Loading ...
Sorry, an error occurred while loading the content.

SF6

Expand Messages
  • David
    So I recall reading that on or more L carrier runs, L-L was forced into pressuring the coax with SF6, not air, on a line that arced over repeatedly. Was it in
    Message 1 of 10 , Apr 4, 2013
    • 0 Attachment
      So I recall reading that on or more L carrier runs, L-L was forced into
      pressuring the coax with SF6, not air, on a line that arced over
      repeatedly. Was it in the California High Desert, maybe?

      I was wondering what effect that had on the line losses and equalization.
    • OZOB99
      Long repeater spans=higher voltage=corona/arcing;details in previous posts, search sf6 in archives.
      Message 2 of 10 , Apr 5, 2013
      • 0 Attachment
        Long repeater spans=higher voltage=corona/arcing;details in previous posts, search sf6 in archives.

        --- In coldwarcomms@yahoogroups.com, David <wb8foz@...> wrote:
        >
        >
        > So I recall reading that on or more L carrier runs, L-L was forced into
        > pressuring the coax with SF6, not air, on a line that arced over
        > repeatedly. Was it in the California High Desert, maybe?
        >
        > I was wondering what effect that had on the line losses and equalization.
        >
      • David
        ... I know that. There s discussion re: L-3 arcing, etc. I thought there was a specific L-4 that also had an issue, but in any case, my question is not why SF6
        Message 3 of 10 , Apr 5, 2013
        • 0 Attachment
          On 4/5/13 9:39 AM, OZOB99 wrote:
          > Long repeater spans=higher voltage=corona/arcing;details in previous posts, search sf6 in archives.

          I know that. There's discussion re: L-3 arcing, etc. I thought there was a
          specific L-4 that also had an issue, but in any case, my question is not
          why SF6 was used but rather what side effects it had re: transmission
          impairment. Just as air-insulated coax has different characteristics than
          foam-core, SF6 insulated will as well. How much so is the issue.

          (This question prompted by a recent walkaround of BG&E's Waugh Chapel
          500/230/110KV yard; and the new SF6 breakers on the 500:230 stepdown
          transformers.)
        • OZOB99
          I don t recall the BSP s on transmission,pilots,equalization,etc having seperate requirements for SF6 cables,so the difference was probably negligible;it s
          Message 4 of 10 , Apr 5, 2013
          • 0 Attachment
            I don't recall the BSP's on transmission,pilots,equalization,etc having seperate requirements for SF6 cables,so the difference was probably negligible;it's likley one of the propeller heads at BTL wrote a paper on the trials.

            This BSP mentions SF6 cables:

            http://www.telephonecollectors.info/index.php/browse/bsps/by-division-number/signaling-testing/doc_view/8148-330-200-501-i1




            --- In coldwarcomms@yahoogroups.com, David <wb8foz@...> wrote:
            >
            > On 4/5/13 9:39 AM, OZOB99 wrote:
            > > Long repeater spans=higher voltage=corona/arcing;details in previous posts, search sf6 in archives.
            >
            > I know that. There's discussion re: L-3 arcing, etc. I thought there was a
            > specific L-4 that also had an issue, but in any case, my question is not
            > why SF6 was used but rather what side effects it had re: transmission
            > impairment. Just as air-insulated coax has different characteristics than
            > foam-core, SF6 insulated will as well. How much so is the issue.
            >
            > (This question prompted by a recent walkaround of BG&E's Waugh Chapel
            > 500/230/110KV yard; and the new SF6 breakers on the 500:230 stepdown
            > transformers.)
            >
          • Robert Getsla
            I have some experience with SF6 in waveguides and cavity resonators, and I have worked with CableTV trunk and branch amplifier hardware, although I do not have
            Message 5 of 10 , Apr 5, 2013
            • 0 Attachment
              I have some experience with SF6 in waveguides and cavity resonators, and I have worked with CableTV trunk and branch amplifier hardware, although I do not have any AT&T Long Lines experience.

              The dielectric coefficient of SF6 gas is considerably higher than air, so the impedance of a coax filled with SF6 gas in the space betweeen the center conductor and the shield will not be the same as an air filled coax.  I believe the SF6 fille coax impedance would be lower than the same coax air filled, and it probably will be somewhat lossier, too, but it will be able to withstand a higher voltage and current in the center conductor for two reasons.  First, SF6 has a higher breakdown voltage than air, and secondly, it offers better heat transfer from a hot center conductor to a cooler outer conductor.  The cable center conductor becomes hot from I squared R heating due to the low frequency current passing through it (powering remote amplifiers distributed along the cable).  The RF signals in the cable are relatively low level (milliwatts at the sending end) and do not appreciably add to the cable heating.  CableTV uses a similar technology of
              power down the cable center conductor (60 or 90 Volt AC power at up to 15 amps of an almost square 60 Hz wave from saturable core voltage regulating transformers) to power remote line amplifiers, but CATV cable uses closed cell foam polyethylene between the center conductor (copper plated aluminum) and the outer shield (extruded seamless aluminum over the foam dielectric) instead of air or SF6 gas so it does not need to be presurized to withstand water intrusion.

