Will we have HVDC in home socket one day?

[PandaBear]

Seems like the way things go in the future we would be better off with high voltage DC for a lot of stuff, for better variable load variable frequency appliances like HVAC, computer, fridge, lighting, microwave, fan, car charging, etc.

I think at least in the US 110V AC is going to eventually hit a limit on what we can affordably power, and if we are going to switch to higher voltage and current why don't we start with a DC, something that we can easily step down and switch to variable load?

 

[Leo99]

Edison vs Tesla Part 2?

 

[supton]

I've long wondered that. Maybe with the move to more solar it would happen? Once people start using battery banks at home (recycled car batteries?) then DC is going to make more sense. In the home.

But I wonder what voltage would be used. Copper costs money. Regular household wiring exists, and is rated for what, 600V? On a full rewire (of changing the wiring in the breakers and then all the outlets) one should have a low loss / high voltage setup already in the house. But... bucking voltage from very high voltage down to a low voltage, I forget how efficient that is. Usually things suffer economy of scale. Buck convertors can be efficient but nothing is 100%.

A good amount of stuff I believe is already just taking 120Vac and first hitting a diode bridge. I'd think they'd happily run on 120Vdc without issue. A number of those devices are 90-240Vac. Not everything, just a good amount of low(er) power items.

DC may make sense outside of the home. We like to think that power transformers are lossless and great devices to step voltage up/down. Which is reasonable. But even at a low 60Hz frequency the bulk of the thick high tension wires does not carry electricity--it can be steel for strength, with something more conductive around it. Skin depth is about a third of an inch at 60Hz, meaning 67% of the electricity is flowing on the outer third--and it drops exponentially after that. But DC can use the whole wire diameter (still would need steel in the middle, for support, but now one could use a thicker layer of aluminum and perhaps less steel). I have to wonder if transmission line losses might go down on high voltage transmission lines that have to run long distances--they may, or they may lose it on step up/down and conversion in/out of 60Hz.

We'll see in the future. Change is hard, and I'm not sure that 60Hz is not getting the job done, and changing outlets and connectors is not trivial.

 

[JohnnyG]

You're kidding right?
There is a very good reason for widely distributed AC current.

However, if you are talking about locally generated electricity, such as solar cells or wind power, you might have a point.
In a different post I said that in Australia, where solar power is cheap, electric water heaters (resistance type) can be powered by DC without a converter/Inverter. Other common items are not quite so easily adapted.

 

[PandaBear]

You're kidding right?
There is a very good reason for widely distributed AC current.

However, if you are talking about locally generated electricity, such as solar cells or wind power, you might have a point.
In a different post I said that in Australia, where solar power is cheap, electric water heaters (resistance type) can be powered by DC without a converter/Inverter. Other common items are not quite so easily adapted.

I am not kidding. I don't think it would suddenly happen one day, but it might happen as a single HVDC enter the home and then transformed locally into the 110V inside the home, with different socket for different voltage.

This of course all depends on what the main load is and whether we can efficiently use 15A 110V like we have gotten by for 100+ years. If anything it will happen in large commercial facilities first and then gradually change to apartment building, condo, dorm, etc before entering older home.

As of now lighting are no longer the main electric user and now it is climate control, refrigeration, cooking, home electronics (using low voltage DC anyways), and in the future water heater (if heat pump being cheaper than natural gas). DC has better variable load and switching efficiency to begin with.

 

[brianl703]

If anything it will happen in large commercial facilities first

Large commercial facilities usually have 480V 3-phase, 277V single phase, 208V 3-phase, as well as the normal 120V single phase.

With such a wide voltage range available, what would HVDC offer to make it worth the trouble of installing?

 

[nomas]

High voltage DC transmission lines already exist .

 

[BeerCan]

Recently I learned that some transmission cables are HVDC. It makes sense in a lot of situations. For example https://en.m.wikipedia.org/wiki/Pacific_DC_Intertie

Edit: nomas beat me

 

[vwmaniaman]

110 v AC is much safer in the home. Because it changes its directional charge 60 times a second. If you had 110 V DC in an outley and your kid or your pet or you became exposed to the power, you would not be able to let go of the wires. DC will hold on to you and can stop your heart.

 

[supton]

Large commercial facilities usually have 480V 3-phase, 277V single phase, 208V 3-phase, as well as the normal 120V single phase.

With such a wide voltage range available, what would HVDC offer to make it worth the trouble of installing?

I'm not sure about commercial facilities like that, it'd depend on the type of motor drives I'd think.

I used to wonder about office buildings. Years ago everyone "needed" a UPS on their work computer. Why bother with a bunch of 12V SLA's that had like 5 minutes of backup time when a building could just invest in a much larger system and keep everyone working through a major outage? but laptops have steadily decreased in price and now I think almost everyone in my office has a laptop.

 

[supton]

110 v AC is much safer in the home. Because it changes its directional charge 60 times a second. If you had 110 V DC in an outley and your kid or your pet or you became exposed to the power, you would not be able to let go of the wires. DC will hold on to you and can stop your heart.

I do not believe that to be true. A quick search seems to indicate otherwise. Link. [I'm not well versed in this aspect so if you have more info I'd be interested to know.]

 

[brianl703]

I'm not sure about commercial facilities like that, it'd depend on the type of motor drives I'd think.

From what I've seen pretty much anything larger than a small building or strip mall is going to have 480V service. Multi-story office buildings and large warehouses, for example.

 

[supton]

From what I've seen pretty much anything larger than a small building or strip mall is going to have 480V service. Multi-story office buildings and large warehouses, for example.

