Often before a power cable goes, it gives off a few subtle signs of distress. Unfortunately, many critical distribution cables are underground, which makes them difficult for people to access and monitor. But now a new cable-crawling robot, developed by researchers at the University of Washington (UW), Seattle, could provide much-needed insight into the health of subterranean power systems.
“Monitoring cable systems is one of the holy grails of the electricity industry,” says Don Von Dollen, program manager for the IntelliGrid Program at the Electric Power Research Institute (EPRI), in Palo Alto, CA. “When you get a cable failure, it’s a real pain to find it, dig it up, and fix it. Coming up with good diagnostics has been a longtime challenge, and it’s a tough nut to crack.”
For decades, researchers and utilities have been working on various ways to monitor power grids. A traditional method, which has been used for 50 years, is called a high-potential test, says Von Dollen. “You basically disconnect the cable and send a big voltage spike across it,” he says. “If there are any problems, this is going to cause the cable to fail.” It’s a brute-force method, he says, but if the cable fails, at least it’s in a controlled setting. More recently, people have used radar to detect malfunctions.
But these methods require a fair amount of human interaction. The UW researchers approached the challenge by designing a robot that can autonomously traverse underground cables buried in pipes and tunnels. The robot, which rolls along on small neoprene wheels and is powered by a battery pack, hugs the cable tightly as its three onboard sensors scan for signs of wear and tear. Only about 10 percent of underground cables are found in pipes or tunnels (the rest are buried directly in the ground). But these cables are often the ones that “experience unexpected conditions” such as water drips, says Alexander Mamishev, professor of electrical engineering at UW and project leader, which makes them more susceptible to failure.
Monitoring these underground power systems is a two-part problem, Mamishev says. First, the terrain is difficult for a robot to navigate autonomously. Miles of cables consist of twists, turns, brackets, and overhangs that can impede progress. These considerations were integral to the design, he says. The robot has a gyroscope to help maintain its balance and stabilizing arms to help right it if it slides off track. The robot is built in segments, somewhat like a train with multiple cars, and it sits three inches above a cable. One segment is devoted to the robot control, and the other one houses the sensors and data-processing units.
The second challenge with monitoring power cables involves the type of sensor that’s used. Mamishev suspects that a combination of sensors can find problems in cables better than one single method can. Therefore, his team’s robot includes a thermal sensor to locate hot spots; an acoustic sensor to locate the quiet snap, crackle, and pop of sparks from a partial electric discharge; and a dielectric sensor to measure the presence of moisture that may have seeped into the insulation. In addition, the robot has a video camera so that its progress can be monitored remotely.
Mamishev and his team recently took the robot to New Orleans to test it at Lockheed Martin’s Michoud NASA Assembly Facility. The goal was to find damage that could have been caused by Hurricane Katrina, as salt water from the floods can seep into cable insulation. They did not find damage in the facility’s wires. However, this first field test was a good proof of concept, Mamishev says, and the robot successfully navigated three miles of cable.
The researchers’ “technology looks very exciting and promising,” says Dave Hawkins, project manager for the integration of renewable resources at the California Independent System Operator, a nonprofit organization that manages the majority of the state’s high-voltage power grid. The robot could “improve electrical reliability by identifying cables that have reached the end of their useful life,” he says. “By forecasting potential failures, cables can be removed from service in a planned way,” saving time and money.
EPRI’s Von Dollen says the research is on the right track to tackle part of the industry’s cable-monitoring problem. “It’s an interesting approach,” he says. However, because the robot sits a few inches taller than the pipes, it might have a difficult time navigating the tight spaces in the ducts and pipes where some cables are housed.
Mamishev says that the robot has the potential to shrink down to a height of about one inch, but no thinner. Yet even at three inches, it can still crawl unimpeded along a number of underground cable systems, he says.
In the coming months, the researchers plan to replace the current rigid stabilizers with new ones that are flexible and adaptable. Then, if the robot encounters a tight space, the arms can bend out of the way; if it tips, the arms can right the robot more precisely. In addition, Mamishev will add more batteries, perhaps by adding more segments to the robot train.
The robots have the potential to be commercialized relatively soon, he says. However, the timeline would completely depend on the interest of commercial partners. “If we had good partners,” he says, “in one year we could offer a cable-checking service.”
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