Switching from AC grids to DC grids

[StuUK]

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.

Indeed, all going round in circles......

[hamdi.tn]

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.

what changed is what you use in the END of the line, and  still you talk about home appliance basically.  Again most industrial equipment need that 3 phases AC.

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.

am quite sure alot of this is as true as inaccurate

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

define easily.

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.

they did ABB did it, and guess what it's full of semi-conductors.
and in the era of quantum computers , overload and protection are still a problem 

Actually AC an pass more easily through your body than DC.

it should be the same cause as a load it's the RMS that matter, but i clairly said "am not sure" cause am not saying it's scientific truth, it's just some cases i saw that mention electric shock burns are much more severe when DC is the cause.
The scientific truth is you can feel a 30mA AC but it's 300mA DC that can hurt you.

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

am not talking about transformers ( used in old linear ps), am talking about DC-DC converter in SMPS they will cause as much power loss as if they are powered from a DC source. Beside a transformer will still be in a DC-DC converter for isolation.

[Syntax_Error]

https://en.wikipedia.org/wiki/High-voltage_direct_current

In the USA, we do use HVDC (400+VDC) for many interstate backbone power grid connections. It is used primarily for it's efficiency.

[MrOmnos]

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.

I was wondering how many of the houses in this world receive 3-phase AC?? Not many!! Most of the motors used in appliances are universal motors. You won't find a 3-phase motor in a house unless the residents are running a small industry from their house. And heaters and kettles are not a problem if you have a 120v or 75v 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.

Do you understand I have posted this because I care about global warming. I am pretty sure if we run a design competition today, tomorrow we will have 100s of design submission of better circuit breakers.  Plus somebody might have a good solution out there if I do a better research. Digital circuit breakers might be the answer.

The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!

I was talking about the step down part and I am pretty sure you knew that but still wanted to make a pathetic argument. You do know transformer efficiency varies?? Then you have the loss in switching circuit. Where as you can completely remove the transformer loss when converting DC to DC. And DC-DC converter can reach the efficiency of over 90%. So, we have less loss in transmission compared to AC, and even less loss in consumption. And if do it only for the houses and skip the industry and other sectors still we will be saving a huge amount of energy.

[Monkeh]

And DC-DC converter can reach the efficiency of over 90%.

So can AC-DC converters.

So, we have less loss in transmission compared to AC, and even less loss in consumption. And if do it only for the houses and skip the industry and other sectors still we will be saving a huge amount of energy.

Like we save energy by boiling kettles slower?

[MrOmnos]

Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!

[Monkeh]

Go read the books, everything I said is true expect I made it as a story and sequential.

Saving energy by boiling a kettle slower is not true.

And why and how would I use an isolation transformer when I am not using AC at all.

Because you are using AC (look up how an SMPS, that is, a DC-DC converter, works.), and because AC or DC, you need to keep the higher, more efficient voltages for transmission away from things people touch.

[MrOmnos]

Like we save energy by boiling kettles slower?

No by using low power kettles. In a DC system we have converters, so we don't wanna decrease the efficiency of those converter by drawing unnecessary current. You didn't think of it that way did you??

[MrOmnos]

Because you are using AC (look up how an SMPS, that is, a DC-DC converter, works.), and because AC or DC, you need to keep the higher, more efficient voltages for transmission away from things people touch.

How will a transformer work in a DC system please explain it to me!!

[Monkeh]

Like we save energy by boiling kettles slower?

No by using low power kettles. In a DC system we have converters, so we don't wanna decrease the efficiency of those converter by drawing unnecessary current. You didn't think of it that way did you??

Think of it incorrectly? No, no I didn't. DC-DC converters tend to be more efficient at their design power, not less.

Because you are using AC (look up how an SMPS, that is, a DC-DC converter, works.), and because AC or DC, you need to keep the higher, more efficient voltages for transmission away from things people touch.

How will a transformer work in a DC system please explain it to me!!

Do you or do you not actually know how an SMPS works?

[janoc]

How will a transformer work in a DC system please explain it to me!!

I suggest you learn something about switching power supplies, namely the flyback topology. If you have used a computer to write the above, then you have one right there. Switching PSU can work with either AC or DC input, with AC there is that rectifier that was mentioned by @hamdi.tn to first change it to DC.

Otherwise, if you don't want to use this type of power supply, I would love to see your solution for the necessary galvanic isolation.

[janoc]

Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!

