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Why DC May Replace AC (2019)

electricalindustry.ca

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Re: Why DC May Replace AC (2019)

#121

Post by sokoloff »

userbinator wrote: post #99
dpierce9 wrote: post #42

The major thing that is unaddressed is the inertia of the extant built environment. We know that lead paint is bad for people, especially kids, but we haven’t remediated it in much of the pre-1976 housing stock. Why? It is expensive and the places where it is worst are not high-value areas. Similarly, there are many homes in America with low voltage knob and tube which is an uninsulated wire. Would you retrofit homes…

K&T is not uninsulated.

Can confirm. My previous house had some still in service for some general hallway lighting and other light loads. It was not jacketed (like modern non-metallic cable is), but it was definitely insulated.

Re: Why DC May Replace AC (2019)

#122

Post by simonebrunozzi »

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…

Youden, your comment is a great example of why I love HN so much. Thanks for sharing this, and clearing a few important things about the article.

Re: Why DC May Replace AC (2019)

#124

Post by The_Double »

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 ?

A lot of industry like datacenters and vehicles are moving to 48V. 48V is right on the limit of what is considered low-voltage and therefor doesn't have special safety requirements. And because it's a multiple of 12V, it can work well with old equipment by putting exiting supplies or batteries in series, and using very efficient constant-ratio down-converters.

see for instance: https://www.vicorpower.com/documents/whitepapers/wp-boosting...

Re: Why DC May Replace AC (2019)

#125

Post by Reason077 »

sandworm101 wrote: post #49
Reason077 wrote: post #35

Earlier quoted context omitted.

> "solar generates DC but wind generates AC." Yes, but a wind turbine is allowed to spin at variable speeds - its rotation is not synchronized to the grid frequency in the same way that hydroelectric and thermal turbines are. In order to get a wind turbine's power output to match the grid frequency, it goes through an AC -> DC -> AC conversion in a component known as a double-fed induction generator (DFIG).

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.

> "That's why huge fields of them all turn in perfect lockstep."

Not true for any modern wind farm. While they might appear to be turning together, that's just because the wind conditions are relatively consistent across the whole area. Not because they are actually synchronised! Most modern turbines will target a certain rated/optimal/maximum rpm once a certain wind speed has been reached, but are free to rotate more slowly (while still generating power) in light wind conditions.

Re: Why DC May Replace AC (2019)

#126

Post by pfdietz »

elihu wrote: post #73
dhdc wrote: post #64

Earlier quoted context omitted.

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

I can believe this is true. It's easier to make reliable AC switches because whatever arcing you get is self-extinguishing because the voltage passes zero 120 times a second. With DC, if you get an arc it'll just keep going as long as conditions allow. That means if you use a designed-for-AC switch in a DC application with the same voltage, it's likely to destroy itself sooner or later. (Apparently this is one of the…

A big capacitor?

Re: Why DC May Replace AC (2019)

#127

Post by jnsaff2 »

R0b0t1 wrote: post #118
jnsaff2 wrote: post #111

The author gets a few things wrong or mixed up (mostly covered in other comments). None of the other comments also talk about galvanic isolation, you need transformers (therefore AC) for that. But the article is trying to sell you on DC, like there are lot of marketing around hydrogen and such. I would say the right approach would use the right technology where it is the best option. HVDC to replace HV AC transmissio…

You don't need AC for isolation, you need transformers. Isolated DC to DC is a big area of power supplies. You've also got cabling size backwards: you need thinner conductors for higher V due to lower I.

Transformers only work with AC. Isolated DC-DC has also transformers where there is really high frequency AC. DC->AC||AC->DC. It's all hidden and due to high frequency quite small. But still AC.

Re cabling: currently we have 230V at home, going to 24VDC would mean 100x more losses. So I would say I got it correctly when I said going to 24V or 48V needs thicker cables due to increased I. As for your sentence, you don't NEED thinner conductors when voltage goes up but you would be quite wasteful if you didn't.

Re: Why DC May Replace AC (2019)

#128

Post by Gravityloss »

mlyle wrote: post #97
Gravityloss wrote: post #81

Earlier quoted context omitted.

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...

You're talking past each other. The electronics perform two key functions: * Choosing pitch angles for efficiency and turbine safety. You can, for smaller turbines, just synchronize the turbine to the grid, but this is becoming uncommon practice. * Converting the produced AC power to DC, and then choosing the proper frequency output and voltage to feed power to the grid, and inverting the DC to make this power. This…

Yeah, I don't know if for example in big wind offshore wind parks, since it's connected with a DC cable to mainland anyway, you would convert all the turbine generator outputs just to DC.

Then at the ground station where it's connected to the grid, convert to AC, whatever kilovolts are needed.

Re: Why DC May Replace AC (2019)

#129

Post by dpierce9 »

userbinator wrote: post #99
dpierce9 wrote: post #42

The major thing that is unaddressed is the inertia of the extant built environment. We know that lead paint is bad for people, especially kids, but we haven’t remediated it in much of the pre-1976 housing stock. Why? It is expensive and the places where it is worst are not high-value areas. Similarly, there are many homes in America with low voltage knob and tube which is an uninsulated wire. Would you retrofit homes…

K&T is not uninsulated.

I misspoke. The K&T I had in my house had gaps in the loom because it failed. You also can’t insulate around it.

Re: Why DC May Replace AC (2019)

#130

Post by sandworm101 »

Upitor wrote: post #101
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.

They are still not syncing directly to the AC grid. They are not going 50 rounds per second

Gearboxes. Windmills of the same design will turn at the same rate in lockstep with each other. A gearbox converts that rate into what is necessary for the generator. The generator turns in sync with the grid.
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