What if we had DC instead of AC in the power lines?

[coppercone2]

so then you are running things in linear region? thats wasteful.

[T3sl4co1l]

Yabbut, how are you going to keep those pulses together?  "Pulses" implies tons of harmonics.  Maybe we filter it, so there aren't many harmonics left; but then we're back where we started, with a lot of signal power in a more-or-less sinusoidal AC component, except we have a pesky DC offset we can't transmit through a passive transformer.

Limit the slew rate so that there would be little EMI at the frequencies that are particularly problematic.

I'm curious -- what slew rate is adequate for limiting EMI from 200 mile HV cables?

Can you ensure the waveform comes out the other end with good fidelity, no dispersion or anything?  Can you ensure it will have a constant impedance, or if not, that it can be compensated with a few line reactors and such?

This is actually a really good exercise, and I hope you take on the challenge of demonstrating it.  I haven't actually done it myself, and would be interested to see. 

Tim

[NiHaoMike]

so then you are running things in linear region? thats wasteful.

Actually, a rectifier load would draw no current until the input voltage goes above the voltage on the capacitor. It's also possible to use a switcher for the transitions.

Yabbut, how are you going to keep those pulses together?  "Pulses" implies tons of harmonics.  Maybe we filter it, so there aren't many harmonics left; but then we're back where we started, with a lot of signal power in a more-or-less sinusoidal AC component, except we have a pesky DC offset we can't transmit through a passive transformer.

Limit the slew rate so that there would be little EMI at the frequencies that are particularly problematic.

I'm curious -- what slew rate is adequate for limiting EMI from 200 mile HV cables?

Can you ensure the waveform comes out the other end with good fidelity, no dispersion or anything?  Can you ensure it will have a constant impedance, or if not, that it can be compensated with a few line reactors and such?

I'm thinking it would make sense for short distances of a few hundred feet at most. As in, most notably, for off grid applications.

[Zero999]

I doubt pulsed DC would do much to solve the arcing problem. I thought one of the reasons why AC is better at quenching the arc is because the current actually changes direction, not just stops for a short period of time. With pulsed DC  there's also the issue of pesky parasitic inductance keeping the current flowing and thus the arc, when the voltage drops.

[David Hess]

Would we have had fluorescent lighting with DC?  They do work with resistive ballasts once started but that's none too efficient.

Most likely they would be "hybrid" lights with a small incandescent bulb acting as a current limiter. At least until solid state becomes economically viable for that application. For that matter, why is it that CFLs are almost always AC inside? Wouldn't it be cheaper to just put in a voltage multiplier with capacitors sized to limit the current?

Fluorescent lamp operating life is compromised by DC operation due to electromigration between the filaments.

One thing not mentioned yet is HVDC transmission which has become quite common for new major interconnects, both for a slight efficiency boost, but also to avoid the need for frequency synchronisation.

Frequency synchronization is convenient but not the original reason high voltage DC lines were used.  Frequency synchronization could always have been done at one end without the high voltage conversion.

Losses are greater than proportional to voltage when AC is used due to factors like corona discharge so for a given cost of the transmission line elements, more power can be transferred for the same loss using DC rather than AC because the crest factor is lower.  This becomes significant with the longest lines at the highest powers making the high voltage and high power AC to DC and DC to AC conversion economical despite its high cost.

[duak]

I suppose electromigration could be minimized by swapping the fluorescent tubes end-for-end at some defined interval.  Chimney sweeps would be retrained as tube swappers.  Clean and swap your tubes, ma'am?

Until Westinghouse worked out the totalizing power meter there was no way to determine how much energy a customer had used.  Edison apparently had some sort of electrochemical cell that used electroplating and weighing of electrodes to determine consumption.

It was pretty obvious at the time that AC was much better than DC for distribution.  That Edison resorted to politics rather than technology shows his position.  If he had won, it wouldn't have been for long.

Can anyone think of any advantage in having DC delivered right to the user by the PoCo?  I suspect that Telco exchanges still distribute 48V DC power internally to simplify UPS systems.  BTW, I once ran across a number of lead-acid cells in a scrapyard which I assume were from the local Telco.  Each was at least a foor or two on a side.  I think they were rated at 800 amp-hours.  I recall thinking whether I could justify getting a few of these for something someday. 

Cheers,

[David Hess]

I suppose electromigration could be minimized by swapping the fluorescent tubes end-for-end at some defined interval.  Chimney sweeps would be retrained as tube swappers.  Clean and swap your tubes, ma'am?

Early DC fluorescent tubes were operated this way but they still suffered from much shorter operating life.

Until Westinghouse worked out the totalizing power meter there was no way to determine how much energy a customer had used.  Edison apparently had some sort of electrochemical cell that used electroplating and weighing of electrodes to determine consumption.

These cells were also used in test instruments as "hour" meters.  Once the cell reaches the end, its connections can be reversed to reset it and have it run in the opposite direction unless it leaked.

Can anyone think of any advantage in having DC delivered right to the user by the PoCo?  I suspect that Telco exchanges still distribute 48V DC power internally to simplify UPS systems.

I do not think there are any advantages.  It would decrease safety because arcs are more difficult to extinguish and DC has no transmission advantages at low voltages.

Transformers are both more reliable and more efficient than DC to DC converters.

[NiHaoMike]

Can anyone think of any advantage in having DC delivered right to the user by the PoCo?

Not having to worry about power factor because there's no such thing in DC. Also one less source of EMI.

[David Hess]

Can anyone think of any advantage in having DC delivered right to the user by the PoCo?

Not having to worry about power factor because there's no such thing in DC. Also one less source of EMI.

Replacing transformers with high power DC to DC converters is going to increase EMI.

[cdev]

We could have a decentralized store and forward power system model with much of the generation capacity for homes at least done via solar energy.

A while back I posted some stuff on a real nightmare scenario which could be caused by a solar storm. Our current system could be taken out for years, by a big solar storm or god forbid atmospherically detonated nuke.  It would in itself be a huge crisis that would cause the breakdown of society but I'm not done, as shortly later the lack of cooling power would lead to a chain reaction of nuclear meltdowns. All at once.

We might never recover, that could cause all sorts of problems.

A DC system could avoid all that.

[Beamin]

how would you float stuff or filter it so you don't have HV on the terminals of the device when you disconnect it? it will also clench your muscles and kill you quick.

and have you ever played with DC plasma?

i dont want fire snakes and lethal stored charges everywhere.

how do you limit current with passives? you would need monstrous MU inductors, which would generate ridiculous magnetic fields. 

everything would need to be complex multi failure mode active devices.

actually look at it with high current levels:


also shit would corrode like crazy for the same build quality.

The answer to this question should be this video. I will never use a c switches for DC again I knew the sparks were harder to quench but that was like an arc welder making almost 1cm spark on 240 which on paper isn't possible but once the conductive plasma forms its actually passing enough amps to power that heater through the air so really hot lethal spark!

[Seekonk]

Some big players have been trying to come up with a standard for DC transmission in the home. One efficient power factor corrected supply to get rid of all these simple half and full wave that are starting the power companies to force power factor correction on the lines. I run HV DC at my camp and run all the wall warts and other home electronics off that.