  OK, I think this explains everything we need to know 

I hope you realize that HVDC stands for high voltage DC (emphasis mine). By definition not anything related to houses. Unless you want 600kV DC coming to your house, that is. It is actually explained in that Wikipedia article! 

You asked about the 3 phase voltage - pretty much every single house in Europe has a full three phase connection. That there isn't a 3 phase outlet in every room doesn't mean that 3 phase power is not available - typically various circuits in the house are connected to different phases to spread out the load and then you have a full 3 phase outlet for large things, such as central heating/air conditioners, electric heating, if installed, etc.

The nonsense about 75-120V being sufficient - just LOL. Why do many places in the US (which is normally a 110V country) have 220V outlets installed when 120V would be sufficient? I guess they don't know what to do with money ...

Did you ever try to calculate how much thicker the cables would have to be to carry the same power compared to a three phase AC for something like an elevator motor in an appartment block? Never mind that even if you used DC to power it, you would still need a big ass converter to change the voltage to multi-phase AC - you certainly don't want to use a brushed motor at those currents!

The rest is pointless to try to reply to because you obviously don't have a clue what you are talking about.

[rx8pilot]

In the early days of power distribution and use, it kind of HAD to start with generation, then distribution, then end user. It took a very long time for the system to develop into what we see today.

The difference today is that we can choose to have major changes at any point in the process from generation to distribution to end-uses. On the end-use side, it requires no government or political hurdles to start the process of creating DC system. I was always thinking that having 2 isolated 48VDC busses per house would be a good solution. Low power devices would be 48VDC and high power would be 96VDC. As for fuses and switches - the talk about relays and contact distance is simply non-sense. I have been designing and manufacturing eFuses and solid state DC switching for a while and its easy and cheap. You can fuse and switch very high current DC in solid state very reliably. Think of any SMPS topology - it is switching DC at hundreds of Khz or more 24/7/365. If you only need a light switch that will operate maybe a 100k cycles over decades, who cares? With 48VDC, most applications would not need galvanic isolation, but it is easy if you need to.

eFuses are the same thing, solid state and easy. Much easier than the mechanical breakers we use today. At household and most commercial current levels, switching and managing DC is easy. Look at the power levels seen in a Tesla car, there is not a single relay or breaker (there are fusible links for safety I think). They are dealing with high voltage and high current and switching it to a super high power vector drive to make the car faster than a Ferrari.

The reason I would want local household DC is for the ease and efficiency of distributed generation. Wind, solar, battery backup, etc are much easier to tie together when everything is DC. Local batteries charged by solar and wind can take peak loads off the grid and provide some level of backup in the event of grid failure. As for commercial use, I recently sold my CNC machine shop. We of course had a ton of high power motors up to 30HP - All of which ran off of HVDC around 320V. The air compressor, the CNC spindles, everything. There was not a single motor in the whole facility that ran off of AC directly. This includes the variable speed blowers on the HVAC system - HVDC also. We did not plan that, it's just the way it is.

As for utility scale generation and distribution, I have few opinions because I have no experience on what it takes to deal with long distance DC. I will say that if there is a push for big growth in local micro-generation, there will be a diminishing need for high-power centralized generation that needs to travel 100's or 1000's of Kms. I have enough sun exposure to generate about 60% of my electricity at my house. If all houses did that, the grid would have a lot of excess capacity. There is a large storage warehouse close to me that has a 1MW solar installation, but they only use about 25% of that. The rest is being pushed back out.

The simplicity and ubiquity of installed AC generation and distribution mean that it will be around for a very long time, but there is no reason the push for change can't start at the end-user side of the equation. Most of the changes would not need any real government or political permissions. The scale will tip at some point while people seek better (more efficient and reliable) solutions to energy needs. The interstate grid system will be around for a very long time, but if it is supplemented with new ideas and technology, it will never need to expand and the demand for it will slowly diminish.

[hamdi.tn]

Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!

well thanks for the advice, i will read a lot about it  i said it's as true as inaccurate because it's true what you said, it's just not the problem and while DC can be a solution for those problem it's not the solution for the all the other problems that lead to this choice of using the AC grid. 
the funny part is the wiki article talking about HVDC explain the particular use of such system and when it can more advantageous than the AC. so i really really suggest that you read some books or at least the same article cause it clearly say how Siemens in unclear about the projects cost (for commercial reasons), if simens write about it , they expect some money to be made.

You aware that even if this happen it will only be useful for 18% of electricity users, this is the pourcentage of residential users.
50% of electricity consumers are industrial
(source US energy information administration)
 
transformer work on DC, by switching. SMPS use it all the time.

