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LIPA

April 2, 2012

LIPA

Q:

I have read a paper from the conference record of the 2008 IEEE International Symposium on Electrical Insulation (ISEI) by some folks at Powertech Labs from my home province of British Columbia. The paper was titled: “Condition Assessment of 15 kV Rejuvenated Underground XLPE Cables.” The cables in question are operated with AC, but the testing method is with DC. Does a DC test have validity on an AC cable? The paper shows results of before-and-after diagnostic testing on two treatment methods, referred to as “method A” and “method B.” Are these results representative of Novinium’s post injection experience?

A:

Other readers may wish to review the full text of the paper to which you refer. The paper is available for a small charge from the IEEEXplore® digital library; click here to view the abstract and full citation. The test method utilized in the paper is the LIpATEST™ technique, proprietary to PowerTech Labs. PowerTech is primarily owned by BC Hydro. The LIPA technique measures the DC leakage current through the cable insulation as a function of applied DC voltage. The 15 kV-class cables described in the paper are subjected to a negative voltage, increased in 4 kV steps of 1-minute duration, to a maximum of 16 kV. The leakage current is recorded at each step. The purveyors purport that the magnitude of the leakage current and its rate of change with applied voltage provide an indication of the quality of the cable insulation.

You asked two questions: Is the test valid and are the results representative? I provide answers to both in four parts, entitled: DC Testing, LIPA Validation, Rejuvenation Methods Tested, and Representative or Not?

DC Testing

The 2001 version of IEEE 400™, “Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems,” provides some guidance and is available from ANSI. Click here to view the abstract and complete citation. Paragraph 4.2 states in part …

“Whenever dc testing is performed, full consideration should be given to the fact that steady-state direct voltage creates within the insulation systems an electrical field determined by the geometry and conductance of the insulation, whereas under service conditions, alternating voltage creates an electric field determined chiefly by the geometry and dielectric constant (or capacitance) of the insulation. Under ideal, homogeneously uniform insulation conditions, the mathematical formulas governing the steady-state stress distribution within the cable insulation are of the same form for dc and for ac, resulting in comparable relative values; however, should the cable insulation contain defects in which either the conductivity or the dielectric constant assume values significantly different from those in the bulk of the insulation [Editor: That would be all aged cable!], the electric stress distribution obtained with direct voltage will no longer correspond to that obtained with alternating voltage. … Furthermore, the failure mechanisms triggered by insulation defects vary from one type of defect to another. These failure mechanisms respond differently to the type of test voltage utilized. For instance, if the defect is a void where the mechanism of failure under service ac conditions is most likely to be triggered by partial discharge, application of direct voltage would not produce the high partial discharge repetition rate that exists with alternating voltage. Under these conditions, dc testing would not be useful. However, if the defect triggers failure by a thermal mechanism, dc testing may prove to be effective. For example, dc can detect the presence of contaminants along a creepage interface.

In the case of joints and accessories, their dielectric properties may differ from that of the cable with regard to conductivity. This may result in a dc stress distribution at the interfaces between the cable and the accessory that is very different from the stress under ac voltage. A careful examination of the system is necessary prior to a dc test in order to avoid difficulties.

Testing of cables that have been service aged in a wet environment (specifically, XLPE) with dc at the currently recommended dc voltage levels (see IEEE P400.1) may cause the cables to fail after they are returned to service (see Fisher, et al. [B23], and Steennis, et al. [B48]). The failures would not have occurred at that point in time if the cables had remained in service and not been tested with dc (see Eager, et al. [B21], and Srinivas, et al. [B47]). Furthermore, from the work of Bach, et al. [B7], we know that even massive insulation defects in extruded dielectric insulation cannot be detected with dc at the recommended voltage levels.”

In short, …

1.    DC testing does not measure the same defects to which the subject cable is exposed in its AC environment.

2.    There is little or no relationship between DC test results and likely AC performance.

3.    DC testing damages the aged cable it seeks to diagnose.

LIPA Validation

If the purveyors of the LIPA test wish to validate their test they simply need to run an experiment with a suitable control. To wit, divide a population of, say 100, homogenously aged cables into a control group of 50 and a test group of 50. Monitor the performance of the control group for future failure history. Submit the 50 cables in the test group to LIPA, and then monitor that group for future failure history. If the purveyor’s claims are accurate, there will not be a significantly higher failure rate in the test group compared to the control group and the failure rate in the subgroup of the test group that tested “bad” should be significantly higher than those of the test subgroup that did not test bad. Since PowerTech is a subsidiary of a utility with a sizable population of appropriately aged cables, it should be a simple matter to arrange such a test. I am unaware of any such test. Without the simple application of the scientific method the claims of efficacy cannot be confirmed by me or anyone else.

Rejuvenation Methods Tested

Novinium can and does utilize both method A and method B. Method A is properly called unsustained pressure rejuvenation or UPR. Novinium has made improvements to the UPR method. The improved UPR method is called iUPR. Method B is Sustained Pressure Rejuvenation or SPR. SPR outperforms UPR and iUPR by any measure of post-injection reliability.

Representative or Not?

Not—for two reasons. First, as mentioned above, the LIPA test should not be used to judge AC reliability. Second, even if LIPA were a valid test, 13 samples for UPR and 4 samples of SPR are not statistically significant.

Novinium is the only rejuvenation vendor in the world that performed a full third-party, side-by-side controlled experiment of rejuvenation technology. The work was executed by NEETRAC and the results are extraordinary. As soon as those results become public you can read about them here.