Summary

Almost all electric power utilities distribute a portion of the electric energy they sell via underground cable systems. Collectively, these systems form a vast and valuable infrastructure. Estimates indicate that underground cables represent 15 % to 20 % of installed distribution system capacity. Cable systems are designed to have a long life with high reliability. However, the useful life is not infinite. These systems age and ultimately reach the end of their reliable service lives. Estimates set the design life of underground cable systems installed in the United States to be in the range of 30 to 40 years. Today, a large portion of this cable system infrastructure is reaching the end of its design life, and there is evidence that some of this infrastructure is reaching the end of its reliable service life. Complete replacement of old or failing cable systems is not an option. Many billions of dollars and new manufacturing facilities would be required. Electric utilities and cable/cable accessory manufacturers are simply not in a position to make this kind of investment.

Thus, the key to managing this process is to find these “bad actors” and to proactively replace them before their repeated failures degrade overall system reliability. Various cable system diagnostic testing technologies were developed to detect cable system deterioration. The results of diagnostic tests are used to identify potential failures within cable systems and then again, after repair, to verify that the repair work performed did indeed resolve the problem(s) detected.

This report summarizes an extensive collaborative effort made to understand how to effectively use the various diagnostic technologies to establish the condition of medium voltage underground cable circuits. The information provided is the result of a collaborative effort between NEETRAC staff, Georgia Tech academic faculty, electric utility industry participants, as well as cable system diagnostic testing service providers and test equipment providers.

The full report is available here.

Report topics include:

• How power cable systems age and fail (section 2.0)
• Diagnostic Accuracy (section 3.1)
• Diagnostic Testing Technologies:
  • Time-Domain Reflectometry (TDR) (section3.2)
  • Partial Discharge (PD) (section to 3.3)
  • Dielectric Loss (Tan d) (section to 3.5)
  • Dielectric Withstand (High Pot) (section 3.8)
  • Monitored Dielectric Withstand (section 3.9)
  • Combined Diagnostics (section 3.12)
• Practical Application of Diagnostic Tewchnologies (section 4)
• Utility Experience with the Cable System Diagnostic Techniques – Case Studies (section 5)

There is no doubt that cable system diagnostic testing can be used to improve system reliability. However, to be effective, the technology should be appropriate to the circuit to be tested. Setting accurate and reasonable expectations is also a critical part of the process.

In general, the work performed in the CDFI led to the following observations:

• Diagnostic tests can work. They often show many useful things about the condition of a cable circuit, but not everything desired.
• Diagnostics are generally unable to determine definitively the longevity of the circuit under test. Cable diagnostics are much like medical diagnostics. They can often tell when something is wrong (degraded), but it is virtually impossible to predict the degree to which a detected defect will impact the life of the system tested.
• The performance of a diagnostic program depends on:
  • Where diagnostic is used
  • When the diagnostic is used
  • Which diagnostic to use
  • What is done afterwards

The above statements do not imply that diagnostic testing should be avoided. In fact, the contrary is true. Users should recognize and consider these issues before a testing program begins. When applied properly, diagnostic testing will provide information that can be used to effectively lower cable system failure rates. There is still much to learn, but cable diagnostic testing is a rapidly developing field. Increasingly useful technologies and new approaches are currently being developed that will increase the effectiveness, understanding, and economic success of performing cable system diagnostic testing programs.

Disclaimer

The information contained in the CDFI report is to our knowledge accurate and reliable at the date of publication.

Neither GTRC nor The Georgia Institute of Technology nor NEETRAC will be responsible for any injury to or death of persons or damage to or destruction of property or for any other loss, damage or injury of any kind whatsoever resulting from the use of the project results and/or data. GTRC and The Georgia Institute or Technology disclaim any and all warranties both express and implied with respect to the services to be performed hereunder and any deliverables results therefrom, including their condition, conformity to any representation or description, the existence of any latent or patent defects therein, and their mechantability or fitness for a particular use or purpose.

It is the user's responsibility to conduct the necessary assessments in order to satisfy themselves as to the suitability of the products or recommendations for the user's particular purpose.

No statement herein shall be construed as an endorsement of any product or process or provider.

Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the Department of Energy.

Acknowledgements

This material is based upon work supported by the US Department of Energy under Award No. DE-FC02-04CH11237.

The information in this handbook came from a number of sources, including various specifications, technical publications, diagnostic test providers and consulting engineers as well as the general experience within NEETRAC.

All of the information in this report has been included at the discretion of NEETRAC.

The Principal Investigators gratefully acknowledge the technical contributions of:

Last revised on August 15, 2011