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Why DC May Replace AC (2019)

electricalindustry.ca

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Re: Why DC May Replace AC (2019)

#31

Post by danhor »

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…

AFAIK wind turbines, as opposed to conventional turbines in power plants, don't directly connect to the 60Hz Grid but go through DC and an inverter. This is done so they can efficiently work at different speeds at not just a few mechanically selectable ones

Re: Why DC May Replace AC (2019)

#32

Post by elihu »

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 voltages isn't as bad as AC, and you mostly get rid of 60-hz RF noise.

One argument for AC though is that it's easier to make AC switches, since those have a self-extinguishing arc. Maybe even household light switches can be replaced by solid-state devices?

Aside from the difficulty of making reliable switches, I'm not aware of anything about DC that makes it inherently more dangerous than an equivalent AC voltage.

Re: Why DC May Replace AC (2019)

#33

Post by danachow »

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 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 levels - it’s hard to understand what you’re trying to even claim.

> its been repeated elsewhere, but transmitting dc power with any meaningful voltage is dangerous, like burn your whole house down dangerous

Would you like to specifically reference in your linked video where that claim is made? Because I didn’t see that, and I am puzzled what you’re referring to. Quite the opposite he demonstrates at household voltage, DC is safer than AC from a shock standpoint (see 2:08). Why do you think high voltage DC is inherently less safe than AC?

This video seems to demonstrate the basic historical concept that AC is superior for transmission due to the typical ease in converting to high voltage low current and back - the key point is that it is high voltage for lower current and lower loss and this has traditionally been easier achieved with AC. It doesn’t really get into modern power conversion which has changed things somewhat.

Re: Why DC May Replace AC (2019)

#34

Post by danhor »

InTheArena wrote: post #18

I keep wondering if there might be some value in a derivative high-efficiently USB-C PD standard (since distance, and other factors come in) for whole house. IE, could you add DC power via USB connections to a bunch of different devices with a high-efficiency power supply for all of the different connections, rather then having low-efficiency power supplies in lots of other devices?

Most modern (GaN) USB-C chargers are already highly efficient. Since USB-C PD can only be used as a point-to-point connection (since a voltage level is negotiated), a sane architecture would likely use a high voltage (>100V to minimize resisitve losses) DC line with local step-down. At that point, the dc-dc step down shouldn't be integrated into the cabeling but into an external unit or into tne device, since the idle power, size and cost of a 500W PC power supply and a 5W headset charger is very different (even using DC).

The losses from the AC conversion aren't very high and the most energy intensive consumers (resisitive loads, ACs, Fridges) don't benefit much from switching to DC.

With high voltage DC safety becomes another concern, with arcing being a huge issue.

Re: Why DC May Replace AC (2019)

#35

Post by Reason077 »

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).

Re: Why DC May Replace AC (2019)

#36

Post by borodi »

jacquesm wrote: post #30
bsder wrote: post #25

Earlier quoted context omitted.

You are simply wrong. Skin effect at 60Hz is about 8mm. Power transmission (especially the long distance ones) conductors are normally quite a lot larger than that. Even the wires coming into your house are probably pretty close to that so there will be some effect even if it's not huge.

8 mm (in copper, which is rarely used for powerlines, if at all, it is super expensive and heavy) is huge for a single conductor, and your typical overhead powerline is concentric shells of tens of conductors. Negligible: has no practical effect on the construction. It's in the 4th significant decimal or so for a typical powerline segment, dwarfed by plain resistive losses. You want those multiple conductor arrangeme…

Reactive power losses due to the cable's inductance are probably the largest factor in the efficiency boost of using DC. Also you can use a somewhat higher voltage since you don't have to account for the AC peak, it does mean however that breaking a DC arc is harder since the is no 0 crossing.

Re: Why DC May Replace AC (2019)

#37

Post by dudeofea »

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…

I'm not sure if that's the moral of that video. Just watched (had seen before), and his conclusion is that you shouldn't transmit power with low voltage because it means you need lots of current.

HVDC has some tradeoffs over HVAC, but you should be able to transmit power just fine with either.

HVDC doesn't suffer from the skin effect that AC does: https://www.allaboutcircuits.com/textbook/alternating-curren...

HVDC is however harder to make/break contact with compared to HVAC as AC crosses zero volts many times: https://electronics.stackexchange.com/a/325608

EDIT: Sorry, didn't mean to pile on this comment with everyone else

Re: Why DC May Replace AC (2019)

#38

Post by shadowgovt »

danhor wrote: post #31
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…

AFAIK wind turbines, as opposed to conventional turbines in power plants, don't directly connect to the 60Hz Grid but go through DC and an inverter. This is done so they can efficiently work at different speeds at not just a few mechanically selectable ones

They have to, because the variable wind means that they need to be spinnable at a continuous range of speeds.

Components connected to the AC grid need to synchronize with the grid's frequency. Since we can't force the wind to blow at a particular rate, we'd either need a lot of fancy mechanics on the turbines themselves to drop their speed (which would waste energy) or we decouple their spin rate from the grid frequency with the AC -> DC -> AC converter (which also wastes energy, but probably less and with much less cost than complicated spin-rate-stabilizing machinery).

Re: Why DC May Replace AC (2019)

#39

Post by elihu »

WorldMaker wrote: post #14
castratikron wrote: post #11

Earlier quoted context omitted.

Question is whether those devices would be better off using brushless DC or some other DC motor? Tesla uses a switched reluctance motor (basically stepper motor) instead of induction motor on their low end vehicles for example.

Just about every electric vehicle at this point uses DC induction motors. Many support AC fast charging with various sorts of battery hacks, but Lithium Ion batteries are kind of inherently DC when it comes to applying power to the motors. At this point DC motors generally seem to out-class their AC counterparts other than the efficiency of using the same current as walled outlets in homes. The article puts it this w…

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.

Fast charging is done with DC. Level 1 and level 2 charging uses 110 or 220 volt AC and is quite slow by modern standards.

Re: Why DC May Replace AC (2019)

#40

Post by b33j0r »

If we discover room-temperature superconductivity and it is industrial-scalable, I could see DC taking over AC transmission lines.

But right now, resistive heat losses make DC a silly solution. That’s why we rectify only when the energy reaches “the edge.”

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