27418Re: SF6 and power down the coax
- Apr 7, 2013Don'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:
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 email@example.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,
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.
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