AltaLink's transmission lines are loaded to near capacity, increasing the risk for conductor failures. Proactively ensuring the integrity of compression fittings will reduce the number of occurrences in the future.
Like other utilities across North America, some of the company's infrastructure is aging and nearing the end of its life cycle. As more load is placed on these already strained lines, asset management is starting to notice a pattern when it comes to sleeve conductor failures.
These failures stem from a variety of reasons, but in many cases, the sleeves were not installed correctly. For example, the steel conductor wasn't properly inserted into the inner sleeve or a lack of inhibitor allowed moisture to get into the steel strands, causing a premature breakdown.
A compression joint failure's financial impact is unpredictable. It can range from thousands of dollars to tens of thousands of dollars and beyond. As such, it is critically important to develop and implement proper quality-assurance procedures. When a utility partners with contractors, this issue becomes even more paramount due to the number of external workers performing work on a facility. The specific work experience of these individuals is not known and needs to be monitored accordingly.
Managing Rapid Growth
AltaLink services about 85% of Albertans and more than half of Alberta's transmission grid, or about 12,000 km of transmission lines and nearly 300 substations. AltaLink's service territory is diverse. The facility winds through the Rocky Mountains, over the flat prairie and across heavily treed geography. Alberta is experiencing rapid growth, and with an increase in residents moving into the province, the load increases by approximately 100,000 people per year.
To meet this demand, AltaLink is investing heavily in capital projects over the next five-plus years. In fact, the utility already has a 500-kV transmission line in development and is also installing numerous large double-circuit 240-kV lines and substations. Over the last decade, AltaLink has nearly tripled its staff members with no sign of slowing down. To take on this workload, the utility has contracted out most of the new construction. Line contractors across North America are being called upon to meet this work load demand.
To aid the growing need for power, AltaLink is interconnecting to wind farms and independent cogenerators. In the southern part of the province, wind farms are providing green energy to AltaLink's lines. With so many independent power producers interconnected, ensuring continual power flow and reliability is always a key focus for AltaLink.
Safety and reliability is on the forefront of AltaLink's operating practice. A result of extreme weather and loading is also placing increased wear on the utility's infrastructure, bringing the demand for quality control and quality assurance to the forefront.
Inspecting the Conductor
In the past, AltaLink's technicians used gamma radiography to inspect the inner portion of compression fittings, but this method was more cumbersome and less user friendly.
Using this approach, technicians would hang the 100-lb machine on the wire. Because it had an active iridium source and a longer potential exposure time to radiation, the field workforce had to follow strict guidelines set out by the Canadian Nuclear Safety Commission. When the machine completed the procedure, it produced a film that had to be exposed and then developed in a water solution. Later, the AltaLink staff would use a high-intensity viewer to determine whether or not the image was successful.
They would then need to store the X-ray films. The films were bulky, making storing them and sharing them with colleagues cumbersome and difficult.
AltaLink also used a digital micrometer to measure the inner and outer components on the sleeves. Using this method, they could ensure the sleeve fell within the manufacturer's tolerances for minimum and maximum compression.
While the micrometer could be configured to work within the air, it was difficult to get a consistent reading when working aloft and in frigid temperatures.
Another option was the go/no-go gauge, but this didn't provide the crews with all the compression readings required. They could see a minimum compression, but in order to be effective, the gauge needed to show both a minimum and maximum compression for inner and outer sleeves.
Discovering an X-ray Solution
The utility then came across Vidisco's digital X-ray system, which is used in Alberta's oil field industry. The system weighs about 40 lb and can be used on almost any site. This process requires three trained workers in the field: one on the ground and one up in the air with a qualified utility worker.
This X-ray machine, which produces an image similar to one in a hospital, uses an X-ray tube. The tube produces X-radiation. This radiation is absorbed on the imager screen. The digital image is sent wirelessly to a laptop on the ground, which has the supplied viewer software. The technicians can then manipulate the image with numerous viewing tools. The quality of the image can be determined instantly. The grey scale and polarity can be adjusted to refine the resolution. By manipulating the image in this way, materials of different density can be examined very clearly.
