer failure made the job all the more difficult.
Scope and Challenges
Northeast Power is a generation and transmission cooperative, serving eight member-distribution cooperatives in northeast Missouri and southeast Iowa. It has 56 employees. The coop’s Henry substation is located in a critical growth area where voltage sag is a problem. Two 161-kV lines, from ITC Midwest, LLC, power the substation and its single 56-MVA transformer. Three 69-kV feeders exit the substation, but there are eight 69-kV deadend structures: one feeds a capacitor bank, one is a transformer low-side breaker bay and the others are for future expansion.
The substation was built in the 1980s and the transformer was 1973 vintage. System studies showed if that transformer failed, voltage regulation in the area could be a problem. In the existing system configuration, Northeast Power needed a second transformer at the substation. Plans were made to add another transformer and install heavier 69-kV conductors, an additional static wire and flying taps. Thus, the 69-kV substation structure towers would have to support a greater load and increased over-turning moment. When the transformer was added, the towers would have to be upgraded, as well.
With an idea of what the scope of the work would be, the next step was to schedule an outage. Because the substation is in a critical area, the coop could only get an outage when the load was low — in the spring or fall. Northeast Power decided to perform the work in the spring of 2013 and arranged for an outage with ITC Midwest and the Midcontinent Independent System Operator (MISO). A significant amount of coordination and planning was needed to schedule the outage, so other substation improvements also were planned.
ITC Midwest’s long-term plans included increasing line tensions, which would increase the load on the 161-kV tower. Therefore, Northeast Power planned to replace it, as well.
Get Ready, Get Set
Northeast Power hired V&S Schuler to design and manufacture the three-bay 161-kV structure and one of the 69-kV towers. CLC Engineering was retained to design the other seven 69-kV towers, which were fabricated by Lehigh Utility Associates Inc.
CLC Engineering designed new single-column steel cantilever towers with one further enhancement. To help speed up the rebuild work, the new towers were designed to handle mechanical load coming from only one side, with no load on the other side. This was recommended so that when workers transferred the lines to the new towers during the scheduled outage, the towers would not be overstressed. The stronger towers also would be able to withstand additional mechanical loads Northeast Power might add in the future.
The existing aluminum lattice towers had a 3-ft by 3-ft (0.9-m by 0.9-m) base plate. The new towers had a smaller 1.5-ft by 1.5 ft (0.5-m by 0.5-m) base plate. However, the existing concrete foundations were not adequate for the new structures and loadings. This put Northeast Power’s engineers in a difficult position. Thirty days was not enough time to remove the 69-kV towers, tear out the existing foundations, pour new foundations and set the new towers. An alternative solution was required.
CLC Engineering suggested the foundations could be augmented and helical piles could be used to support the new foundation sections. Helical piles can be drilled into the soil with a drive motor, and they are faster and easier to install than a traditional poured-concrete foundation. This seemed like a good solution. More importantly, it was the only viable solution, given the scope of work and time constraints.
However, Henry substation is not a big, spacious substation. Workers could not get drilling equipment into the south side of the 69-kV foundations because of energized bus work and underground duct work. The only option was to extend the foundations to the east, west and north.
While Northeast Power was designing the augmented foundations, it received news from one of its distribution cooperatives that a new fertilizer plant was being constructed in its territory, and the plant would add 35 MW of system load. The utility decided that system stability made installation of a transformer close to the new load the best solution. This was done and allowed the installation of a second transformer at Henry substation to be deferred. The second transformer at the Henry substation is now scheduled for a 2020 installation.