by Thermo 2. April 2012 19:19

LIPA

Dear Felicitous Frog-

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?

Currently,

AC in BC

Dear AC-BC:

Other frog fans 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. This frog is unaware of any such test. Without the simple application of the scientific method the claims of efficacy cannot be confirmed by this, or any other frog.

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. In the mean time, actual post-injection performance of better than 99.6% on millions of feet of cable can be viewed at …

www.novinium.com/Lessons.aspx

Always skeptical of claims without data,

T. B. Frog

by Thermo 1. September 2011 17:28

DC Testing

I don’t reflexively follow the crowd, but the consensus is that DC testing is destructive and does not provide useful data anyway!Dear Ms. Frog,

We have been using DC testing for centuries without any problems. How do you explain that, oh wet and wise one?

Oregon Anomaly

Dear Anomaly-

DC testing of medium voltage AC cables comes up only occasionally.  I discussed the warranty impact in an April 14, 2010 post, titled “DC Testing to sectionalize faults – Warranty Impact.”  Click here to check out that post. There are many anomalies in Oregon that may never be explained, but it seems unlikely that the laws of physics are different in The Beaver State. I suppose there is a chance that everybody else is wrong and you are right. It happens to me all the time that I defend a position that is contrary to conventional wisdom.

I am actually not an expert on DC testing, so I have to rely on those that are.  One way I can be pretty sure that DC testing is inherently destructive is that the folks that manufacture and sell DC testing equipment croak along on the same chorus.  To wit, HV Diagnostics, Inc., one of the most respected suppliers of high voltage test equipment pronounces in its brochure:

“On Medium Voltage Extruded (XLPE, PE, EPR) cables, DC is no longer recommended by most international standards. DC has been found to be both destructive, causing premature failure of aged MV cables, and/or ineffective in detecting many types of serious pending insulation defects in new and old cable installations.”

You have to take that kind of pronouncement pretty seriously, because the supplier sells DC testing equipment.  As for your anomalous experience, I would recommend that you compile a comprehensive data set and use randomization and suitable controls.  Anecdotal results are inherently misleading. Also make sure that the equipment operator’s job does not depend upon the results – the guys that do testing for a living seem generally to be proponents of testing.

In cahoots with the beavers,

T. B. Frog

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Potpourri

by Thermo 11. May 2011 19:44

Middle East Query – Diagnostic Testing Timing

Dweller of the Desert asked 22 questions in his post …

Middle East Query – 22 Questions.

In this installment I address question 13.

13.   Can the customer test the cable right after injection? Could it be done immediately? If not, after how many days, weeks or months?

You are not going to like my answer, but this frog is incapable of subterfuge. There are no cable testing methods that will tell you when a cable will fail, which is what you really want to know. At best, testing will provide you a number you can track over time. An extensive U.S.A. study, the Cable Diagnostic Focused Initiative (CDFI) led by NEETRAC (The National Electric Energy Testing Research & Application Center at Georgia Tech’s College of Engineering) of commercialized cable testing methods came to this conclusion—see CDFI slide 41. The CDFI is the largest and most comprehensive study ever undertaken. All cable testing methods, except online PD testing, can cause damage to the cable insulation and shorten cable life.  Some cable testing methods such as Tan Delta or Power Factor are not comparable before and after injection, because cable injection alters the chemistry and physics of the cable, changing the measured parameters in ways counter to the claims of the diagnostic supplier.  See my 2010, September 10, post, “Cable Rejuvenation Impact on Loss Factor (tan-Delta).” In one example from that post, the tan-Delta at 0.1Hz increased after treatment, even though dielectric strength increased substantially – just the opposite of what the proprietors of the test predict. Because available diagnostics do not provide useful information and/or the tests are inherently destructive, Novinium discourages cable testing before or after cable injection. If a circuit owner chooses to test its cable despite overwhelming evidence of futility and counter productivity, before or after injection, Novinium will suspend, but not extend, any warranty for 120 after the test. Make sure you check out my 2010, November 12 post: Diagnostic Testing – Should I do it? One sneaky way to test the veracity of diagnostic firms’ claims is to request a warranty for cables which test as good, but fail. And when I say warranty, I don’t mean a credit for future diagnostics – I mean money back.  In a three-year experiment undertaken as part of the CDFI to measure the accuracy of online partial discharge testing, false negatives were about 9.5% and 19.5% for accessories and cable respectively.  False positives were 69% and 56% respectively.  I can do better than that with a role of the dice!

For now, Ma’a salama (مع السلامة/Good bye)

T. B. Frog

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Operational Considerations | Potpourri | Warranty Reflections

by Thermo 14. April 2010 17:15

Dear Thermo-

 

We use a 10kV hipot DC to sectionalize URD faults. We actually use this on several sections at a time so it will go through good sections of cable until it reaches the bad section. We split the run of cables fed by a common fuse in half and hipot one way (could be 4 or 5 sections of cable) and if it is not found we hipot in the other direction. Do we have to wait 120 days after this and if so how do we track it?

 

Assessing in Alberta

Thermo

Dear Alberta-

We have corresponed before … frogs might start talking, and I am not comfortable with interspecies relationships.

There are no record retention requirements imposed by our warranty. The individual who submits a warranty claim might query the operations folks to learn if there has been any fault activity over the past 4 months in the adjoining circuits.   It is well established in the literature and in IEEE standards, that applying 10kV DC on aged polymeric cables is inherently destructive.  If that body of literature is not easily available to you, I can provide you with the appropriate references. (Just write to me again, but use a different pseudonym.)  There are less destructive ways to sectionalize faults.  For example, if you use the same approach, but at 1kV there is no issue with our warranty. The amount of damage from imposing DC is proportional to the voltage squared times the time, so 1kV is 100 times less damaging than 10kV imposed over the same length of time. There are also sectionalizing arc reflection TDRs that can identify the failed section with a few very brief pulses.

The best idea is to avoid this situation entirely. Treat or replace 100% of the at-risk cable segments in a loop and the probability of a failure in the loop will be dramatically lower. Avoid rejuvenating only the easy-to-treat cable, as half-steps will only get you to half-reliability.

 

Love and kisses,

Thermo

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