After the X-ray of the fittings is complete, the images can be sent via e-mail. The entire process from shot to laptop happens remotely and in seconds.
With gamma radiography, the workers had to spend 40 to 55 minutes to develop the image to see if it was of good quality. With the digital X-ray technology, however, this can be determined immediately, and if it's not the right resolution, the process can be repeated quickly. Using the supplied viewer, the technicians have the ability to use the measuring tool to take measurements of various parts of the sleeves' internal sections to ensure tolerances are met.
If one of the technicians has a concern about how an X-ray looks, he or she can e-mail it to an operational engineer for further interpretation. Because the laptops are equipped with air cards, it can bring all office resources out to the field.
The company contracts out its digital X-rays to an external vendor, but AltaLink team members try to be on site to interpret and decipher the X-rays. That way, one can determine a pass/fail on the compression sleeve. The skill set in deciphering these images is continually getting better with time and experience.
In most cases, AltaLink requests a quality-control/quality-assurance staff member be on site when the critical splicing is to be done. Splicing has been identified as a critical part of the project. As a result of this stringent quality-assurance procedure, AltaLink facility specialists have rejected 10 sleeves because of improper installation.
Identifying Problem Sleeves
Before changing over to this method, AltaLink first performed a trial with some compression sleeves. The utility physically prepared sleeves with common deficiencies and discovered that the problems show up quite clearly. AltaLink wanted to build its knowledge base of what actual deficiencies look like when viewed as an X-ray.
AltaLink insists on certain quality-control standards with its compression fittings. For example, 90% of the sleeve needs to contain the conductor, a void in Penatrox needs to be less than 12 mm, crimps are to be overlapped by 5%, and a space is required between the steel inner and the aluminum stranding. It has been documented that the sleeves are either squeezed incorrectly or the steel inner is not inserted properly inside the inner sleeve. Another workmanship installation error occurs when the aluminum outer is not centered over the steel inner. This results in the mechanical and electrical integrity of the fitting to be less than that of its designed specification.
Another widespread problem is a lack of Penatrox. If there is lack of inhibitor, the conductor eventually will generate excess heat between the aluminum outer sleeve interface with aluminum stranding causing unwanted heat and a premature breakdown.
Technicians wanted to see if they could catch these installation errors with the viewer. All deficiencies on the sleeves and dead ends could be identified internally using the digital X-ray image.
Rolling the Method to the Field
AltaLink has adopted digital X-ray technology as its preferred method of X-ray inspection because of the ease of storing and sharing images and the instant response. Getting a second opinion for clarity is just a click away.
The company is using implode sleeves but is not required to X-ray these types of sleeves. In all cases, the technicians can tell whether or not they have been done correctly just through a visual inspection.
At AltaLink, the company's current standard is to X-ray all full-tension, self-dampening compressions and any common conductor 1033 mcm and larger.
While the technology is available, AltaLink is reluctant to X-ray the compression sleeves while they are energized. A company in China, however, has an imaging plate that allows utilities to perform X-rays while they are energized and then remove the plate from the live line to view the image. A trial has yet to be done with this particular digital system.
AltaLink is currently using the digital X-rays on new construction; that way, they know the integrity of all the sleeves, and they can retain them on the line file and noted location on their as-built drawings. The company is also experimenting with taking pictures of polymer insulators and porcelain units to check for deterioration in efforts to assess the longevity of these attachments. AltaLink is also trying to get underground cable sleeves X-rayed to see if installation errors and deteriorating sleeves can be identified.
AltaLink is striving to be an industry leader when it comes to building and maintaining lines. By being proactive and doing due diligence, AltaLink is able to ensure the quality of lines being built today.
Darryl Rempel (firstname.lastname@example.org) is a transmission facilities specialist with AltaLink. He started in the line trade with Manitoba Hydro and then moved to Alberta, where he worked within a small group tasked with line inspections, facility acceptance and quality assurance. He then became a technical line specialist with the asset management group.
AltaLink | www.AltaLink.ca
Vidisco | www.vidisco.com