Light rail trains transport passengers through downtown Minneapolis. Adjacent to the tracks, an underground vault houses four 115/13.8-kV transformers, which power 70% of the downtown.
Xcel Energy installed the four transformers between 1961 and 1986. Last October, Transformer 4 failed due to an internal short, violently shifting the coils and creating a slight bulge on the side of a steel tank. The fully dressed transformer, which weighed about 220,000 lb, moved 2 inches across the floor. Because of the redundancies built into the substation, no customers lost power.
At this point, Xcel Energy had no other choice but to replace the transformer. The new custom-built transformer took seven months to design and manufacture. Because the transformer was located underground and adjacent to the light rail tracks, Xcel Energy used this time to devise and execute a plan for the project that would minimize impact to train operations.
Shutting Down the Train
During the project, Xcel Energy had to find a way to install a new 180,000-lb transformer from Waukesha Electric in the underground vault. The vault, located in front of the company's corporate headquarters, is covered with six panels measuring 6 ft by 25 ft by 2 ft and weighing 42,000 lb each, waterproofing, 4 ft of sand and a sidewalk.
To perform routine maintenance, Xcel Energy's linemen can enter the vault through the basement of the office building. The opening is large enough for the workers to maneuver a man lift into the vault. Replacing a transformer, however, presents a completely different set of challenges.
Eighteen years after Xcel Energy installed Transformer 4 in the underground vault, Metro Transit built the light rail line through downtown. As a result, Xcel Energy had to work with both the transit team and the city of Minneapolis to shut down the train during the construction project.
At first, the utility, the transit agency and the city of Minneapolis expected the train to be shut down for a week. Through teamwork, project management, and advanced planning, the construction crews only had to shut down the train for 32 hours.
Complicating matters, the utility had to find a weekend when the Minnesota Twins weren't playing at Target Field, the downtown baseball stadium. Because the transformer had to be custom built, the utility had to wait until after June 15, and it only had three weekends in July and August from which to choose. The utility then scheduled the transformer replacement for the weekend of July 8.
Replacing the Transformer
A few weeks before the project, Xcel Energy brought in a crew to demolish a portion of the sidewalk and block off one lane of the street. In addition, the company had to identify and mark underground utilities and make sure a crane was available on the weekend of the project.
On the morning of Friday, July 8, Metro Transit closed down the light rail at 10 a.m. and shifted passengers to buses. It took about five hours for Metro Transit to remove the light rail wires. Then the crews constructed a crane pad, which consisted of sand and polymer mats over a weight-sensitive area on the track. This helped to distribute the weight as the heavy equipment drove over it.
The construction crews then removed the roof panels, lifted up the old transformer and placed it on the sidewalk. The workers then lifted the new transformer off the trailer and placed it into the vault. Afterwards, they picked the old transformer off the sidewalk and placed it on the trailer to be hauled away. These moves were carefully choreographed to reduce the duration of the work.
Working Down Inside the Vault
When the transformer was successfully lowered down into the vault, the workers focused on dressing the transformer by installing the insulator bushings, filling it with oil and going through the moisture removal procedure. In addition, they had to wire the transformer into the control system. Because it was underground, the fire control system had to be re-piped as well as the cooling system.
While most transformers are air-cooled, the new transformer is water-cooled. In addition, the new transformer is also larger than the former one. When the workers lifted the new transformer in place, there were only inches to spare at the end of the transformer. As a result, no more than one crew could work down in the underground vault at a time.
Due to the limited space, wiremen also had to wire the control cabinet on to the transformer. Normally, the wiring can be done at the factory, but in order for the new transformer to fit inside the existing vault, it had to be done on site. In addition, the team also had to work on the electrical and control connections as well as the water control and fire-suppression piping.
Handling Safety Concerns
While working both above ground and underground, Xcel and its contractors focused on safety. As part of the transformer replacement, the utility reviewed historic photos to determine potential safety challenges. During the initial phases leading up to the weekend, Xcel Energy held safety meetings to clearly outline what activities were going to take place. One of the key items of safety was fall protection and safety in the work zone.
The company's safety concerns included fall hazards when the vault roof panels were removed. To address this hazard, the utility had to determine appropriate anchorage points to attach a retractable harness system. When standing on the roof panels, the workers had a retractable cable system so they could tie off and freely walk around.
In addition to fall protection, the workers also had to take care when working within feet of the active light rail. To ensure safety on the job site, all workers completed safety training by Metro Transit Light Rail staff before accessing the work zone. Xcel Energy also conducted daily light trail safety briefs to review light rail protocol and construction activities that would occur next to the light rail. One of the key strategies to keep the crews safe was having a spotter, which was provided by Metro Transit. This person would call a break when the train would pass, and then everyone could get back to work.
Other precautions were the usage of high-visibility clothing and other personal protective equipment. Minneapolis police officers provided security to keep onlookers from getting too close to or passing through the construction zone during the weekend work. It also provided workers the ability to focus on the work at hand.
Restoring the Site
It took two-and-a-half months for Xcel Energy and its contractors — Carl Bolander & Sons, C25 Construction, Armstrong Crane and Ulteig Engineering — to do the site demolition, site restoration and energize the transformer.
After the workers successfully installed the transformer inside the vault, they had to put the roof back on, waterproof it and then backfill it. As part of that process, the workers added a foot of concrete on top of the waterproofing to act as a protective layer. They backfilled the vault to the sidewalk elevation, repaired the roadway and replaced the sidewalk.
While the project faced a lot of challenges, Xcel Energy was able to overcome the obstacles and adhere to a tight construction schedule thanks to good planning and a cooperative working relationship with Metro Transit and the city of Minneapolis. When the utility has to perform a transformer replacement project in the future, it will have the knowledge, experience and support to get the job done quickly and safely.
Grant Stevenson is a senior project manager for Xcel Energy and has been with the company since 1984. Stevenson is a Project Management Institute-certified project management professional.
Joe Samuel is a senior project manager for Xcel Energy and has been with the company for eight months. Before joining the utility, he worked in the civil engineering industry and worked for a variety of municipalities and private developers. He is a professional engineer.
Editor's note: To view 32 hours of work in 3.2 minutes, please visit http://tinyurl.com/Xcel5thStreet for a time-lapse video.
Armstrong Crane and Rigging Corp. www.armstrongcrane-rigging.com
Carl Bolander & Sons www.bolander.com
C25 Construction, LLC www.construction25.com
Ulteig Engineering www.ulteig.com
Waukesha Electric www.waukeshaelectric.com
Xcel Energy www.xcelenergy.com