A NEW CABLE TESTING DEVICE will help electric distribution utility workers discover and fix weaknesses in cables under controlled environments, before being returned to service. Central Hudson purchased a very-low-frequency (VLF) AC Hipot cable tester from High Voltage Inc. earlier this year, and it has proved to be very useful. Amir Hammad, junior engineer in distribution planning, is monitoring the results of the program, which is being used to test aerial and underground cables in the Fishkill, Newburgh and Poughkeepsie, New York, U.S., divisions.
“A problem with the former method of testing is that it caused a treeing effect, which is a void in insulation. Charges that build up within these voids using the DC input method of testing would want to dissipate to ground, thus causing a breakdown in the cable's insulation, which would ultimately cause a failure,” says Pete Harpolis, operating supervisor.
The instrument applies an AC voltage at a much lower frequency than the normal AC operating frequency, and it is possible to test miles of cable with a small test unit. A typical test lasts about 30 minutes per phase at three times line-to-ground voltage, which stresses the cable above normal operating voltage. The VLF rapidly causes any defect to grow to the point where it causes the cable to fail. Cables that can handle the higher voltage and pass the test have greater assurance of reliable service.
“The purpose of this testing is to cause defects to occur during testing before placing a cable back in service,” says Harpolis. “It forces weak cables to fail during testing.” For example, during proof testing of the cable, a fault was identified near the Poughkeepsie Municipal Building. It was found that if the cable had been placed in service, Central Hudson could have experienced an additional outage due to the collapsed conduit run feeding a network transformer in the area.
While VLF Hipot testers are widely used in Europe, they have only recently caught on in the United States. Central Hudson's new VLF AC testing device was first used to test the WN cable as a trial in the Newburgh division, between Taft Avenue and St. Luke's Hospital. “That was in 2002, and to date, we haven't had any failures since we proof tested this cable,” says Harpolis. The long-term plan is to test all the cables in Central Hudson's service territory and fix problems as they are discovered. A database of test results also will be produced to assist in the creation of a new maintenance program for cable testing.
Central Hudson purchased the new device through the work improvement technologies initiative, which provides research and development funds to demonstrate and evaluate work-saving productivity improvement and data-gathering technologies to support day-to-day electric and gas operations. “Through this program, we get an opportunity to see for ourselves if this particular instrument makes sense and what it can do for us,” Harpolis said. “It opens us up to other technologies that may prove beneficial.”