By Michael Valenti, Senior Editor

The electric power industry has gotten together with instrumentation companies to develop technologies that will assist utilities in maintaining their transmission lines, whether buried underground or strung overhead.

The challenge in maintaining or laying electric, gas, or steam transmission lines under city streets is particularly difficult. Work crews must often dig their way through a bewildering array of utility networks built up over the years as urban districts grew and changed. Often no single map shows their location. Inadvertently damaging these lines is costly and, in the case of gas or steam, hazardous as well.

The Electric Power Research Institute in Palo Alto, Calif., is trying to take the guesswork out of utility excavation with a ground-penetrating imaging radar system. EPRI developed the system with Witten Technologies in Washington, Mala Geoscience in Mala, Sweden, and Schlumberger-Doll Research of Ridgefield, Conn. The system creates three-dimensional images of underground features. The Gas Institute of Chicago supported the creation of the new radar system, which can be used to create up-to-date maps of the subsurface down to 10 feet, or 3 meters.

Ground-penetrating radar uses radio waves to detect objects buried in nonmetallic material. The technology was first used in Austria in 1929, five years before the term "radar"—from "radio detection and ranging"—was even coined. Its first use was to sound the depths of a glacier without drilling.

Consolidated Edison construction workers were able to dig around buried utilities by using the 3-D ground-penetrating radar developed by EPRI, Witten Technologies, and Mala Geoscience.

For several decades, ground-penetrating radar systems were used mostly by universities conducting noninvasive subsurface research. All those systems involved emitting radar pulses into the earth over an area being studied, receiving the echoes reflected by buried objects or different strata, and using the signal's time of flight to calculate depth.

These systems were commercialized in 1972 to probe a variety of applications. Ground-penetrating radar has helped archeologists unearth artifacts, military engineers detect land mines, and construction crews map the location of reinforcing bar in roadbeds. Today, computers are incorporated to create images of subsurface cross sections as described by the radar echoes.

"About three years ago, EPRI was approached by its member utilities, asking us to develop ways to prevent underground accidents and reduce the costs of excavating, maintaining, and repairing gas and electric lines," recalled Ralph Bernstein, an electrical engineer and former technical leader of the radar project. "We determined the best way of addressing this concern was developing a device that combined proven ground-penetrating radar technology with 3-D data collection possessing precise positioning control and advanced imaging software."

EPRI sought the assistance of Witten Technologies, based on its experience in designing ground-penetrating radar imaging systems for utilities, and Mala Geoscience, which has been building radar systems for geophysical study since the 1930s.

According to Mike Oristaglio, a geophysicist and chief scientist at Witten, "We devised an array of nine transmitting antennas and eight receiver antennas, compared to a couple of transmitter and receiver antennas typically employed, to multiply measurements and create a more accurate underground image."

During field demonstrations held in the Bronx last December, the ground penetrating radar discovered additional trolley tracks that did not appear on Con Edison's maps.

The antennas are encased in a durable, wheeled plastic shell mounted on a trailer so that a vehicle can tow them over excavation sites at up to 2 kilometers per hour as close to the surface as possible. The antennas are grouped to take up only 2 meters in width, so that the system will not block traffic as it is driven through city streets.

In order to improve the software, Witten and Mala worked with Douglas Miller and Jakob Haldorsen, on leave from Schlumberger-Doll Research. They were instrumental in adapting the software Schlumberger had developed years earlier for seismic oil and gas exploration to focus the raw radar echoes and convert them into realistic 3-D images of underground objects.

"This is the same mathematical lens principle that is used in sonograms: By bouncing many radar pulses off different sections of objects, the computer will use the myriad echoes to create a high-resolution image of those objects," Oristaglio explained. In addition, the utility software ensures that the nine transmitting antennas are activated consecutively so each signal does not interfere with its fellows.

The original prototype of the radar system was developed in 1998, and a precommercial unit was tested by EPRI member utilities in New York City, Seattle, San Diego, Dallas, Paris, and Antalya, Turkey, during 1999 and 2000. Consolidated Edison Co. of New York tested a prototype last December on six contiguous blocks in the Bronx, to anticipate utility conflicts before a major roadway reconstruction was performed.

The Bronx as Proving Ground

The Bronx site was a fitting proving ground for the 3-D radar because it is congested with utilities, "not only our own electric and gas lines, but telephone lines, drains, water lines, and abandoned trolley tracks," said Len Toscano, a civil engineer and section manager at Consolidated Edison's Construction Management Group. Toscano explained that his department identifies buried utility infrastructures and tries to avoid damage to them by new construction in the city of New York. The task is complicated because older records of a site may be limited in detail, and many refer to curbs and buildings as points of reference that often have changed considerably.

Con Edison and Witten technicians set the ground-penetrating imaging radar, or GPIR, at 200 megahertz to capture several thousand square feet in a single image. By building layers of images, they compiled information for approximately 60,000 square feet of the Bronx site. "A key achievement of the GPIR was enabling us to effectively identify a specific type of utility, for example, oil-o-static lines," said Toscano.