I'm sure. My thinking was, what's the incentive to change? Well, if they could get cheaper motor drives, or otherwise justify the cost of a changeover, then it works. But if all the motors are AC motors of some sort, or otherwise tied to 60Hz, then isn't much of an incentive.

 

[brianl703]

I'm sure. My thinking was, what's the incentive to change? Well, if they could get cheaper motor drives, or otherwise justify the cost of a changeover, then it works. But if all the motors are AC motors of some sort, or otherwise tied to 60Hz, then isn't much of an incentive.

I wouldn't think that a motor drive designed to run off DC is going to be very much cheaper than one designed to run off AC. And yes, there is a huge installed base of 60Hz AC motors..

 

[Leo99]

I do not believe that to be true. A quick search seems to indicate otherwise. Link. [I'm not well versed in this aspect so if you have more info I'd be interested to know.]

I thought Edison did an exhibition electrocuting various animals with unAmerican AC to prove his good old American DC was safer.

 

[supton]

I thought Edison did an exhibition electrocuting various animals with unAmerican AC to prove his good old American DC was safer.

Apparently so. Link.

Brown paid local children to collect stray dogs off the street for his experiments with direct and alternating current.[59] After much experimentation killing a series of dogs, Brown held a public demonstration on July 30 in a lecture room at Columbia College.[60] With many participants shouting for the demonstration to stop and others walking out, Brown subjected a caged dog to several shocks with increasing levels of direct current up to 1000 volts, which the dog survived. Brown then applied 330 volts of alternating current which killed the dog. Four days later he held a second demonstration to answer critics' claims that the DC probably weakened the dog before it died. In this second demonstration, three dogs were killed in quick succession with 300 volts of AC.[61] Brown wrote to a college that he was sure this demonstration would get the New York Board of Electrical Control to limit AC installations to 300 volts. Brown's campaign to restrict AC to 300 volts went nowhere but legislation did come close to passing in Ohio and Virginia.[62]

Link 2.

Edison’s reaction to AC’s success was to show how dogs and cattle could be killed by AC but remain unharmed by DC of similar powers, fuelling media scare stories of accidental electrocutions from faulty AC installations. Edison even helped New York state authorities to find a new way of executing prisoners using AC, with convicted murderer William Kemmler in 1890 becoming the first person executed in an AC electric chair. The end of Williams’ life was, by all accounts, gruesome and slow, which suited Edison well, who dubbed AC the “death current”.

Let's be clear: 1kV of AC or DC will kill you, if it can make a path through your body. It's the current through the heart that will stop it (although enough current through limbs can do all sorts of damage).

 

[MolaKule]

I think at least in the US 110V AC is going to eventually hit a limit on what we can affordably power, and if we are going to switch to higher voltage and current why don't we start with a DC, something that we can easily step down and switch to variable load?

How do you step down a DC voltage unless you use a large and inefficient, power robbing rheostat to do so? What constitutes a variable load and how do you control it?

 

[PandaBear]

I'm not sure about commercial facilities like that, it'd depend on the type of motor drives I'd think.

I used to wonder about office buildings. Years ago everyone "needed" a UPS on their work computer. Why bother with a bunch of 12V SLA's that had like 5 minutes of backup time when a building could just invest in a much larger system and keep everyone working through a major outage? but laptops have steadily decreased in price and now I think almost everyone in my office has a laptop.

Laptop certainly eliminates the need for UPS of that machine. We don't alway have everything on UPS though and mainly it is for safe shutdown without data loss than keep running through blackout. I think having the whole building powered with battery backup seems overkill when the major need is likely a few systems only.

I have seen some data centers switching their power supply from 1 phase to 3 phases and that improves their efficiency in single digit percentage, everything adds up of course. I wouldn't be surprised those build next to solar farm might be fed with HVDC directly, but that's likely a special case.

 

[PandaBear]

How do you step down a DC voltage unless you use a large and inefficient, power robbing rheostat to do so? What constitutes a variable load and how do you control it?

Assuming it is for DC to DC, and from hundreds of volt to say 12V, that'll likely be using MOSFET with PWM and capacitor and inductor (SMPS). rheostat like resistance voltage control robs too much power and will not be used these days. There are DC->DC converter, but only from high to low voltage, not the other way around without things like charge pump.

Variable load would likely be an inverter driven load like ECM motor, with variable frequency or current derived from a DC source, without going from household 110V AC to DC then invert back to variable amp and freq AC.

 

[supton]

Laptop certainly eliminates the need for UPS of that machine. We don't alway have everything on UPS though and mainly it is for safe shutdown without data loss than keep running through blackout. I think having the whole building powered with battery backup seems overkill when the major need is likely a few systems only.

I have seen some data centers switching their power supply from 1 phase to 3 phases and that improves their efficiency in single digit percentage, everything adds up of course. I wouldn't be surprised those build next to solar farm might be fed with HVDC directly, but that's likely a special case.

I'm not surprised, going from single phase to three, less ripple.

Assuming it is for DC to DC, and from hundreds of volt to say 12V, that'll likely be using MOSFET with PWM and capacitor and inductor (SMPS). rheostat like resistance voltage control robs too much power and will not be used these days. There are DC->DC converter, but only from high to low voltage, not the other way around without things like charge pump.

Indeed. My mind boggles at what it'd take buck a few kV down, I'd think the problems would kill it--but it's likely doable. FET on resistance, inductor losses... imagine what kind of caps would be used! and their size! [Imagine the EMI too!]

Anyhow. Transformers aren't 100% efficient either--they have to have means of cooling too. It's not like an SMPS has to be >99% efficient to be in the running.