Switching from AC grids to DC grids

[MrOmnos]

The old yellow lights bulbs are lone gone and even the refrigerators are smart and everything we look around these days is DC powered. So, why are we still using AC? Why are we taking DC power from the large solar panel fields, converting them to AC again and dealing with losses in double digits? When we are turning all our electrical appliances "smart" , why are our grids still in the 50s?? Were the engineers so focused on the transistors that they forgot about the transformers?? What the hell happened??

[rob77]

take less (fill in what makes you high) .... and you will see the reasons..

just a few of them:
- vast majority of generators producing electricity are 3 phase AC
- vast majority of electric motors in factories are 3 phase AC
- vast majority of the grid uses transformers - again AC

you solar panels and your house - which might be ready to accept DC are just a small fraction of the system (like a grain of sand in a desert).
and don't forget that many home appliances are still AC.

[Rerouter]

Distance, and corrosion across dissimilar metals,

How do you step up a DC voltage from 300V to 11KV at 100A?, With DC, it involves multiple electronic parts with a low margin of overload, with AC its a block of metal with wire, that has an efficiency bloody close to 100%

what happens when you mix aluminum clad steel wire with copper wire or joiners, with AC, as the current flow reverses many times, you end up with only environmental corrosion, with DC, it is amplified and things break down faster,

And finally the most important one for the power grids, AC sparks are self extinguishing in most cases, the current falls to zero 50 or 60 times a second, and providing there is enough gap it will stop, with DC it wont, this is not just relevant in faults, think what happens when you open a switch contact, you need a more complex design to ensure the larger required gap is provided when the contacts are separated,

For local in house, sure rectify and use to your hearts content, but play with big power and its not worth it, even a little,

[HAL-42b]

There are no technical reasons to keep the AC grid. The only reasons are economical and political.

Technically one advantage of the AC grid was its ability to deliver 3 phase rotating field to motors. These days nearly all motors are driven from inverters anyway, despite the availability of 3 phase power. So this advantage of  AC is gone.

Second advantage is switching. Switching big DC currents is difficult. The contacts of the switch tent to arc and weld together. AC power passes trough 0V 100 times a second. This tends to extinguish arcs and makes switching easier.

The political aspect is centralization. Politicians like centralization and hate decentralization, despite what they say on TV.

The economic aspect is mainly related to coal and steel. You need three tons of coal to produce a ton of steel. For this to work you need cheap coal.

If the demand for coal fell the price of extraction would go up, which would make steel expensive and the industry would grind to a halt. Many coal burning coal powerplants keep the demand for coal high and price of coal low.

A DC distribution grid would encourage decentralization which would make coal burning plants unnecessary. This simply won't be allowed to happen.

[dannyf]

"why are we still using AC?"

For historical reasons, as the end devices are designed to work with a.c..

Long distance transmission is hvdc now. To make matters worse. Lots of wasted energy in a.c. to dc conversions.

[necessaryevil]

You say historical reasons, but DC transmission is only just getting economically viable in some situations. The length of the transmission line is a factor. And, despite being around for ages, photovoltaics are only becoming more common in the last few years. A few years... I wouldn't call that 'historical'.

[MrOmnos]

I don't think you guys got the point I am trying to make. It's not that we can't generate DC power. Why not generate DC power?? AC transmission lines were outdated long time ago. They were never designed to for high power transmission. Only reason we have large AC grids because small grids got interconnected with each other and grew over time. As the population grew the power demand increased and today we are dealing with losses in double digits. According to Siemens for a power input of 3500Mw, at 600miles and 2x500Kv volts the power output of an AC line is around 2,975Mv i.e loss of around 15% per 600miles. Where as High-Voltage dc lines at 600Kv has the output of 3,342Mv i.e loss of about 5%. I understand the High Voltage DC conversion equipment is costly, but the overall cost when the losses are factored in are less than AC lines. And with the advent of better technology it is possible to decrease the conversion costs as well.  A DC line would require less conductor, no need to deal with phases. No skin effect. We wouldn't have to worry about big pylons to put the cables on. We could run them underground. And it would be compatible with all other forms of energy generation. I think if we had done the same amount of research in electrical systems as we did in the semi-conductors department than we would have better grids today. We left it as it was because it was working but it won't for long. I mean think about it, how convenient it would be to have 75v DC line running in your house. With the efficiency of DC-DC converter, we could save lots and lots of energy. No need of those crappy AC adapters with crappy step down transformers.The possibilities are endless.

[HAL-42b]

I get the point and I do agree but I have no say in power distribution research 

Google had some articles published about running their servers with 48V DC. They did some analysis on that. Might be interesting reading.

It would be interesting to see how a 75V DC grid would stack up economically with the ever increasing prices of copper.