Oil-o-static lines are buried high-voltage cables encased within an oil-filled steel pipe to cool and insulate them. "We were able to identify how the oil-o-static lines traverse through the job site, thanks to the information provided by the 3-D image."

Radar verified the location of some of the known utilities, and discovered "additional trolley tracks not listed on our maps," said Toscano, who was impressed by the system's ability to verify the position of plastic objects, both a polyvinyl chloride communication cable duct and a polyethylene gas pipe.

Con Edison continues to experiment with the radar system to see how it can be used. "For example, when operating at higher frequencies, the GPIR is able to provide greater resolution, which will be needed for more sensitive applications," said Toscano.

There are six commercial units introduced in 2001 serving the greater New York City area.

Making Corona Visible

Corona, phenomena located in the opposite direction from underground transmission lines, are the subject of another maintenance device. Corona are electrical discharges that occur on transmission lines and sub-station components, including insulators, conductors, lines, cable terminations, bushings, and transmission line surge arresters, and can indicate faulty equipment.

Utilities are typically made aware of corona by complaints of faulty radio or television signals. Because corona are invisible in daylight with the naked eye, maintenance crews will investigate by aiming a television or radio antenna at suspect lines and substations, and track corona by signal interference.

The Tennessee Valley Authority used the DayCor camera from a helicopter to inspect miles of overhead transmission lines, substations, and switchyards.

According to Andrew Phillips, an electrical engineer and project manager for transmissions and substations at EPRI, "This practice can only indicate the vicinity of the corona source, not the exact location and hence the offending component. Nor does corona activity always mean components need replacing. For example, an accumulation of bird droppings can cause corona."

Using night vision goggles to inspect corona activity is not practical either, because maintenance crews would have to be paid overtime to inspect lines. "Another problem is that nearby artificial light, whether street lights, highway lights, or marking lights on the substations or lines themselves, will render night vision devices inoperable," explained Phillips, who earned his doctorate researching corona.

EPRI researchers decided to develop the DayCor camera to depict transmission line corona in broad daylight at a safe distance, so that utilities could take corrective action swiftly. They sought the assistance of Ofil Ltd. in Nes Ziona, Israel. Ofil developed a dyed doped polymer technology to design solar filters to depict phenomena, such as hydrogen and alcohol flames, that are ordinarily invisible in direct sunlight or in fully illuminated electric light.

EPRI's DayCor camera makes corona, the invisible electrical discharges that can indicate faulty electrical equipment, visible to inspectors during the day.

EPRI also organized a working group of member utilities to bring the power company perspective to the project, and to fund it. Those utilities were the Tennessee Valley Authority, Southern Co., East Kentucky Power Cooperative, Bonneville Power Administration, New York Power Authority, City Public Service of San Antonio, Allegheny Power, and Central Hudson Gas and Electric.

The DayCor is aimed at its target much like a conventional video camera; that is, the inspector points the camera at a target and observes the image in a liquid crystal display. However, in order to view corona images in full daylight, the DayCor camera uses Ofil's bispectral visible- solar blind UV imager to divide the incoming image into two stereoscopic images. The imager transmits one image through the solar filter to block out the sunlight, then sends the filtered image through an image intensifier and a charge-coupled device.

The second image is sent directly to a standard video camera. The two images are pro-cessed and combined in a mixer that produces a visible image of the target with the corona superimposed upon it. Based on the type, mag-nitude, and location of the corona activity observed, the DayCor operator can determine the con- dition of the component and whether any corrective action is needed.

"This is a breakthrough technology," said Fisher Campbell, a project manager at the Tennessee Valley Authority who worked with EPRI on the DayCor camera project. "It allows daytime corona inspections that were not previously possible. Ultimately, we would like to employ the DayCor camera in airborne inspections. That would allow us to inspect our 17,000 miles of transmission lines for corona and to work the camera into our routine line inspection program."

Two Camera Versions

There are two versions of the DayCor camera. The Mark I is tripod-mounted, commercially available, and can capture 500-picocoulumb corona up to 30 meters away. The Mark II is a more sensitive handheld version, capable of spotting 100-pC corona from a distance of 50 meters.

The Mark II has a narrower field of vision for higher magnification, digital signal processing to quantify the corona activity, and noise rejection algorithms. Both versions operate at temperatures down to -20°C and up to 60°C.

"We began using the Mark II DayCor in October 2000, on TVA substations, switchyards, and trans-mission lines, and found some corona activity that proved valuable to us," said Barry Gore, an electrical engineer and manager of operations and maintenance projects at the TVA in Chattanooga, Tenn. "For example, we found cracked porcelain and nonceramic insulators in switchyards, as well as loose connections at substations that produce audible noise that irritates our customers."

Gore said the TVA had used the Mark I DayCor from a helicopter, and would use the Mark II to conduct airborne inspections this spring. With an initial price of about $80,000, the Mark II may be beyond the reach of smaller utilities, but Gore suggested that power companies invite EPRI Solutions, the institute's spin-off company, to survey their substations. "Demonstrating the savings in inspection costs and preventive maintenance that DayCor offers might make the system more attractive," he said.


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