Neutral Corrosion & Novinium Warranty

Dear Informed Frog-

I have been asked to pursue something in writing concerning the 50% neutral corrosion and Novinium’s warranty. I respectfully request to have this from Novinium by quitting time this Friday, January 25, 2013. Thank you in advance for your cooperation in this matter and feel free to call me with questions or problems.

Signed,

Corrosion Concern in Colorado

Dear Concerned-

Short Answer

Go to …

http://www.novinium.com/Warranty.aspx

… and search for the word “corrosion” … you won’t find it! The warranty is purposely silent on neutral corrosion.

Less Short Answer

To understand why the short answer is so short, it is really useful to understand the purpose of the neutral and the consequences of various levels of neutral corrosion on the performance and reliability of the cable. To this end, view the recording of the September 2012 Webinar or read the webinar’s companion paper, “Neutral Corrosion – Significance, Causes & Mitigation”  prepared and presented by some of the cable experts at the Insulated Conductors Committee of the IEEE that created IEEE 1617™, the IEEE "Guide for the Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium Voltage Underground Cables."

Based upon IEEE 1617 and the aforementioned webinar many circuit owners have chosen 50% corrosion as their cutoff point for rejuvenation. But you get to make that determination yourself. You need only to communicate your wishes to the Novinium masters that proide your services. If you would like to discuss your choice with an expert … you can find them at Novinium. As you will learn in the webinar the 50% rule-of-thumb is a “glass half-full” proposition, because the rate of corrosion in direct buried cables declines over time. The pessimistic “glass half empty” perspective is not supported by observation.

Even if you have the occasional neutral corrosion in excess of 50%, don’t despair. The corrosion can be pinpointed and repaired, generally at a fraction of the cost of replacement. The webinar explains that option too.

Neutral on neutrals,

Thermo

Galvanic Corrosion of Neutrals

Oh greenest of greens,

My copper neutrals look a lot like you – green! I was wondering if our practice of bonding neutrals to the rebar in our vault structures might lead to neutral corrosion.

Califorrosion

Dear Califorrosion-

Yes, copper carbonate does have similar hue to me ... a touch bluer though. And like me, copper carbonate is your friend. The copper carbonate patina protects the native copper underneath from corrosion. It’s also true that bonding two metals can cause corrosion under the right circumstances. In fact this type of corrosion, galvanic corrosion, is the first mechanism mentioned in IEEE 1617™ (Paragraph 6.1 of “IEEE Guide for Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium-Voltage Underground Cables”). For galvanic corrosion to occur five conditions are required …

1. One of the two metals must be more anodic (inclined to be less negative). In your query that would be the steel rebar.

2. One of the two metals must be cathodic (inclined to be more negative). In your query that would be the copper neutral.

3. There must be a metallic connection. That would be the bonding hardware together with the neutrals and rebar.

4. There must be an environment for ions to flow. Wet soil provides such an environment, wet concrete works too, but not nearly as well.

5. Finally there has to be oxygen present. This last assumption is usually true unless the area is an anaerobic swamp.

The bottom line to your question is that the steel rebar is the anode and the copper is the cathode and hence any galvanic corrosion that does occur will occur to the detriment of the rebar. The rate of galvanic corrosion on the rebar will likely be quite slow, because concrete, even submerged concrete, does not allow for the rapid transport of ions.

My colleagues will be presenting a webinar on the subject: “Neutral Corrosion: Causes, identification & Mitigation” on September 21, 2012. To register for this learning opportunity navigate to www.novinium.com/events.aspx.

Green wishes,

Thermo

Failure Causes III

In my January 24^th post, “Failure Causes I,” I provided a partial answer to an inquiry from Colorado Querier. Colorado sought to understand if rejuvenation technology was appropriate for the “many types of aging factors” from which his firm’s circuits might suffer. We learned that 39% or more of all circuit failures are component failures and that these reliability issues are directly addressed with a rejuvenation program.  In yesterday’s post, “Failure Causes II,” we learned that more than 78% of the cable failures, which represent over 60% of the circuit failures are directly caused by water trees.  78% times 60% yields 47%. Water trees are the root cause of more than 47% of circuit reliability issues. Taken together (39% plus 47%) component issues and water trees account for more than 86% of all circuit reliability issues. We could stop right there, because 86% could be characterized as the vast majority. We could stop right there, because of the over 100,000,000 feet of cable rejuvenated over the last two-and-a-half decades, over 99% continue to provide reliable service. Cables treated by Novinium enjoy a post-injection failure rate less than half that of the industry-wide figure. We could stop there, but we won’t. The Novinium masters of reliability strive for post-rehabilitation reliability perfection.