[Seekonk]

This will be the future.  Some large companies are working on a home DC standard for home electronics.  The utilities will soon become fed up with devices that only take power at the peaks.  A single PFC converter for each home will solve that.  That demand may be driven by LED lighting.  Getting tired of my LED lights flickering with every surge in the grid.  The grid won't get more reliable.  Considering people would rather starve than have their cell phone and internet go dead, a few solar panels on each house feeding a DC buss will have momentum.  I live in a community where 1 in 4 homes has solar panels.  In US tanks of hot water are still the norm. Heating water with a few hundred watts of PV is Is the most cost effective use of PV. I heat all my water with just the PV energy that would be normally wasted. Think of the effect of every home having just a few solar panels powering the wall wart suckers of our existance.  It is a huge dent in electrical usage.  Grid tie may be the sensible option, but the cards are quickly being stacked against it.  Payouts will continue to drop when you feed power back.  The home DC grid is likely the future.

[HAL-42b]

Electronics yes. But how would you go about designing an ordinary 2500W kettle? It would have to draw 33.3 Amps.

The on-off switch would have to interrupt this much current without arcing and without welding shut, and it would have to cost pennies. Same for the thermal cut-off.

[Seekonk]

I don't see why it has to be one or the other.  Utility power is here to stay for big items.  A secondary 500W DC grid for the home that is solar ready is in everyones future.

[MrOmnos]

Smart kettles!! Solid state relays/switches (cost few bucks these days and can handle 40 amps with ease), use digital temp sensors and controllers. Make everything digital. Doesn't cost much!! Encourage people to not use stuff that uses so much power. Why use a 2500W watt kettle to boil water when you can do it with a 1000w kettle and save money by waiting 5 more mins. 

[Monkeh]

Why use a 2500W watt kettle to boil water when you can do it with a 1000w kettle and save money by waiting 5 more mins.

More trolls!

[hamdi.tn]

with AC its a block of metal with wire, that has an efficiency bloody close to 100%

what you said is correct but 100% efficiency transformer really !!!

AC transmission lines were outdated long time ago. They were never designed to for high power transmission.

really !!

a debate 200 years old ! i think Tesla and Edison argued about this 

1-historical reasons, first distribution grid were DC and abandoned for power loss. and making higher voltage require a lot of equipment as well. AC easier to generate and distribute.
2-while we can't argue much about solar power futur use, we still can argue about it's efficiency in a large scale implantation.
3-Most massive electric load run easily on AC.
4-protection circuit on a DC load are tricky, your circuit breaker will not work fine on a DC line
5-i think but not sure, 400VDC will certainty kill you or cause much more damage than the AC.
6-Your appliance convert AC to DC with a single bridge rectifier, most power is lost in the isolated step down converter , and you will still have to use it even if your grid is DC

[Monkeh]

with AC its a block of metal with wire, that has an efficiency bloody close to 100%

what you said is correct but 100% efficiency transformer really !!!

He said close to. 98% is quite reasonable..

[HAL-42b]

I don't see why it has to be one or the other.  Utility power is here to stay for big items.  A secondary 500W DC grid for the home that is solar ready is in everyones future.

This is quite a sensible idea actually.

[hamdi.tn]

with AC its a block of metal with wire, that has an efficiency bloody close to 100%

what you said is correct but 100% efficiency transformer really !!!

He said close to. 98% is quite reasonable..

well, yes am sorry for this but it's true it's close to 100%, i always knew transformer had horrible efficiency due to joule loss and core losses. but yap they are pretty much efficient

[dannyf]

AC transmission lines were outdated long time ago.

The point you seem to have trouble comprehending is that the grid is the least of the problem. The consumption is. Most of today's electronics is designed to take ac power, because of historical reasons. While the grid is mostly DC now from a transmission point of view, the delivery is still AC, because the demand is in AC. And that's not going to change any time soon.

A viable solution is for a transitional period with both AC/DC deliveries, with AC being phased out.

[MrOmnos]

really !!

a debate 200 years old ! i think Tesla and Edison argued about this 

1-historical reasons, first distribution grid were DC and abandoned for power loss. and making higher voltage require a lot of equipment as well. AC easier to generate and distribute.

First of all, this is not a 200 year old debate.  Telsa and Edision argued about which was better for their time. This is a debate about which is better for our time. As you can see, things have changed lately.
I don't think you read what I wrote. Let me explain it to you as a story. Easy enough in the beginnings, wires weren't that long so losses seemed acceptable. Today though, we have so high power demands and few sources we need to shuffle big power long ways. What they discovered was that with long wires came really big losses because of the AC. Every shift in polarity needed some extra current that was dissipated as capacitive losses due to the proximity to ground and other wires. With really long transmission wires NO power could be delivered! It was all lost in capacitive charging. So they began experimenting with thicker wires and they soon found out about skin effect, the current that is rising or falling prefered to travel mostly in outer parts of wires, so the wire conduction capacity didn't scale with cable thickness. They tried to increase voltage, but that lead to corona discharges, meaning more losses. So their only remedy was really tall and wide pylons to carry the thick cables that had to be finely stranded and partly isolated from each other strands(to combat skin effect) and just live with the losses of about 30-40% per 1000km. I have mentioned the data from Siemens. When you use dc, you need less conductor, no need to worry about phases, no skin effect, no large pylons.

