Do the Math..

[rstofer]

But we are in the 21st century now, and high-power semiconductors are practical for large-scale and widespread utility applications. As I said in a previous post here, it is not difficult to imagine a future where we could see DC distribution in the "last mile" and the end of those big "pole pig" transformers at the top of wooden power utility poles.

I doubt that DC will ever be useful in the home or workplace.  Induction motors are just far superior to DC motors and it really isn't practical to do DC->AC conversion for every single motor.

The ordinary wall clock is dependent on the accuracy of the 60 Hz line.  Sure, we could convert to GPS clocks but that's a stretch for a microwave.  Yes, we could network all the appliances and have them all use NTP to set their clocks but that's going to be a while.

Voltage level conversion of one kind or another will still be required.  Transmission and distribution voltages will always be much higher than utilization voltages.  Substituting commutators for transformers will still leave something on the poles (unless the utility takes the opportunity to underground everything).

[dom0]

But we are in the 21st century now, and high-power semiconductors are practical for large-scale and widespread utility applications. As I said in a previous post here, it is not difficult to imagine a future where we could see DC distribution in the "last mile" and the end of those big "pole pig" transformers at the top of wooden power utility poles.

Well technically most of Europe never had those in the first place.

[SeanB]

Conversion is easy, you convert it to a chopped DC and use a transformer, then rectify it again and send down the line. At high power you tend to use the good old fashioned SCR stacks, using a set of 6 banks ( 3 phases in star, 3 in delta to get the 30 degree phase angle so you reduce ripple), each with their own commutation stack. Then you simply use a large LC filter on the output and send it out as low ripple DC. Incoming again you use the SCR banks and the same transformers and LC tanks, and the same transformers with delta and Y windings. That way you can have a bidirectional power transfer, with the one side acting as either a synchronous rectifier or a power switching stage, using identical stages each side.

Efficiency can easily go over 90% at full load, helped as well by the line losses being lower as there is no inductive effects and skin current with DC, plus the extra 30% more power transmission available on the same line over AC, along with only needing a single conductor with earth return, or just a pair of conductors.

And in any case, your typical vacuum cleaner, which uses an universal motor, will quite happily run on either AC or DC.

[IanB]

Last time I checked power transformers, both for transmission and distribution were about 90% efficient.

If a 240 MVA power transformer were only 90% efficient it would have heating losses of 24 MW. Don't you think it might get a little bit hot?

[dom0]

MW-range transformers go beyond 99 % efficiency...

[commie]

You don't change the DC voltage, you change the AC voltage before it is rectified.

What's hard about that?  A hydro plant probably generates at 12 kV (I'm just guessing) because higher levels would cause massive dielectric issues at the generator.  It absolutely does not generate 500 kV directly.  The generator output voltage is stepped up to 500 kV by ordinary transformers.  No big deal.  Just a step-down transformer in reverse (more or less).

Now, the 500 kV AC is rectified and, voila', we have 500 kVDC.  Send it on its way.

At the receiving end, there needs to be a commutating setup to convert it back to AC.  Still not a big deal.  It's been done since '65.

So you generate 12kv ac then rectify it and send it on its way, could you be a bit more technical how you turn it back to manageable Vac with 90% efficiency that a transformer does?, and lastly, why rectify it to average dc what advantages have you achieved?Why not leave the electricity ac?
 

[commie]

MW-range transformers go beyond 99 % efficiency...

Big sorry, I'm 9% out.

[IanB]

So you generate 12kv ac then rectify it and send it on its way, could you be a bit more technical how you turn it back to manageable Vac with 90% efficiency that a transformer does?, and lastly, why rectify it to average dc what advantages have you achieved?

Please do a little bit of research before posting. It will make you look a bit less ignorant and you will also benefit by learning something.

[commie]

Please do a little bit of research before posting. It will make you look a bit less ignorant and you will also benefit by learning something.

Come now, don't get upset, I want to hear about how you change 1000000Vdc back into ac. you can't use a transformer so what do you use?, I'm so intrigued to know.

[Richard Crowley]

Where did you get this 1000000V number you are tossing around?   Did you pull it out of your ..um.. hat?   

If you spent as much time reading on the interweb as you do posting, you might learn what actual practical voltages are used for both AC and DC transmission and distribution. And you will see a history of how conversion between AC and DC (in both directions) has evolved over more than 80 years.

[commie]

Where did you get this 1000000V number you are tossing around?   Did you pull it out of your ..um.. hat?   

Here.....

'is to be used'? HVDC transmission IS used. In multi-gigawatt scale all over the world.

There is an active 270kVDC 2GW link between the UK and France.
There is an active 250kVDC 250MW link between Scotland and Northern Ireland.
There is an active 450kVDC 1GW link between the UK and the Netherlands.
There is an active ±200kVDC 200MW link between the UK and Ireland.

