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Why DC May Replace AC (2019)

electricalindustry.ca

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Re: Why DC May Replace AC (2019)

#111

Post by jnsaff2 »

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 transmission, sure if it makes sense for your use case. DC-ify all homes and appliances just because? Definitely not. DC-ify parts of home lighting? Why not. Tho lighting is usually at 24VDC or 48VDC and then you are going to need either very thick cables (insane waste) or DCDC converters and where is the advantage there?

Besides transmission line level of DC, rest of the article is pretty much noise, generation is where we have big problems at the moment.

Also AC frequency is currently used as signaling mechanism to regulate grid power-balance and stability, spinning reserve and all that jazz. With DC you need some other signaling mechanism, first to mind is Voltage but with all the smart semiconductor devices my guess is that the signal gets lost as these devices tend to compensate. So you'd need a dedicated signaling mechanism like software...

Re: Why DC May Replace AC (2019)

#112

Post by KennyBlanken »

Reason077 wrote: post #35
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…

> "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).

I have personally worked on a tens-of-kW-scale wind turbine which was directly connected to the grid and was fixed speed and blade angle. It's quite common at lower power ratings because the cost/complexity of pitch control isn't justifiable.

It wasn't much more complicated than a three phase electric motor bolted to high-angle-of-attack blades, connected to the grid via contactors controlled by a microcontroller and a grid-intertie monitor.

Re: Why DC May Replace AC (2019)

#113

Post by twobitshifter »

DoingIsLearning wrote: post #98
Skunkleton wrote: post #76

Earlier quoted context omitted.

But DC is used for distribution, just at extremely high voltages.

HVDC is used in distribution for specific transfer between point A and point B, HVDC is not used _for_ distribution. Distribution implies a network that keeps getting downstepped until it reaches consumer nodes.

San Francisco has a “secret dc grid serving 900 customers” - the problems described are now solvable.

https://news.ycombinator.com/item?id=26431569

Re: Why DC May Replace AC (2019)

#114

Post by alduin32 »

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…

Yes, opening a DC circuit under load can be very surprising. The arcs can just stay on forever, and pose a big fire hazard.

Re: Why DC May Replace AC (2019)

#115

Post by dreamcompiler »

coding123 wrote: post #103
dreamcompiler wrote: post #85

Earlier quoted context omitted.

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…

That's going to be some thick ass wires for the oven, water heater and a few other high wattage items.

The big loads would have to remain at 120 or 240vac.

Re: Why DC May Replace AC (2019)

#116

Post by danachow »

Arainach wrote: post #47
danachow wrote: post #33

Earlier quoted context omitted.

> 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. First you say AC is way better for transmission, which is false - you should look into what HVDC is.. and second what you’re talking about “curb to home” is power distribution, not transmission - so your post is confused on a few le…

https://www.youtube.com/watch?v=S7C5sSde9e4&t=3m18s https://www.youtube.com/watch?v=S7C5sSde9e4&t=7m34s The thermal properties of AC for comparable amounts of power in a conductor are significantly better. Copper is expensive, and having dealt with just the difference between connecting 12 gauge and 14 gauge to outlets and pulling it through conduits I'm certainly not enthusiastic about needing larger conductors.

He’s demonstrating the use of a transformer to step up the voltage. The thermal properties are because of the lower current - not the line frequency. The transformer requires AC to work, but that is not the only way to convert power, and once the voltage is stepped up, it can be rectified back to DC and will actually be more efficient than AC - hence the reason that HVDC is a thing.

High power DC-DC conversion equipment is still more expensive than AC and the technology simply didn’t exist during the current wars.

Re: Why DC May Replace AC (2019)

#117

Post by kkfx »

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…

The case for DC is named microgrids: keeping the frequency on a microgrid is hard, inverters react too slowly for most loads (that does not ramp up/does down power slowly) while DC devices can be far quicker. The case for AC is large grid and safety.

Re: Why DC May Replace AC (2019)

#118

Post by R0b0t1 »

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.

Re: Why DC May Replace AC (2019)

#119

Post by sokoloff »

picture wrote: post #87
zw123456 wrote: post #74

Earlier quoted context omitted.

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.

The EU safety regs have a cutoff of 50VAC and 75VDC. (Those are only ~5% different in peak voltage.)

Below those levels, you have only general product safety regs to comply with. At/above those and up through all “reasonably household” voltages, you (probably) have to comply with the EU low voltage directive. “Probably” because the LVD itself isn’t law but member states have generally implemented it in their laws.

Re: Why DC May Replace AC (2019)

#120

Post by R0b0t1 »

picture wrote: post #87
zw123456 wrote: post #74

Earlier quoted context omitted.

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.

You break skin at about 50VDC. We could keep ~120V for distribution. There are problems tuning control loops of buck converters dropping more than about 30V (though you can just have multiple buck converters in a row).

Correcting the power factor from 120VAC gives you a boost circuit that gives you 360-400VDC. Some motor control and battery technology standardizes around this voltage. Cars are a big one, but also PFC direct to inverter motor control, which is becoming popular in white goods.

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