Harry E. Orton
OCEI
PO Box 38715 Metropolitan PO
North Vancouver, BC V7M 3N1
Canada
E-mail address: h.orton@ieee.org
Abstract: Diagnostic testing of in-situ
power cables is an emerging technology.
Many diagnostic systems are being developed, both off-line and on-line
that are providing much data. These
data need interpretation and understanding as the technology emerges. This paper will address some of the
commercially available diagnostic methods and cover the field experiences with
these diagnostics. Both PILC and
polymeric insulated cables are included in the discussion.
The age and size of most utility underground installed plant is
increasing and the overall failure trend will continue to grow unless a
directed maintenance program is implemented.
For example, Colorado Springs Utilities has 62% of their distribution
network underground, with an average age of failed cable of 21.6 years, that
were first installed 33 years ago.
Utilities need a predictive maintenance tool, that will allow proactive
maintenance to occur before unplanned customer outages. By using a diagnostic it is hoped to
accomplish cable, termination or splice replacement before failure and to
provide scheduled cable system maintenance.
Additional budget information will be gathered to reduce costs and to
target replacement money .
Both off-line and on-line diagnostics are commercially available to
determine the condition of cable and cable accessory insulation. Two major types of insulation degradation
occur in cable systems. One is an
average or overall condition caused by chemical aging and/or water treeing. The diagnostics used for this type of aging
includes dissipation factor (loss angle), harmonic analysis, return voltage,
isothermal relaxation current, dielectric response or dc leakage current. A summary of these methods is provided in
the table.
The second type of degradation is discrete or incremental condition
assessment, that utilizes dissipation factor measurements or partial discharge
(PD) level measurements. No matter
which type of diagnostic is used, it should be applied in a non-destruction
manner, so that the diagnostic itself does not reduce cable or accessory life.
Cable accessories are treated quite differently from cables. For example, the accessory design is not
always properly tested, they are man-made in the field, so workmanship is a
concern, and they are not properly tested after installation. Most cable accessory materials are more
resistant to partial discharge activity than the cable and will withstand PD
and treeing activity longer than the adjacent cable insulation. However, there are likely to be more defects
in a cable accessory than in a cable, so PD detection is more applicable to
cable accessory assessment
Cable diagnostics, particularly partial discharge detection and
location is still undergoing development.
Interpretation is difficult due to the number of variables involved.
Future diagnostics should:
·
Look at specific activities over time or Trend Analysis that includes
operational parameters, such as voltage, load, temperature and humidity,
·
Distinguish between PD types, and especially between harmful and
harmless PD,
·
Data interpretation improvement is necessary.
The full paper has been
published in the Transmission &Distribution World Magazine, June 2002, Page
43.