Six utilities performed field testing of underground distribution cables to locate partial discharge (PD) sites. They removed sections from field service for laboratory testing at the BICC Marshall Technology Center. Corona detection was performed at 60 Hz, while dissipation factor was performed at 0.1 Hz.

The 30-ft (9-m) sections underwent accelerated life tests in water-filled tanks. Current was cycled through the conductor to elevate the temperature. The cable specimens were simultaneously subjected to a voltage stress equivalent to two, three or four times the normal operating voltage until failure.

Ultra Power Technologies Inc., a licensee of Instrument Manufacturing Co. (IMCORP, Storrs, Connecticut, U.S.) performed the field PD test by raising the voltage gradually until a PD or reaching a maximum voltage level of three times operating voltage. The tests detected PD at 18 sites, and more than 80% of these sites had a PD lower than three times nominal rating. Laboratory testing verified the cables neared end of life, as all cable specimens with identified PD failed within one day. The test equipment predicted the failure location within 3 ft (0.9 m). A few specimens with no identified PD sites also failed during laboratory testing, possibly because the equipment sensitivity was lower than the prevailing system sensitivity.

Not all tests were positive. Accelerated life testing of cables provided by one utility showed results that deviated significantly from the field diagnostic predictions.

Cable samples from one utility were retested with advanced IMCORP technology to better pinpoint the PD sites. These were dissected and examined by microscopy. All sites were found to have large water trees (bowtie and vented varieties) and electrical trees initiating from or across the water trees.

The EPRI-funded research shows that PD diagnostic testing at power frequency identifies weak spots in installed cables where a near-term failure is likely to occur. Laboratory testing indicates that removing portions of cable with PD sites significantly increases the reliability of the remaining cable. By averting the total cable replacement, the utilities achieve improved reliability at a relatively low cost.
Circle 185 on Reader Service Card

2002 Electrical Transmission Conference
“Electrical Transmission in a New Age”

Sponsored by the Structural Engineering Institute of the American Society of Civil Engineers (ASCE)

September 9-12, 2002
Omaha, Nebraska, U.S.
Phone: (800) 548-2723 (U.S.) or (703) 295-6300 (International)
Fax: (703) 295-6144

This two-and-a-half day conference includes 10 sessions, each with multiple presenters who are leaders in the field, and also offers tours where attendees can see an H-frame destruction test, modern fabrication and an operational wind turbine.