And to put all those to shame, there is an active 800kVDC 8GW link in China. Coming online next year, a 1.1MVDC (yes, MEGAVOLT) 10GW link in China. That one will be 3,333km long.

Obviously DC transmission isn't viable, that's why China's spending more money than you can comprehend building a 10GW DC transmission system.

And no, you don't use "bjt's and opamps", you use massive fucking thyristors.

[Richard Crowley]

So what did you learn when you read this reference?

https://en.wikipedia.org/wiki/Ultra-high-voltage_electricity_transmission_in_China

I was interested to learn that the HVDC line near me (The Pacific DC Intertie) was originally designed with a 1.3MV line to Hoover Dam on the Colorado River.

Note that all those high-voltage devices (both vacuum-tube and solid-state) are assembled from stacks of lower-voltage components. They simply string enough of them in series to handle whatever voltage is required.  Not nearly as mysterious as you seem to think it is.

[rstofer]

Please do a little bit of research before posting. It will make you look a bit less ignorant and you will also benefit by learning something.

Come now, don't get upset, I want to hear about how you change 1000000Vdc back into ac. you can't use a transformer so what do you use?, I'm so intrigued to know.

The exact same way a UPS system works.  You commutate the DC voltage with a bunch of SCRs (really BIG ones, and not SCRs exactly) and convert it to AC.  As I have said before, this is old news, the technology is at least 50 years old.  Yes, the particular devices have evolved but the techniques, not so much!

This is an explanation:
https://en.wikipedia.org/wiki/HVDC_converter

Notice that converters are inherently bidirectional.  They convert AC to DC or DC to AC.

Just think about the average UPS.  There are 3 blocks.  First we convert AC to DC, next we store DC in a battery and discharge it when the AC fails, and, finally, we convert the DC back to AC.  There are probably millions of these systems around.  Some small, some large, but they all do the same thing.  You can't store AC and you can't power a PC with DC.

Even variable speed drives do the same thing.  They take in some line voltage (and phases), convert it to DC and then control the conversion frequency back to AC.  Thus they can control the speed of an induction motor.  On the way by, they can convert from single phase input to 3 phase output.  It's done every day!

[Richard Crowley]

Note that more and more appliances and tools for consumer use (which nominally operate on AC mains power) actually use DC motors internally, even converting the mains power to DC and using poly-phase drivers for the motors. Clearly, they wouldn't go to this expense and complexity if there weren't some benefit.  And, of course, we have seen switch-mode power-supplies (SMPS) now for decades in all manner of consumer and industrial goods. These days old-school "heavy-iron" power supplies with mains-frequency transformers are getting rare.  Much to the dismay of the "golden ears" audiophools.

To see some practical examples of poly-phase DC motors in consumer appliances, recommend this series of videos from The Post Apocalyptic Inventor...

https://www.youtube.com/channel/UCDbWmfrwmzn1ZsGgrYRUxoA/search?query=motor

[commie]

Note that all those high-voltage devices (both vacuum-tube and solid-state) are assembled from stacks of lower-voltage components. They simply string enough of them in series to handle whatever voltage is required.  Not nearly as mysterious as you seem to think it is.

Why transmit electrical power using dc when ac is alot more manageable?
I'm still waiting to hear about the dc receiving end, and how to change 1MVdc back into usable ac?

Stacks you say, what are these?.. diodes.. transistors what....?

We have already established that transformers are 99% efficient at ac, these 'stacks' you talk about, efficiency converting 1MV dc into 117Vac sinusoids??, what about chopping byproduct harmonics etc and I can go on....   

[dom0]

Richard: Although it is hard to refute that the massive influx of extremely low cost SMPS cause strong pollution of the radio spectrum. Not to speak of things like powerline communications...

You can't store AC and you can't power a PC with DC.

Well technically you can and in larger installations this isn't at all unusual. It's just that the small voltages used in smaller UPSes (24 V or even 12 V) are impractical.

Stacks you say, what are these?.. diodes.. transistors what....?

SCR-like devices.

efficiency converting 1MV dc into 117Vac sinusoids

The output AC is for transmission, ie. >100 kV AC.

what about chopping byproduct harmonics etc and I can go on....

Filters.

[dom0]

In case you haven't noticed almost all HVDC projects are point-to-point (sometimes with a "tap" station or two), while AC transmission are mesh-like with a great many stations interconnected. Think a little about it and you might just get behind why that is.

[IanB]

Anyone get the feeling we are being trolled?

[Richard Crowley]

Why transmit electrical power using dc when ac is alot more manageable?

You are starting with a false assumption.

