Georgia Transmission takes an inside look at compression-sleeve installations during construction.
When it comes to electric reliability, consumers don't think much about it. More often, they expect it. And these days, consumers have more reasons than ever to rely on a constant and extremely reliable flow of power.
At Georgia Transmission Corp. (GTC), which builds and maintains the high-voltage power infrastructure for 39 of Gerogia's 42 electric membership cooperatives, new technologies and the grid have a symbiotic relationship. GTC employs high-tech x-ray equipment to evaluate the integrity of compression splices and deadend fittings on new transmission projects. This inside look at key conductor connections is GTC's latest initiative to improve the reliability of the grid by reducing the margin for error during construction.
Inspection Measures Taken to New Heights
At the turn of the century, there were numerous upgrades on the horizon for Georgia's transmission infrastructure. Though processes were in place, the size and scope of upcoming projects would require rigorous standards to ensure the soundness of the new lines and towers before they were energized.
Previously, GTC had developed a construction notebook for all new projects. The notebook, which is maintained by the on-site contractor, serves as a detailed record of the project. Inspection activities and notes about the construction are documented and ultimately scanned in as part of the corporate record.
The most recent addition to the notebook is a checklist articulating the process for compression sleeve and deadend fitting installations. The checklist states GTC's expectations and requires the name and signature of the worker and the date of the installation. It also holds the contractor responsible for the proper installation of compression sleeves and deadend fittings. Contractors must immediately replace sleeves that fail testing or inspection.
In GTC's experience, the majority of compression-sleeve and deadend-fitting failures are due to improper installation. Compression sleeves are the connection between the end of the conductor from one reel and the beginning of another. These splices are installed about every 2 miles (3 km) along the line route. The sleeves have steel and aluminum components. The steel core of the conductor is exposed, cleaned and inserted into the barrel of the steel sleeve. It is then compressed with a custom-sized die, starting with the knurl mark in the center of the sleeve and moving outward. An aluminum outer sleeve — slid onto the conductor before the steel sleeve is installed — is similarly pressed on to complete the installation.
A similar process is used on deadend fittings. The compression sleeve functions as both a mechanical and electrical device. Proper installation is crucial to both the physical stability and electric reliability of the line.
Failure Leads to a New Procedure
In late 2007, a 25-mile (40-km) 230-kV line experienced a mechanical failure due to an improperly installed compression sleeve. The line was originally constructed in 2003. Once the issue was identified, GTC consulted with Haverfield Aviation Inc., a company that conducts comprehensive inspections; Alcoa Inc., the sleeve manufacturer; and the National Electric Energy Testing and Research Application Center (NEETRAC). Based on these discussions, GTC decided a more rigorous sleeve inspection program was needed.
Haverfield has used x-ray equipment on transmission lines for more than a decade and pioneered the use of it by helicopter. In early 2008, Haverfield flew the length of the suspect line to inspect all the compression splices with x-ray equipment. Splices that exhibited the potential for mechanical failure were replaced by the original construction contractor.
This line component failure impacted the construction in 2009 of GTC's Thomson-Warthen project, a 39-mile (63-km) 500-kV line in east-central Georgia. It was proposed that a comprehensive x-ray inspection for splices and deadends be implemented as part of the new line construction. GTC approaches inspections with the philosophy of “trust but verify” — using inspections to validate that the work meets standards. All involved knew the added level of inspection would increase contractor accountability and final product integrity.
X-ray Inspection in Action
During construction, every compression sleeve on the 39-mile line was x-rayed for integrity. At each splice and deadend fitting, Haverfield aerial lineman Paul Trenton x-rayed the sleeve with a 150-kVp machine manufactured by Golden Engineering. A specially trained technician, Trenton operates the equipment according to Haverfield's robust radiation safety policy and applicable regulatory compliance. With a Logos Imaging Inc. digital processing unit containing phosphorous plates, a laser scans the image, digitizing it so it can be downloaded to a field computer. It is labeled with the date, location and diameter of the sleeve, as well as the name of the installer and the date of installation. Image processing takes approximately four minutes.
The inspection team was then able to evaluate each splice on site. When uncertain about an issue arising from the x-ray, the image was sent to Alcoa for an expert opinion. At the peak of construction, Trenton produced 35 to 50 images a day.