If component issues and water trees represent the frog’s share of reliability root causes, what are the secondary issues? And how does rejuvenation technology address, or not address, these issues?

Neutral Corrosion

The occurrence of neutral corrosion within the population of bare neutral cables is 100%.  But don’t despair, the occurrence of neutral corrosion that creates safety or reliability issues is an order of magnitude less significant than circuit failures from all other causes – that is, generally 1-2% of cables suffer substantive neutral issues. Click here to check out my July 7, 2010 post along with its links to other published works. Even though the neutral corrosion issue is less significant than many assume, the good news is that neutral corrosion is both detectable and addressable. In fact, the Novinium masters routinely detect and repair neutral corrosion.

Thermal Issues

When cables are heavily loaded over sustained periods the insulation loses anti-oxidants and plasticizers. Oxidative degradation and polymer embrittlement contribute to a decrease in dielectric strength and in severe, but rare, cases may lead to cracking of the insulation. Designed to stay in the insulation for decades after injection, Novinium’s Ultrinium™ 73X fluids include anti-oxidants (AOs) and plasticizers. These materials all but halt oxidative degradation and embrittlement. Anti-oxidants have also been proven to slow the rate of water tree growth and increase the inception voltage of electrical trees. Click here to learn more about anti-oxidants in my March 14, 2011 post, “AO, AO … It’s home from work we go.” If the insulation gets hot enough the conductor may migrate and the insulation will become eccentric. These eccentricities usually manifest themselves at tight bending radii. The Novinium masters identify and remove most excessively bent cable sections. These most commonly occur near terminations or accessible splices and these areas are inspected during pre-injection preparation. Novinium^® brand rejuvenation addresses all of these thermal issues.

Halo

Halos are unavoidable when a cable is thermally cycled in the presence of water. Thermal cycling creates micro-voids in the middle radius of the insulation driven by the “Molecular Thermodynamics of Water in Direct-Buried Power Cables.” Click here to view the paper by the same name from IEEE Electrical Insulation Magazine (Nov/Dec 2006). The collection of voids formed this way is referred to as a halo. In the absence of water trees or some other defects, a halo does not lead to failure, because the halo size is limited by the molecular thermodynamics of water in the polymer. None-the-less, rejuvenation reverses most of the dielectric degradation caused by halos by filling the micro-voids with more compatible organo-silicones. Novinium^® brand rejuvenation addresses halos.

Manufacturing Defects

Voids, protrusions, contaminants, eccentricities, and skipped shields are “unwanted features” of a new cable. With the possible exception of skipped shields all of these unwanted features are in every cable. Fortunately for your newer purchases the magnitude of the defects is low enough that the cable can provide reliable service for its design life. For both your new cable purchases and your 30- and 40-year-old legacy purchases if the defects are large enough the cables will fail early in their lives … these kinds of defects yield what statisticians call infant mortality.  Your decade-old cables have been screened by operation of substantive manufacturing defects – those that will actually cause a failure without an accompanying water tree. In short, manufacturing defects are everywhere, but in legacy cable their manifestation is a water tree growing from the defect. Rejuvenation directly address the water tree and Novinium Ultrinium™ 73X brand rejuvenation includes patented stress grading components, which directly address stress-enhancing defects. Click the links below to learn more about stress grading …

Installation Defects

Excessively tight bending radius, excessive pull force, and exterior abuse rendered during installation are analogous to manufacturing defects. Serious problems manifest themselves shortly after installation. If an installation defect survived for several decades it is not so serious that it cannot be addressed by rejuvenation technology, particularly technology that includes Novinium patented stress grading chemistry.