The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!

I was talking about the step down part and I am pretty sure you knew that but still wanted to make a pathetic argument.

no it's not a pathetic argument, he just thought that you knew what you talking about and you knew about switching transformer  that run on DC that what you expect from someone sure that all our appliance are in DC ! you not talking about 5V USB grid voltage don't you ?!

[MrOmnos]

How will a transformer work in a DC system please explain it to me!!

I suggest you learn something about switching power supplies, namely the flyback topology. If you have used a computer to write the above, then you have one right there. Switching PSU can work with either AC or DC input, with AC there is that rectifier that was mentioned by @hamdi.tn to first change it to DC.

Otherwise, if you don't want to use this type of power supply, I would love to see your solution for the necessary galvanic isolation.

Don't they converted back to AC by the chopper part in a SMPS?? Transformer can't work in DC that is what I am trying to say.

[hamdi.tn]

all this been said , it's not wrong that in a particular case of home , a DC power supply can be much useful, TRUE. but can't be as easy for everyone else, and not as easy to distribute so basically that's why power grids still in AC.

[MrOmnos]

Go read the books, everything I said is true expect I made it as a story and sequential. In fact you can read it on Wikipedia. Search HVDC And why and how would I use an isolation transformer when I am not using AC at all. I am not talking about industrial sector here. Only the houses. Like houses get 220v in my country, but it might be different for industries!!

well thanks for the advice, i will read a lot about it  i said it's as true as inaccurate because it's true what you said, it's just not the problem and while DC can be a solution for those problem it's not the solution for the all the other problems that lead to this choice of using the AC grid. 
the funny part is the wiki article talking about HVDC explain the particular use of such system and when it can more advantageous than the AC. so i really really suggest that you read some books or at least the same article cause it clearly say how Siemens in unclear about the projects cost (for commercial reasons), if simens write about it , they expect some money to be made.

You aware that even if this happen it will only be useful for 18% of electricity users, this is the pourcentage of residential users.
50% of electricity consumers are industrial
(source US energy information administration)
 
transformer work on DC, by switching. SMPS use it all the time.

The transformer is not there because of AC-DC conversion but because there must be galvanic isolation!

I was talking about the step down part and I am pretty sure you knew that but still wanted to make a pathetic argument.

no it's not a pathetic argument, he just thought that you knew what you talking about and you knew about switching transformer  that run on DC that what you expect from someone sure that all our appliance are in DC ! you not talking about 5V USB grid voltage don't you ?!

If you read my previous post, I have mentioned houses.
I am aware about switching transformers. I was not aware thy were called "switching transformer". I was thinking of indcutors all this time. Never met anyone calling them transformers. May be I should start talking to people more often.

[rx8pilot]

I am aware about switching transformers. I was not aware thy were called "switching transformer". I was thinking of indcutors all this time. Never met anyone calling them transformers. May be I should start talking to people more often.

In inductor is half a transformer and used for inductive storage. Sending pulsed DC into a transformer is a normal everyday thing to isolate one DC circuit from another.

[Monkeh]

If you read my previous post, I have mentioned houses.
I am aware about switching transformers. I was not aware thy were called "switching transformer". I was thinking of indcutors all this time. Never met anyone calling them transformers. May be I should start talking to people more often.

Or read on the subject..

As far as houses go.. why would I want to use an awkwardly low DC voltage to feed my larger loads?

My oven doesn't want to run on 75VDC. Nor does my computer (although my laptop would be quite happy on a sane value like 48VDC), my microwave, my welder (yes, people use welders at home!).. Don't get me started on the aircon.

[HAL-42b]

I was always thinking that having 2 isolated 48VDC busses per house would be a good solution. Low power devices would be 48VDC and high power would be 96VDC. As for fuses and switches - the talk about relays and contact distance is simply non-sense. I have been designing and manufacturing eFuses and solid state DC switching for a while and its easy and cheap. You can fuse and switch very high current DC in solid state very reliably.

Didn't know this about the reliability of DC switching. If the reliability and the price is right it is definitely feasible.

You are also right about the motors. All motors are installed with inverters these days so the form of the input energy doesn't matter any more.

The only thing lacking is standard outlets and wiring code.

[rx8pilot]

I guess the incoming voltage for the house could be anything - ~300VDC.