X-rays can reveal a number of failures, both mechanical and electrical. For example, if the steel core is not fully inserted into the compression sleeve, the gap between the conductor ends can result in an electrical failure from insufficient surface area to carry the amperage. Errant strands of conductor wire outside of the core grip have the potential to interrupt the flow of power through the conductor. X-rays also can show an angled conductor. During the installation process, pressure applied to insert the conductor in the sleeve, if not applied in the correct way, can cause the conductor to twist. Though a ridge on the exterior may be visible to the naked eye, only the x-ray can conclusively determine if the angled conductor will be a problem.
The finished sleeve diameter is measured using a caliper and recorded on the x-ray image. The proper diameter indicates the sleeve was compressed at the appropriate pressure. The conductors in this instance required a 60-ton (54-metric ton) press that must reach a pressure of 10,000 psi. GTC also performed subsequent checks on the press using its own pressure gauge to ensure it maintained adequate pressure during operation.
In the 39 miles of line, 409 compression splices and deadend fittings underwent x-ray inspection. The quality construction by Irby Construction, the primary construction contractor on the project, was reflected by the fact that just one splice had to be replaced. In addition to boosting the reliability of a line, on-site corrections ultimately represent a cost savings. Replacing improperly installed sleeves and deadend fittings during construction costs far less than after the line is energized.
An Inside-Out Approach
Few issues can remain undetected in the x-ray inspection. But even with the most advanced technology, a qualified on-site inspector is still needed — and so is a comprehensive strategy. For GTC, that strategy begins with stringent standards for contractors, a thorough documentation of line construction and an emphasis on the accountability of those working on its projects. GTC follows through on that strategy with an x-ray inspection of every compression splice and deadend fitting in the line and a comprehensive third-party aerial inspection by one of the industry's premier inspectors.
With every step, the construction and inspection teams work to reduce the margin of error. With increasing precision, they are accomplishing their goal. The tools put in place verify that when a line is cleared to be energized, it was built correctly. This sets a benchmark for ongoing maintenance on the line once it goes into service. When an issue arises, the origin of its construction — from the date to the responsible party — is clearly documented and attributed.
The GTC maintenance team, in fact, has begun adopting the technology for selective inspection of existing lines. GTC's inside-out approach to inspection is a comprehensive process. It's the inspection team's version of signed, sealed and delivered.
Jay M. Horsley (firstname.lastname@example.org) brings nearly 30 years of industry experience to his role as the manager of construction inspection for Georgia Transmission Corp. He started his career at Oglethorpe Power in 1988. Initially, he performed substation and transmission line inspection services, which prepared him for becoming lead construction coordinator and, subsequently, manager of construction inspection. His current duties include providing functional management services for the construction inspection department, which includes ensuring quality control and quality assurance on all GTC capital projects.
Gregory Starks (email@example.com) is a project manager for bulk transmission line projects for Georgia Transmission Corp. His 30 years of industry experience ranges from substation design for both transmission and distribution to construction and project management. For the last decade, Starks has overseen substations and transmission projects ranging from 115 kV to 500 kV. Starks graduated from the Georgia Institute of Technology with a BSEE degree. He is a member of the IEEE and is a registered professional engineer.
Failure to achieve adequate penetration of the conductors into the sleeve is the most common issue identified by the x-ray inspection. The two x-ray images above show this issue. In the top photo, the right conductor appears to be at the edge of the steel core. In the center image, the left conductor is almost at the edge of the core. These issues could result in a future failure. Both sleeves were replaced after x-rays were reviewed in the field.
What X-rays Reveal
The bottom image reflects a properly installed compression splice. Both conductors were inserted correctly, are well aligned in the sleeve and have adequate contact with the steel core to sustain the amperage that will flow through the line.
Alcoa Inc. www.alcoa.com
Companies mentioned in this article:
Georgia Transmission Corp. (GTC) www.gatrans.com
Golden Engineering www.goldenengineering.com
Haverfield Aviation Inc. www.haverfield.com
Irby Construction www.occusoft.com
Logos Imaging Inc. www.logosimaging.com
National Electric Energy Testing and Research Application Center www.neetrac.gatech.edu