              In the CATV world, newer systems use two (or more) "pilots" (reference signals injected at known levels at the sending end) with selective level detectors at each amplfier listening for each pilot.  The lowest frequency pilot is for overall AGC or temperature related cable loss compensation, while the higher pilot(s) operate after the AGC has restored the low frequency pilot level.  These higher frequency pilot detectors then provide automatic high frequency (or "slope") compensation control. 


              In an SF6 pressurized system, if the gas pressure ever becomes high enough, the SF6 could liquify (even at room temperature) and that will cause serious cable frequency response detuning, because liquid SF6 has a higher dielectric constant than SF6 gas.  So, I believe AT&T watched their SF6 pressures very closely to make sure they never reached the point where any SF6 gas might condense in the coaxes, because that would cause impedance discontunities in the cable and increase the "span" loss between amplfiers radically due to changing cableimpedance. 

              Bob Getsla aka LinearBob    WA6WHT
              Transmitter engineer for KTSF-TV26 and KTSF-DT27, San Francisco, CA  (1 day per week)
              Analog, RF, and Network engineer for WattMinder, Sunnyvale, CA (the rest of the week)
              Linux evangelist
              Inventor



              ________________________________
              From: OZOB99 <ozob99@...>
              To: coldwarcomms@yahoogroups.com
              Sent: Friday, April 5, 2013 6:39 AM
              Subject: [coldwarcomms] Re: SF6


               
              Long repeater spans=higher voltage=corona/arcing;details in previous posts, search sf6 in archives.

              --- In coldwarcomms@yahoogroups.com, David <wb8foz@...> wrote:
              >
              >
              > So I recall reading that on or more L carrier runs, L-L was forced into
              > pressuring the coax with SF6, not air, on a line that arced over
              > repeatedly. Was it in the California High Desert, maybe?
              >
              > I was wondering what effect that had on the line losses and equalization.
              >




              [Non-text portions of this message have been removed]
            • David
              ... Thanks for the education. On L4, the DC current is 520mA, so not sure how much I^2R there is to worry about. L5 was 910 mA. I know far less about L3, where
              Message 6 of 10 , Apr 5, 2013
              • 0 Attachment
                On 4/5/13 6:34 PM, Robert Getsla wrote:

                > I believe the SF6 fille coax impedance would be lower than the same coax
                > air filled, and it probably will be somewhat lossier, too, but it will
                > be able to withstand a higher voltage and current in the center
                > conductor for two reasons. First, SF6 has a higher breakdown voltage
                > than air, and secondly, it offers better heat transfer from a hot center
                > conductor to a cooler outer conductor. The cable center conductor
                > becomes hot from I squared R heating due to the low frequency current
                > passing through it (powering remote amplifiers distributed along the
                > cable).

                Thanks for the education. On L4, the DC current is 520mA, so not sure how
                much I^2R there is to worry about. L5 was 910 mA.

                I know far less about L3, where ~2000VAC was used. There, far longer
                distances between repeaters and power stations seemed to be the norm. That
                seems to be where the SF6 use was prevalent.
              • widebandit
                SF6 replaced nitrogen in those sections of L3 coax where the AC power potential was 1,200 V or greater. It was used mainly to suppress corona noise rather
                Message 7 of 10 , Apr 5, 2013
                • 0 Attachment
                  SF6 replaced nitrogen in those sections of L3 coax where the AC power potential was 1,200 V or greater. It was used mainly to suppress corona noise rather than to prevent arcing.

                  SF6 is discussed briefly on page 829 of the July 1953 BSTJ L3 Coaxial System Design paper:
                  <http://www3.alcatel-lucent.com/bstj/vol32-1953/articles/bstj32-4-781.pdf>
                  There is mention of extensive testing to determine that SF6 did not physically harm the cable, as well as the need for safety precautions when entering manholes or using soldering torches in the presence of SF6 (though SF6 itself is inert, it has the potential of producing toxic derivatives at high temperatures), but there is no mention of SF6 altering coaxial transmission properties and I would presume its effect was insignificant.