Physical Damage (post-installation)

Frost thrust, dig-ins, and critter attacks can occur at any time. At Novinium we have seen insect attacks and rodent attacks. Amphibians have never been a problem. In the case of critter attacks, these usually occur near terminations and hence are often discovered and rectified as a routine matter during a rejuvenation program.  Dig-ins and frost thrust are generally not discoverable, but follow a pattern similar to manufacturing and installation defects. Cables struck with significant damage fail shortly after the event, insignificant damage may be mitigated by rejuvenation. In summary, rejuvenation mitigates, but does not prevent all failures resulting from post-installation physical damage. Rejuvenation with stress grading technology such as that found in patented Novinium Ultrinium™ 73X brand rejuvenation fluids provides superior mitigation.

Testing Induced

My faithful readers know that this frog is not a devotee of diagnostic testing. The fundamental problem can be summed up thusly:  None of the technologies can reliably discriminate between cables which will fail in short order and those which will not. The rejuvenation program alternative puts a final nail in the diagnostic coffin, because components will all be changed anyway. What sense does it make to find out if the components are good or bad? Since over 99% of rejuvenated cables don’t fail when no diagnostics are utilized and the extension of life is 5-20 times longer that the retesting horizon, paying for a diagnostic is difficult to justify.  If all of that were not enough many diagnostics test induce defects! Electrical trees can be initiated directly by high voltage methods such as off-line partial discharge or indirectly by inducing space charge with DC methods. Even though it makes no technical sense to test, rejuvenation does mitigate the damage testing inflicts on cables if rejuvenation is given some time to improve the dielectric performance of the cable.  For SPR that is about a week; for UPR it is best to wait for at least a year. To explore diagnostic testing further do a key word search on my blog for “diagnostic testing.”

Insulation Shield Separation

Loss of adhesion between the insulation shield and the insulation is a rare occurrence and is the only fault mode not addressed or at least mitigated by rejuvenation. This frog can count on one front paw, and I only have four toes on that paw, the number of failures where the loss of insulation shield adhesion was the cause of failure. These few observed failures suggest that chemical contamination of the soil causes swelling of the shield material and loss of adhesion. Transformer oil or motor oil spills are suspected culprits. If you have a bunch of these kinds of failures on your hands, you have a potential Love Canal situation and you are going to be excavating the whole neighborhood.  No need to treat the cable.

Summary

Advanced cable rejuvenation provided by the masters at Novinium has a proven track record of 99.4% post-rejuvenation reliability. Almost all known causes of solid dielectric underground cable reliability problems are either directly addressed or mitigated. The sole exception is insulation shield separation, which is incredibly rare.

Broad Spectrum Reliability,

T. Bull Frog

Reflections on a TDR

Dear Thermo,

The blog entitled "Neutral Corrosion - How much is too much?" includes a waveform from a TDR (time domain reflectometer, often called a radar) that is used to pinpoint bad sections of cable neutral. The TDR is also used to pinpoint splice locations on the cable. Please provide the details of how the TDR determines the neutral corrosion and splices on the cable and how the wave form is read to tell them apart and to pinpoint their locations.

Reflective in MD 

Dear Reflective-

Step-by-step instructions for how to identify and pinpoint neutral corrosion and splices on concentric medium voltage power cables are provided in Novinium Rejuvenation Instruction 12 entitled, “Electronic Cable Diagnosis and Pinpointing.” Click NRI-12 to view the document as a PDF. The TDR sends a low voltage (10-20 volts), short wave length (1-20 nanoseconds) pulse down the cable. A portion of the wave is reflected when it encounters a change in impedance. There are four main types of impedance changes encountered along the length of a test cable.  Remember – impedance includes three elements, resistance, capacitance, and inductance. 

(1)       Instrument-Cable Interface

The first impedance change that is encountered results from the mating of the test instrument lead, an RG59 coaxial cable, which has a characteristic impedance of 75 ohms, with the power cable, which has a characteristic impedance of 8 to 38 ohms depending upon its geometry and polymer system. To minimize the reflection from this unavoidable impedance change, the masters of reliability at Novinium use a proprietary impedance streamliner. This is akin to an aerodynamic sports car versus a squarish pick-up truck. The impedance streamliner is like the smooth curves of the sports car, reflecting less of the input pulse, minimizing signal attenuation and dispersion. Attenuation is the reduction of signal amplitude and dispersion is the smearing of narrow pulse into a broader, less discrete pulse. Both are undesirable. Some reflection is unavoidable. The signature of Novinium’s impedance streamliner shown in red is superimposed upon the green signature of an older impedance technology device (ITD) in the image nearby. Untoward noise and reflections avoided improve the usability and hence the sensitivity and accuracy of the TDR.