Very high power devices like stoves, heaters, welders, etc can operate at high voltages. Maybe take the incoming HVDC and create 4x isolated 48VDC. The stove would run from all four legs at 192v. While the coffee pot uses 2 legs at 96v and the TV uses a single leg at 48v.
Could be best to just have a single HV line for the stove, heater, welder. If you start from 300VDC or so, you can make any voltage that makes sense. Solid state switching and fusing.

[HAL-42b]

I like the idea of isolated rails in 48V increments. But if you connect all 4 to a single resistive element you break the isolation for the other loads. It has to have 4 separate heating elements so the isolation is maintained.

Also this would make the power outlet design very complicated. I suppose each outlet would supply all 4 rails and the wiring arrangement on the load side will decide which voltage is selected. This means at least 4 contacts per outlet + Ground for non-isolated or 8 contacts for isolated outlets. Too complicated (read expensive).

[TassiloH]

Do the DC fanboys here realize that a typical 16A household circuit breaker (of which you will have dozens in a typical distribution box for a family home) for 230V AC here is rated to reliably interrupt short-circuit currents of up to 10000A, has the size of a box of cigarettes and cost 2.95EUR retail. I wonder how you do that with DC? Keep in mind, matching interrupt capability, size, and price!
Oh, and so far noone has mentioned the shaded-pole ac motors used in fans, pumps (washer, dishwaher...). For sure converting all of them to brushless motors with electronics will offer new opportunities to shorten their lifetime by design 
Oh, and regarding power conversion: Also with a DC network you would need to convert voltage levels and provide isolation for low-voltage DC. So the power supplies in all electronic devices basically would look the same as nowadays. The input rectifier might be omitted and the input capacitor might be smaller. A few cents saved for a few hundred to thousand items you might buy in a lifetime.  Have an electrician come and install a second distribution network in you house: Couple thousand $/EUR minimum. Savings -> negative. Oh, and modern power supplies ar quite efficient also with 120/230VAC input, not much power to be saved if one supplies them with 48VDC instead...

[rx8pilot]

I like the idea of isolated rails in 48V increments. But if you connect all 4 to a single resistive element you break the isolation for the other loads. It has to have 4 separate heating elements so the isolation is maintained.

Also this would make the power outlet design very complicated. I suppose each outlet would supply all 4 rails and the wiring arrangement on the load side will decide which voltage is selected. This means at least 4 contacts per outlet + Ground for non-isolated or 8 contacts for isolated outlets. Too complicated (read expensive).

That is true about the isolation, and maybe an argument for a dedicated high power rail. Although 4 element resistive heater is not that big of a deal. For the connectors, there is no need to have all of them available on all outlets. Only the high power loads need the high power. Normal around the house outlets would be fine with only 4 wires - unfortunately 1 more wire than normal household wiring.  Running all new 4 wire system is not practical for millions of homes.

I am not sure if its practical, but I have a system I designed with a multi-voltage output power supply and smart loads on the other side. The power supply outputs only a pilot voltage so that the load can communicate what it is and how much voltage/current it needs. The power supply gets this confirmation and send the correct power. It's for a commercial application and works well, requiring no effort on the part of the user. It only uses 3 wires - 2 for power and 1 for communication. Could work on a large scale and dynamically allocate the outlet to multiple voltage options with DC switches in a central controller similar to a fuse box. When not being used, the outlets are only 12v at a few mA for the communication so danger is non existent as the entire run is cold until a proper load is verified.

That would be enough to cover any time of lighting, TV, vacuum cleaner, etc. High power appliances already have dedicated plugs on dryers and stoves so it would not be too crazy. There are only a few of those in most houses and they rarely get touched/moved/replaced.

I am not really advocating a move to DC, just enjoying the conversation so far. Blindly sticking with old ideas will get us nowhere while change for the sake of change is not helpful either. The question is: With all the new tech and knowledge from the past 100 years, is AC outdated so bad that there is a real tangible benefit to such a massive change? Clearly there are some benefits and some drawbacks.

[vodka]

Do the DC fanboys here realize that a typical 16A household circuit breaker (of which you will have dozens in a typical distribution box for a family home) for 230V AC here is rated to reliably interrupt short-circuit currents of up to 10000A, has the size of a box of cigarettes and cost 2.95EUR retail.

That a PIA* (1 phase or 2 phase)cost 2.95€ will be from the china, because in Spain it costs over 12€ of the Merlin-Gerin corporation

http://www.portalelectricidad.es/fabricante/merlin-gerin-automaticos

* PIA = Small Automatic Interrupt