Presented by Tom Sandri, Shermco Industries
Over the years there have been several methods and/or philosophies regarding the acceptance and maintenance testing of medium voltage underground electrical power cable in the field. Time honored methods such as direct current (DC) High Potential, Insulation Resistance (IR), and Dielectric Absorption (DA) testing have been employed for many years by utilities and industrial facilities for both field acceptance testing and periodic maintenance evaluation of medium voltage insulated power cables. Insulation Resistance provides a simple test for identifying gross defects in the cable and accessories, but offers little to no diagnostic value. A test like dielectric absorption is useful because it can be performed on even the longest of cables, and yields a self-contained evaluation based on relative readings rather than absolute values.
In the early 1990s the benefits of using DC high potential testing for maintenance purposes of extruded dielectric cables has been questioned and written out of most industry guides. Since the days of high potential DC, other testing methods have been developed that may provide a better indication of the integrity of cables, splices, and terminations. These methods include Very Low Frequency (VLF) high potential testing, Tan Delta, and both Online and Offline Partial Discharge testing.
So where do we stand today? What are the modern techniques and what value do they offer? Do we treat Acceptance testing the same as Maintenance testing? What is the difference? This paper will explore developments in testing and evaluation of medium voltage cable seen over the last 20 years. The intended application of each technique along with the advantages and limitations of the technique will be reviewed providing the knowledge necessary to develop an effective cable testing program.