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Why DC May Replace AC (2019)

electricalindustry.ca

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Re: Why DC May Replace AC (2019)

#81

Post by Gravityloss »

upofadown wrote: post #54
sandworm101 wrote: post #49

Earlier quoted context omitted.

Wind turbines, the big ones, regulate themselves to turn in sync with the grid. The adjust thier blades as needed to keep pace/power. That's why huge fields of them all turn in perfect lockstep.

From some random reading on the net, it is the smaller ones that can be directly connected. Above 2MW things are not as easy and it is better and more efficient to convert to DC.

Wind turbines have frequency converters. They are semiconductor devices. In one 3 MW turbine I visited, it was at the base of the tower.

The blade angle is adjusted to get optimum power extraction. Rotor RPM is completely independent of produced power frequency.

References: https://www.vestas.com/en/products/offshore/V236-15MW and https://library.e.abb.com/public/bf09cdf11d234241845c79ac343...

Re: Why DC May Replace AC (2019)

#82

Post by dreamcompiler »

Youden wrote: post #16

The author seems very confused about whether they're talking about the grid or devices. "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. "DC motors and appliances have higher efficiency and power to size characteristics." -> B…

> "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant.

DC is more efficient for transmission too.

The reason the grid uses AC is that high voltage is always[1] more efficient for transmission than low voltage--regardless of whether you're talking about AC or DC--and in 1900 the only way to step voltages up or down was to use transformers, and transformers require AC. That's unfortunate because for a given voltage, it's more efficient to transmit it as DC than AC.

But today we have power electronics which can step DC voltages up or down, which means we are free to convert the grid to DC.

[1] The single exception is superconducting cables which can transmit low voltage power just as efficiently as high voltage power. But they are not yet cheap enough to be practical except over short distances.

Re: Why DC May Replace AC (2019)

#83

Post by dreamcompiler »

jbay808 wrote: post #19
Youden wrote: post #16

The author seems very confused about whether they're talking about the grid or devices. "DC power is significantly more energy efficient than AC power." -> the examples go on to specify end points for electrical energy but we already use DC there, AC is mainly used in transmission, so the claimed advantages of DC are irrelevant. "DC motors and appliances have higher efficiency and power to size characteristics." -> B…

In terms of motors, yes -- virtually all motors need to provide AC to the coils to run, so DC motors need to use an inverter. But increasingly these days, even AC motors are being run from variable-frequency drives, in order to squeeze out a bit more efficiency, because the savings from better matching the load more than makes up for the losses in the drive. Many jurisdictions are starting to incentivize or require V…

Indeed. There seems to be a lot of confusion about the types of AC. "AC" can mean "50 or 60 Hz sinusoid synchronized to the grid" or it can mean "a waveform that is not a constant voltage." Most motors require some form of the latter. Older motors required the former, but there are very few reasons to build motors that way today. The ones that remain in service can easily be driven by electronic inverters from a DC supply.

Re: Why DC May Replace AC (2019)

#84

Post by analog31 »

arcticbull wrote: post #78
zw123456 wrote: post #59

There is some validity to using DC distribution within a building or domicile, it can make it cheaper and more reliable to have a central power supply and battery system. That is not a new idea obviously, that exact approach has been in use in the telecom industry for decades. There is a AC feed to a system of rectifiers that converts the AC from the grid to -48VDC and continuously charge a string of batteries. There…

Correct me if I'm wrong but isn't the whole point of using AC that it's easy to convert voltages super-efficiently with transformers? Having DC power at home seems like the worst of both worlds. Low voltage DC at home causes large losses in the wiring, and high voltage DC causes large losses in the step-down regulator (leaving you with a buck-type regulator operating at the low extreme of its duty cycle). After all,…

Switchmode power supplies already tend to rectify and filter the high voltage AC before stepping it down. At high frequency, the transformer can be a lot smaller.

