A Cable Too Long

Dear Felicitous Frog,

I am currently reviewing a URD circuit with a cable segment that is 6,200' per our one-line and GIS.  It is our regular #2 db [direct buried] cable.  I am considering the installation of pull-boxes to break up the many long segments prior to attempting injection. What is the practical limit to TDR [time-domain reflectometer] testing?

What is the practical injection limit?   I assume in the best case that there at least a construction splice every 2000 feet, but there may be more that have to be replaced. 

Curious about cable  

Dear Curious- 

For background on how TDRs work, check out my October 1, 2011 post, “Reflections on a TDR”. A TDR sends a radar-like pulse down a cable. Like radar, a portion of the pulse is reflected when it “hits” objects along the cable path. Objects are anything that has a modestly different impedance (resistance, capacitance, and/or inductance) from that of the cable. Splices, cable ends, and neutral corrosion are generally identifiable. These objects are often called impedance anomalies, because the impedance varies locally from that of the cable.

There are two phenomena that reduce the acuity of the TDR – attenuation and dispersion. In the image nearby I illustrate the practical effects of attenuation and dispersion. As a wave travels along a cable its amplitude decays because no cable is without loss. When a male bullfrog croaks into a pipe, the volume decreases with distance because the sound wave amplitude attenuates. The attenuation is due to the imperfections in the molecular collisions. A portion of the sound waves are converted to heat. The same thing happens in the cable as electrons bounce among the cloud of conductor d-orbitals.

The second effect is dispersion and it too is the result of imperfections. Instead of loosing energy, dispersion smears energy because the rate that the signal moves through the cable is not uniform through its cross section. Skin effects and twisted stranding act to disperse the wave. Copper tape neutrals have a particularly nasty dispersion.

Simply recognizing these two effects helps the skilled operator interpret the observed wave shapes. Longer cables and those with more splices or corrosion will have shorter and more dispersed reflections.

Tactics to Improve Acuity

Of course the operator can use the TDR from both ends of the cable. This tactic effectively doubles the TDR’s resolution.

The next choice in the operator’s toolbox is to increase the pulse width. At the expense of resolving smaller or closely spaced impedance anomalies, wider pulse widths are the brute-force way to overcome both attenuation and dispersion.

A third choice is to divide and conquer.  Each construction or repair splice that is excavated provides an opportunity to TDR the two subsegments of cable from the splice in each direction.

Every cable is different, but more than likely the TDR should be able to identify all of the splices on a 6,200-foot run of No.2 URD cable employing just one or two of the aforementioned tactics.

Injection Length Limits

At Novinium, we know no bounds. We have a variety of patented injection paradigms to address long cable lengths. The preferred method for such a cable is sustained pressure rejuvenation (SPR). With SPR subsegments of cable are injected from termination-to-splice and from splice-to-splice. As you suggest, the longest run would likely be 2000 feet, the typical length of a cable reel. We have a model that allows us to predict injection times with great precision. Assuming your No.2 cable has round strands and 175 mils of XLPE insulation, a 2000-foot run utilizing SPR would require about 46 hours of injection time. We have treated cable subsegments that are several miles long and we have additional tools available for the most challenging circumstances.

With SPR the cable subsegments are typically deenergized during the injection process. If having the circuit deenergized for several days is not palatable, Novinium has still more tools at its disposable including flow-though splice technology that supports SPR. Of course, your suggestion of creating shorter subsegments by installing intermediate pull-boxes is another choice that can reduce the injection time.

Flow through really long runs of cable using older approaches is problematic. The challenges and the solution for really long cables, like submarine cables, are included in U.S. Patent 7,976,747 and in the paper, “Advances in Chemical Rejuvenation of Submarine Cables” presented at the Jicable conference in Versailles France in June 2007. To review this paper click here.

In short, we have many tools to address every conceivable situation. Talk to our crack field engineering team to explore all the options, or write back to me with more details and constraints. Check out our senior field Engineer Norm Keitges splashing in Puget Sound. He must have thought for a moment that he was a frog, because we generally frown on humans swimming on the job.

Bending boundaries,

T. B. 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