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  •  Ah, but what about power factor? (6+ / 0-)

    Dusting off my old EE degree, and remembering all those months in power class... (some of the lab sessions could get kind of exciting), when AC is involved you can never neglect the power factor.  This is a bad thing in electrical distribution systems, and it happens when the sine waves of the current flow are out of phase with the sine waves of the voltage.

    Numerous parts of the generation and distribution network become less efficient when the power factor is large.  Utilities go to great expense to counteract the effects of power-factor-boosting things.   Impedance of power lines, transformers, and the loads those annoying customers insist on connecting, all contribute to an increase in the out-of-phaseness.   The motor in your refrigerator, for instance.

    If I remember correctly (it has been some decades) power factor arises from the current phase being retarded behind the voltage phase, and the retardation effect goes up with the amount of current in the lines.  So increasing the transmission voltage and thus decreasing the current, could have the additional benefit of decreasing the power factor effect of the lines.

    I can't remember how much this amounts to in the total schema of things - my summer jobs in the electrical industry were limited to the Distribution Department (from the sub-station to your house), not the long distance Transmission Department, where all these high voltage thingies happen.   We are talking upwards of a million volts, here.   Quite spectacular when one of those insulators fails.   We had a lab where such things were made to happen on purpose.  Like a scene out of Frankenstein.

    A really interesting case is Japan, where half of the country runs on 50 Hz power and the other half on 60 Hz.  You can't easily convert one to the other at these power levels, and if you change the frequency, you also change our old friend the power factor.  So to enable power sharing around the country, Japan has had to build some really big motor-inverter sets, and also really big vacuum tubes to do this.

    It works out that actually using DC for long distance transmission has some benefits.  No power factor losses at all.  But again, big converters at both ends.

    Though ones hates to think that Edison might have been right about something.

    •  Poor power factory simply means (5+ / 0-)

      that you need to increase the current to deliver the same amount of usable power to the load(s), because the voltage and current are out of phase. The usable (real) power delivered is P = IR cos p where p is the phase angle (how many degrees out of phase the voltage and current are), and cos p X 100%  is the power factor - bigger is better, perfect is p = 0, cos p = 1, PF = 100%.

      So if you need 120 watts at 120v, in phase you need 1 Amp. With a phase angle of 30 degrees, PF = 87% and you now need 1.15 amps (15% more current) to service the same 120 watt load. If the wire's resistance in 1 ohm, you'd lose 1 watt in transmission with 100% PF, but 1.3 watts (30% worse) with 87% PF. (There are also 18 watts of "imaginary power", or the nameplate rating on a single device with 87% PF would 138 VA instead of 120 watts - VA is Volt-Amps).

      In places that use a lot induction motors (like factories), they install large banks of capacitors to bring the voltage and current closer to in phase. They do this not because they want to save energy, but because the electric utility charges them for three things: the power they actually use, the power factor they use it at, and demand (sort of the peak power they draw).

      Computer power supplies also have terrible power factor, but residential and commercial users usually don't pay for it, directly anyway. You can buy power factor corrected computer power supplies to replace the original equipment. If decent power factor were mandated, given the huge number of computers and other electronic equipment, there would be a large energy savings.

      Frequency affects power factor only because it affects impedance - the imaginary or reactive part of impedance varies with frequency, so that affects the phase angle between V and I.

      Edison was still wrong, because the technology to step up or step down DC efficiently has only existed in the last 30-40 years or so. But DC transmission lines don't suffer from reactive losses like AC lines do. They still have to deal with ohmic losses.

      Modern revolutions have succeeded because of solidarity, not force.

      by badger on Tue Jan 22, 2013 at 01:24:37 PM PST

      [ Parent ]

      •  I agree with this discussion. Power factor (0+ / 0-)

        can be corrected at the user, and also at the electric switchyard.

        Republicans are like alligators. All mouth and no ears.

        by Ohiodem1 on Tue Jan 22, 2013 at 09:22:38 PM PST

        [ Parent ]

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