Re: Why DC May Replace AC (2019)

#85

Post by dreamcompiler »

zw123456 wrote: post #59

There is some validity to using DC distribution within a building or domicile, it can make it cheaper and more reliable to have a central power supply and battery system. That is not a new idea obviously, that exact approach has been in use in the telecom industry for decades. There is a AC feed to a system of rectifiers that converts the AC from the grid to -48VDC and continuously charge a string of batteries. There…

I think 24vdc is pretty much ideal. It's low enough not to be dangerous (48 volts is right at the threshold of danger) while it could supply enough power via the 12 or 14 gauge copper that's already in most US homes to handle all lighting needs plus probably a decent fraction of other loads.

When you decrease the voltage to 12 you start having to think about fatter wire--especially in larger homes--and that retrofit would be expensive.

Re: Why DC May Replace AC (2019)

#86

Post by distortedsignal »

dhdc wrote: post #64
distortedsignal wrote: post #61

Earlier quoted context omitted.

> The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line for the same distance. The cost of the converters may not be offset by reductions in line construction cost and lower line loss. https://en.wikipedia.org/wiki/High-voltage_direct_current#Di... Not to mention that circu…

> Not to mention that circuit breakers are harder in DC. [Citation needed]

https://en.wikipedia.org/wiki/High-voltage_direct_current#Hi...

Re: Why DC May Replace AC (2019)

#87

Post by picture »

zw123456 wrote: post #74
dhdc wrote: post #70

Earlier quoted context omitted.

> But then there is the tradeoff of the popularity and availability of 12VDC devices due to RV/Boating. Which is exactly why we need to get away from those 12V old garbages asap. 12V is so bloody wasteful, in terms of both wasted power and wasted conductor material. The only reason why its still around is people got used to it and don't wanna change.

what is the optimal voltage based on empirical data ?

> highest we can get away with

Assuming this means highest voltage that's still mostly safe and does not require a whole bunch of insulation, we might be looking at something from 80 to 100 VDC? No idea where I read it from but apparently it takes about that much DC voltage for people to "feel something" when touching conductors with dry skin.

Re: Why DC May Replace AC (2019)

#88

Post by dreamcompiler »

arcticbull wrote: post #78
zw123456 wrote: post #59

There is some validity to using DC distribution within a building or domicile, it can make it cheaper and more reliable to have a central power supply and battery system. That is not a new idea obviously, that exact approach has been in use in the telecom industry for decades. There is a AC feed to a system of rectifiers that converts the AC from the grid to -48VDC and continuously charge a string of batteries. There…

Correct me if I'm wrong but isn't the whole point of using AC that it's easy to convert voltages super-efficiently with transformers? Having DC power at home seems like the worst of both worlds. Low voltage DC at home causes large losses in the wiring, and high voltage DC causes large losses in the step-down regulator (leaving you with a buck-type regulator operating at the low extreme of its duty cycle). After all,…

Not sinusoidal: Boost and buck converters use PWM square waves. Because the power transistors are almost always fully on or fully off the I2R losses in the transistors are quite low.

Re: Why DC May Replace AC (2019)

#89

Post by ncmncm »

elihu wrote: post #32
seanalltogether wrote: post #17

AC is still way better for power transmission, and I don't mean giant power lines spanning from one city to another, I mean from the curb to your home, or within the walls of your home. Electroboom has a video on this topic https://www.youtube.com/watch?v=S7C5sSde9e4 and its been repeated elsewhere, but transmitting dc power with any meaningful voltage is dangerous, like burn your whole house down dangerous, and if i…

AC won out historically because it's easy to change the voltage of AC using a transformer. If we reach a point where modern DC-DC converters are cheaper or better than a traditional transformer, then I don't see why we wouldn't just use DC everywhere. (I don't know if we're actually there yet.) With DC, you can transmit more power over the same wire you'd use for AC (no skin effect), electric shock from moderate DC v…

The physical design of the switch ought to be easy enough to make physically interrupt the arc.

Re: Why DC May Replace AC (2019)

#90

Post by CyberDildonics »

chrisp_how wrote: post #45

An AC computer is more efficient that a DC one, because the machine operates on cycles, or repeating interval-segments.

After skimming this post history I'm not sure if it's AI, a joke, a troll or something else all together. AC computers and sin + cos = consciousness are just the beginning.
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