(2)       Splice 

In the image nearby I am standing next to a very typical splice during a recent coffee break. The neutrals are all dirty as they are prone to be in a pit, but if you look carefully along the orange annotation, you can see how the neutrals are close to the conductor on the cable, then are pig-tailed together and lay farther from the conductor as they jump across the molded splice body. On the far end of the splice the neutrals again come back to intimate proximity. This change in the separation of the two signal conductors – the conductor and the neutral – changes the circuit impedance. The resistance is not significantly changed, the already low capacitance decreases with increasing distance, but that capacitance change is trivial compared to the change in inductance. The inductance and hence the impedance skyrockets as the neutrals leave the insulation shield and then plummets when the neutrals return to the cable. I have superimposed the actual TDR image of a splice, a characteristic sine wave, in the lower-right-hand corner.

(3)       Neutral Corrosion

The physics are even simpler for neutral corrosion. The capacitance and inductance components are insignificant. A good old-fashioned resistance increase is displayed as an impedance increase. Check out the nearby image.

(4)       End-of-cable

Simpler still, the end of the cable is characterized by either an infinite impedance increase if the circuit is open or an infinite impedance decrease if the conductor is grounded to the neutral. When used, grounding devices add some more color to the wave shape, but the basic idea remains the same.

The TDR signal is reflected by each of the above impedance changes and the time the signal takes to travel to and then from the impedance change can be used to estimate the distance to that change. Note that the TDR is not a pinpointing technology, it provides a location estimate. To pinpoint splices and corrosion a second technology, radio-frequency (RF) locating, is utilized. If you desire, I will be happy to explain how that works too. NRI-12, described earlier, provides step-by-step instructions to accomplish RF pinpointing.

Your adroit amphibian,

T. B. Frog

Neutral Corrosion Progression

Two of my disciples inquired of my July 7, 2010 post, Neutral Corrosion – How much is too much?

Dear Sheila and Kurt-

There are several different mechanisms for cable neutral corrosion.  The mechanisms are enumerated in Section 6 of IEEE 1617-2007 "Guide for Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium-Voltage Underground Cables." My colleague, Glen Bertini, was one of the participants in the ICC C7 working group that developed that document. The identified mechanisms of cable neutral corrosion in section 6 are:

1.   Galvanic corrosion

2.   Single metal corrosion

3.   Soil corrosion

4.   Differential aeration

5.   Stray currents

6.   Galvanic corrosion resulting specifically from tin coated neutral wires

Of these possible causes of neutral corrosion, soil corrosion and differential aeration are by the far the most predominant and problematic.  In both cases differences in soil chemistry along the path of the cable lead to differences in potential on exposed neutrals. Current flows in a loop through the soil and along the neutral. Copper is oxidized where the electrons leave the neutral and enter the soil and where there is oxygen (or sulfur) present. The rate of corrosion is a function of the current flow and is constrained by the availability of oxygen. Current flow, in turn, is proportional to the potential difference caused by the local differences in soil chemistry and inversely proportional to the resistance of the loop.  In the illustration nearby I show how this all works. Either from differential aeration or differences in the local soil chemistry the electrochemical potential is higher at point A than it is at point B.

As the neutral corrodes, the resistance in the loop goes up, which slows the rate of corrosion. The loss of the metallic copper itself leads to an increase of resistance. Less obviously, the non-conductive corrosion by-products (i.e. copper oxides) coat the copper surface and increase the resistance between the neutral and soil. Another set of chemical processes determine how quickly the copper oxides are transported off of the native copper surface below them. This copper-oxide transport mechanism is typically very slow in direct buried environments as the oxides are not appreciably water soluble.

At the same time the resistance is increasing, the second law of thermodynamics is at play reducing the chemical potential difference between A and B. The homogenization of chemical potential over time would occur whether or not a cable was present. Nature abhors chemical potential differences so chemical species migrate through the soil toward equilibrium – zero chemical potential difference.