Most massive electric load run easily on AC.

They can be made to run easily on DC as well.

protection circuit on a DC load are tricky, your circuit breaker will not work fine on a DC line

People are making quantum computers and you are worried about circuit barkers. I think we humans are advanced enough to design a good circuit breaker. Most probably they already have because DC lines are already being considered by some countries. Some countries already have HV DC lines.

i think but not sure, 400VDC will certainty kill you or cause much more damage than the AC.

This is just stupid.  220 volts AC is not going to kill you?? Actually AC an pass more easily through your body than DC. (That doesn't mean DC isn't dangerous)

Your appliance convert AC to DC with a single bridge rectifier, most power is lost in the isolated step down converter , and you will still have to use it even if your grid is DC.

No!!
DC-DC power conversion is more efficient than AC-DC. Low power transformers are very inefficient.

[MrOmnos]

AC transmission lines were outdated long time ago.

The point you seem to have trouble comprehending is that the grid is the least of the problem. The consumption is. Most of today's electronics is designed to take ac power, because of historical reasons. While the grid is mostly DC now from a transmission point of view, the delivery is still AC, because the demand is in AC. And that's not going to change any time soon.

A viable solution is for a transitional period with both AC/DC deliveries, with AC being phased out.

I know that bit. That's why I am asking the question. We discovered semi-conductors in the 50s. After a decade or so,most of the appliances had started using DC. After that we had 4 decades to change things. But for some reason we didn't. It's like the entire engineering community just ignored the transmission bit. We have made no progress. Now the grid is so complex and broad that it going to be hard to change things.     

[nctnico]

There are no technical reasons to keep the AC grid. The only reasons are economical and political.

There is. As rerouter pointed out switching DC at high current is a royal pain in the ass. A small relay can easely switch 220V AC at 8A but the same relay is usually limited to 100mA or so at 30V DC.

[StuUK]

with AC its a block of metal with wire, that has an efficiency bloody close to 100%

what you said is correct but 100% efficiency transformer really !!!

AC transmission lines were outdated long time ago. They were never designed to for high power transmission.

5-i think but not sure, 400VDC will certainty kill you or cause much more damage than the AC.

Back to those cruel animal demos.....

http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/

Is the OP a descendent of Edison? 

[Seekonk]

Still not sure if I get your point, DC HV transmission lines are pretty old news.  I first heard about them in the 70's.  In the early 70's the IEEE contest for most innovative use for a microprocessor was for a toilet.  Look how long that took to become everyday knowledge.

Trivia question.  According to the IEEE, where was the first high voltage transmission line.  If you don't know, you will never guess it. 

[MrOmnos]

Back to those cruel animal demos.....

http://knowledgenuts.com/2013/10/19/edison-publicly-tortured-animals-to-discredit-ac-power/

Is the OP a descendent of Edison? 

This time it's opposite. AC is accusing DC to be dangerous.

[janoc]

Most massive electric load run easily on AC.

They can be made to run easily on DC as well.

Really? DC motors are much less efficient than three-phase AC ones. Alternatively, you need a complex driving circuitry which converts the DC to multiphase AC (brushless motors ...). The largest loads in a home are typically various heaters and motors - exactly where DC doesn't have any advantages.

protection circuit on a DC load are tricky, your circuit breaker will not work fine on a DC line

People are making quantum computers and you are worried about circuit barkers. I think we humans are advanced enough to design a good circuit breaker. Most probably they already have because DC lines are already being considered by some countries. Some countries already have HV DC lines.

So basically your argument is that because we can fly to the moon we shouldn't be worried about global warming. The argument makes about that much sense.

Some countries do use DC high voltage lines for point to point distribution connections, indeed. But that is not the same thing as delivering DC to premises!

Your appliance convert AC to DC with a single bridge rectifier, most power is lost in the isolated step down converter , and you will still have to use it even if your grid is DC.

No!!
DC-DC power conversion is more efficient than AC-DC. Low power transformers are very inefficient.

The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!
So you will always have that transformer there, you can't connect your low voltage appliance to the 200-400V mains directly only using a non-isolated converter, regardless whether it is DC or AC! This is a complete red herring unless you are also proposing using safe low voltage - and enormously thick cables to carry the currents that go with it.

BTW, those "inefficient" low power transformers (= switching supplies in common appliances like computers or TVs) have efficiences above 80%.