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Why DC May Replace AC (2019)

electricalindustry.ca

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Re: Why DC May Replace AC (2019)

#131

Post by mlyle »

julosflb wrote: post #110
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...

Blade pitching first use is to reduce the aerodynamics power above the rater power point of the turbine and avoid overload.

Yup, that's why the word "safety" is in there. I probably could have put that better. (It also includes e.g. feathering rather than just trying to stay close to rated power, speed, and torque limits)

Re: Why DC May Replace AC (2019)

#132

Post by mlyle »

Gravityloss wrote: post #128
mlyle wrote: post #97

Earlier quoted context omitted.

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.

> you would convert all the turbine generator outputs just to DC.

You could. But you probably still need a DC/DC conversion step or boosting in order to let the power flow from each of the turbines fairly. (The synchronous conversion to DC from AC provides opportunity to slightly change the voltages you get out, but not terrifically so).

Re: Why DC May Replace AC (2019)

#133

Post by picture »

R0b0t1 wrote: post #120
picture wrote: post #87

Earlier quoted context omitted.

> 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…

> You break skin at about 50VDC.

What does breaking skin mean in this context? My understanding was that humans largely act like a resistor with some parasitic capacitance and inductance. Wouldn't more voltage equal more current in a mostly linear relation?

Re: Why DC May Replace AC (2019)

#134

Post by thehappypm »

unnouinceput wrote: post #108
thehappypm wrote: post #95

Earlier quoted context omitted.

What do you mean by that exactly?

Get a capacitor that supports 700V minimum as highest applicable voltage, and a wire. Go to your socket outlet, put the capacitor in one hole, the wire in the other hole and see if you get shocked or not. Do the same (if you're still alive) with a DC source. Comeback here and let me know how the experiment went and also tell me why you skipped physics class in high-school when this was taught.

I know all about RLC circuits, I just fail to see how this makes it any safer than DC. Conversely, DC can flow through inductors, AC can’t.

Re: Why DC May Replace AC (2019)

#135

Post by R0b0t1 »

jnsaff2 wrote: post #127
R0b0t1 wrote: post #118

Earlier quoted context omitted.

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'…

Yes, but it's not AC in the sense that most people think of AC, where it's oscillating around 0V. Cable comment was related to you mentioning 12 and 24, it didn't seem compared against 120V in your comment.

Re: Why DC May Replace AC (2019)

#136

Post by elihu »

dreamcompiler wrote: post #92
elihu wrote: post #39

Earlier quoted context omitted.

Actually, induction motors seem to be losing popularity in EVs, being replaced by permanent magnet motors which are more efficient (which also makes them easier to cool). And they're usually regarded as AC motors because they're fed 3-phase AC power from a motor controller (also called an inverter). The entire motor controller / motor system runs on DC power, so sometimes it's referred to as a brushless DC motor. Fas…

All charging is done with DC internally. Level 1 and 2 charging uses a rectifier inside the car to convert AC to DC. Fast charging simply bypasses the rectifier. The reason Level 2 charging is current-limited by your car -- even if you had a very high-current AC source available -- is that to take advantage of a high-current AC source your car would have to carry around a bigger, heavier rectifier. Which would decrea…

Yep, that's all true. I meant that L1 and L2 use an external AC power source.

Re: Why DC May Replace AC (2019)

#137

Post by R0b0t1 »

picture wrote: post #133
R0b0t1 wrote: post #120

Earlier quoted context omitted.

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…

> You break skin at about 50VDC. What does breaking skin mean in this context? My understanding was that humans largely act like a resistor with some parasitic capacitance and inductance. Wouldn't more voltage equal more current in a mostly linear relation?

You'll feel it shock you. It overcomes the surface resistance of not particularly moist skin and makes your nerves tingle or your muscles twitch.

Re: Why DC May Replace AC (2019)

#138

Post by contingencies »

DoingIsLearning wrote: post #2

> In China and Europe, new cities and villages are being envisioned that will be entirely DC powered. Citation needed? Other then HVDC links or micro-generation I can't see a practical use for DC unless you are entirely off-grid.

I am aware of at least one American managed company here in China selling DC-all-the-things.

Re: Why DC May Replace AC (2019)

#139

Post by unnouinceput »

thehappypm wrote: post #134
unnouinceput wrote: post #108

Earlier quoted context omitted.

Get a capacitor that supports 700V minimum as highest applicable voltage, and a wire. Go to your socket outlet, put the capacitor in one hole, the wire in the other hole and see if you get shocked or not. Do the same (if you're still alive) with a DC source. Comeback here and let me know how the experiment went and also tell me why you skipped physics class in high-school when this was taught.

I know all about RLC circuits, I just fail to see how this makes it any safer than DC. Conversely, DC can flow through inductors, AC can’t.

AC can and will definitely flow through inductors, especially the 220V/60Hz one you have at outlet. I challenge you to connect an inductor, even one of several Henry's as value, to your outlet and to your hand and comeback here and tell me if it shocked you or not.
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