Corrosion of bare concentric neutrals is highest when the neutrals are new and the soil was disturbed when the cable was installed. As some corrosion occurs and the second law reduces the chemical potential, the rate of corrosion decreases over time. In practice if the neutral has at least partially survived for several decades the rate of continuing degradation is trivial.

The other causes of neutral corrosion are much less prevalent. With the possible exception of stray currents impressed upon neutrals by active cathodic protection systems of neighboring structures (e.g. gas pipelines), all are similarly mitigated by the partial corrosion of the neutral and the equilibration of chemical potential due to the inexorable second law of thermodynamics.

If cows dragged their bulging bellies across the ground as I do they would be safe from stray current. Even with a substantial potential at the ground surface, I remain equipotential. If cows or other mammals are getting electrocuted the neutrals are entirely destroyed either locally or systematically. If the corrosion is systematic, the cables must be replaced. The source of the systematic corrosion should also be identified and eliminated – it’s not a natural phenomena; it’s man-made. Dead cows are the odd cases, but these cases get media attention, so the anecdotes are oft repeated.

In the real world, most concentric neutral corrosion is incredibly local. One or two feet of neutral become corroded. It turns out this problem is easy to diagnose and easy to repair. Diagnostic techniques are described in the aforementioned IEEE 1617-2007. A step-by-step and state-of-the-art procedure is available for free from Novinium Rejuvenation Instruction 12 (NRI 12), Electronic Cable Diagnosis and Pinpointing. Also free are step-by-step instructions (NRI 80), Neutral Corrosion Repair, that make fixing local corrosion a piece of cake. Once a local corrosion site has been pinpointed, chemistry can be employed to protect the location of the identified chemical potential difference. A suitably sized magnesium anode that has a chemical potential well above that of copper is installed as a sacrificial anode. The anode size can be adjusted to prevent neutral corrosion for any desired life.

Over twenty years of rejuvenation experience with a dearth of warranty claims in general and even fewer neutral corrosion issues specifically, provide direct evidence that whatever post-injection progression of neutral corrosion that does occur is of little practical significance.  At Novinium we have had zero failures and zero warranty claims that involved an increase of neutral corrosion after the cable was treated.

Equipotentially yours,

T. B. Frog

Neutral Corrosion - How much is too much?

Dear Ms. Frog,

I am looking for some guidance on what percent of bare concentric neutral corrosion can be tolerated on an underground primary cable before it needs to be replaced.  I wondered if the technical staff at Novinium happens to have any ideas as to where I may get some information on this subject.

Colorado Corrosion Concern

Dear Concerned-

I have access to millions of feet of records of treated cable.  Together, my colleagues and I have analyzed over 70 million feet of rejuvenated cable that had been scanned with a time-domain reflectometer (TDR).  The incidence of neutral corrosion is way less than many suppose.  In an August 1996 article, “Neutral Corrosion Problem Overstated” in Transmission & Distribution World, Bob Gurniak of Pennsylvania Power & Light (PP&L) described this overstatement using data from the AEIC Cable Report and IEEE ICC Task Force on Cable Neutral Corrosion (6-21).  There are two notable exceptions in North America … the Appalachian Mountain Region (in PP&L territory) and Wisconsin suffer more than the normal amount of corrosion because of the low soil electrical conductivity in those regions.  The T&D article is available at …

http://tdworld.com/mag/power_neutral_corrosion_problem/index.html

... without the Table and Figures in the printed version.  I have recreated that table below and provided similar Figure 1 and Figure 2 illustrations.

Fig. 1. Technician Analyzes the TDR readout to pinpoint bad sections of cable.

Fig. 2. Waveform from the TDR.

As a rule of thumb, most circuit owners with 100% neutrals accept up to 50% local loss of neutrals.  What I mean by local loss is that neutral corrosion is almost always limited to just a few feet as shown in the photograph above.  The purposes of the neutral, enumerated in Section 4 of IEEE 1617-2007 (Guide for Detection, Mitigation, and Control of Concentric Neutral Corrosion in Medium-Voltage Underground Cables) are not compromised.

The reason for the locality of typical neutral corrosion is that the predominant cause of concentric neutral corrosion is differential aeration which is an inherently local phenomenon.  See Section 6.4 of IEEE 1617-2007.

Kindest corrosion-free regards,

Thermo