                  - waw -

                  --- In coldwarcomms@yahoogroups.com, David <wb8foz@...> wrote:
                  >
                  >
                  > So I recall reading that on or more L carrier runs, L-L was forced into
                  > pressuring the coax with SF6, not air, on a line that arced over
                  > repeatedly. Was it in the California High Desert, maybe?
                  >
                  > I was wondering what effect that had on the line losses and equalization.
                  >
                • David
                  ... The recently mention BSP notes 0.270 s coax s center conductor was 10.2 ohms per mile, and 0.375 was half that. So on L4, I^2R loss was 0.014 watts/mile
                  Message 8 of 10 , Apr 7, 2013
                  • 0 Attachment
                    On 4/5/13 6:34 PM, Robert Getsla wrote:

                    > The cable center conductor becomes hot from I squared R heating due to
                    > the low frequency current passing through it (powering remote amplifiers
                    > distributed along the cable).


                    The recently mention BSP notes 0.270's coax's center conductor was 10.2
                    ohms per mile, and 0.375 was half that.

                    So on L4, I^2R loss was 0.014 watts/mile per tube. On a 150 mile segment,
                    there would be 400v of IR drop, and ?74 or is it 75? repeaters at 24vdc
                    drop [if all 3 repeater types had that voltage Zener..] or 1800V; for a
                    total power segment drop of ~~2200V.
                  • widebandit
                    Don t forget that on a 150-mi power-feed span up to +1800-VDC was applied at one end, up to -1800-VDC was applied at the other end, for a maximum available
                    Message 9 of 10 , Apr 7, 2013
                    • 0 Attachment
                      Don't forget that on a 150-mi power-feed span up to +1800-VDC was applied at one end, up to -1800-VDC was applied at the other end, for a maximum available drop of 3600-V across the span, regulated to maintain a constant line current of 520-ma. For power-feed spans of 75-mi or less only one set of converters was used. The power system was regulated to a constant current of 520-ma, which means the supply voltage varied as required to maintain the current.

                      Presuming about 5.3-ohms/mi for the 100.3-mil copper center conductor (1.6-mil smaller than #10-AWG) and 520-ma current, 150-mi of center conductor had a total DC resistance of 795-ohms, a total voltage drop of about 413-V, and the center conductor dissipated about 215-W over that distance or about 2.9-W per 2-mile cable span. I would say the I^2xR temperature rise in the center conductor was negligible.

                      At 24-VDC and 520-ma, each repeater dissipated about 12.5-W, for a total repeater power of about 940-W on a 75-repeater power-feed span.

                      The total power dissipation of a normally operating 150-mi L4 span was about 1,155-W per single coax tube, 2,310-W per duplex pair, 23.1-kW for a fully loaded coax-20. That figures out to 713-mW per voice-circuit per 150-mi span, or about 19-W per voice-circuit over 4000-mi.

                      In the BSTJ April 1969 L4 papers, reference to the 2200-VDC long-term voltage drop corresponding to a DC ground voltage of 400-V may be found on page 1045:
                      <http://www3.alcatel-lucent.com/bstj/vol48-1969/articles/bstj48-4-1041.pdf>

                      With 2200-VDC nominal voltage drop over a 150-mi power-feed span and 1400-VDC of overhead, it would seem that the power-feed system had plenty of reserve regulating capacity.

                      The power converters also had a DC output resistance of 6,000-ohms or 12,000-ohms for two converters; which made the 795-ohm line resistance a rather small component of the overal power-feed resistance - an important factor in maintaining stable regulation.

                      - waw -

                      --- In coldwarcomms@yahoogroups.com, David <wb8foz@...> wrote:
                      >
                      > On 4/5/13 6:34 PM, Robert Getsla wrote:
                      >
                      > > The cable center conductor becomes hot from I squared R heating due to
                      > > the low frequency current passing through it (powering remote amplifiers
                      > > distributed along the cable).
                      >
                      >
                      > The recently mention BSP notes 0.270's coax's center conductor was 10.2
                      > ohms per mile, and 0.375 was half that.
                      >
                      > So on L4, I^2R loss was 0.014 watts/mile per tube. On a 150 mile segment,
                      >
                      Actually:
                      P = I^2 x R
                      P = 0.520A^2 x 5.3-ohms/mile
                      P = 0.2704 x 5.3
                      P = 1.43W per mile
                      P = 2.86W per 2-miles
                      P = 214.97W per 150-miles
                      >
                      > there would be 400v of IR drop, and ?74 or is it 75? repeaters at 24vdc
                      > drop [if all 3 repeater types had that voltage Zener..] or 1800V; for a
                      > total power segment drop of ~~2200V.
                      >
                    • David
                      SF6 was used in some coax cable segments:
                      Message 10 of 10 , Jun 14, 2013
                      • 0 Attachment
                        SF6 was used in some coax cable segments:

                        <https://www.nytimes.com/2013/06/14/us/department-of-energys-crusade-against-leaks-of-a-potent-greenhouse-gas-yields-results.html>

                        Department of Energy’s Crusade Against Leaks of a Potent Greenhouse Gas
                        Yields Results....
                      Your message has been successfully submitted and would be delivered to recipients shortly.