AC is NOT "alot more manageable" for very long distances because you start turning the transmission line into a transmitting antenna and "broadcasting" the power out into the air where it is completely wasted. 

AC is NOT "alot more manageable" for  cases where you have high capacitance lines like under-sea where you would lose a large fraction of the power in capacitive loading. 

AC is NOT "alot more manageable" for situations where you have different frequency (or phase) at the two ends of the path. There is a very short ("back-to-back") DC intertie in South America between Brazil and Argentina because Argentina uses 50Hz power and Brazil uses 60Hz.  And even in places where the same frequency is used, if the interconnection is BETWEEN REGIONS that are not phase-locked, DC completely eliminates the phase conversion problem.  For example the Pacific DC Intertie near me interconnects between the Northwest grid and the Southwest grid.  While they are both 60Hz, they are not phase-aligned because they are not interconnected with AC.

I'm still waiting to hear about the dc receiving end, and how to change 1MVdc back into usable ac?

Perhaps you missed the part where it explained that the converters are INHERENTLY BIDIRECTIONAL. Exactly the same devices that convert AC to DC are used for converting DC back into AC.

Stacks you say, what are these?.. diodes.. transistors what....?

https://en.wikipedia.org/wiki/Thyristor
https://en.wikipedia.org/wiki/Thyristor#HVDC_electricity_transmission

We have already established that transformers are 99% efficient at ac, these 'stacks' you talk about, efficiency converting 1MV dc into 117Vac sinusoids??, what about chopping byproduct harmonics etc and I can go on....

Perhaps you missed the part about using poly-phase conversion (6-pole is quite common) to reduce harmonics. Sooner or later people will grow weary of spoon-feeding you information that you should have looked up for yourself before coming here.

[Pjotr]

These days old-school "heavy-iron" power supplies with mains-frequency transformers are getting rare.  Much to the dismay of the "golden ears" audiophools.

Hello there, I am not an audiophool (whatever that me be). And my ears have no opinion on those PSU's. But those modern ironless wall warts have the annoying habit to last only for 2 years or so. Some a bit longer but alas. Those old-school "heavy-iron" power supplies appear to last forever.

[Richard Crowley]

... But those modern ironless wall warts have the annoying habit to last only for 2 years or so. Some a bit longer but alas. Those old-school "heavy-iron" power supplies appear to last forever.

Landfills are clogged with examples of cheap mains-frequency power supplies that lasted only a short while.
And, conversely, there are thousands (millions?) of examples of properly-designed and maintained SMPS that are still going strong for several decades.

"Old-school" mains-frequency power supplies tended to be conservatively designed back in the "good old days". While we see everywhere cheap knock-off products from certain parts of the planet with minimalist designs and thrown together with floor-sweeping components.  No surprise that we perceive a higher failure rate.

[rstofer]

Note that all those high-voltage devices (both vacuum-tube and solid-state) are assembled from stacks of lower-voltage components. They simply string enough of them in series to handle whatever voltage is required.  Not nearly as mysterious as you seem to think it is.

Why transmit electrical power using dc when ac is alot more manageable?
I'm still waiting to hear about the dc receiving end, and how to change 1MVdc back into usable ac?

Stacks you say, what are these?.. diodes.. transistors what....?

We have already established that transformers are 99% efficient at ac, these 'stacks' you talk about, efficiency converting 1MV dc into 117Vac sinusoids??, what about chopping byproduct harmonics etc and I can go on....

Do your own homework!

You have already been given links to many documents that explain exactly how these systems work and Google has another million references or so.

It is being done, it has been done for more than 50 years and the fact that you don't understand it doesn't make it less viable.  You just need to get with the program and read the documentation.

With the 3 phase converter, harmonics were a problem.  With the 12 phase converter, harmonics begin at the 11th and 13th and are considerably diminished.  So the industry seems to be settled on the 6 phase inverter with the significant harmonics at the 5th and 7th.

The electrical grid tends to smooth things out.  Particularly the motor loads.

This has been done for decades and it seems to work!  And it must be cost effective or it wouldn't be done at all.

[Zero999]

Efficiency can easily go over 90% at full load, helped as well by the line losses being lower as there is no inductive effects and skin current with DC, plus the extra 30% more power transmission available on the same line over AC, along with only needing a single conductor with earth return, or just a pair of conductors.

I've always wondered about electrolysis when using earth as a return for DC? How do they stop the anode becoming corroded and the emission of gasses and caustic chemicals such as NaOH?

And in any case, your typical vacuum cleaner, which uses an universal motor, will quite happily run on either AC or DC.

Lots of mains appliances nowadays can work off DC because they have switched mode power supplies and even devices containing induction motors can work fine off DC because they already have an inverter for speed control.

[Mr.B]

Here's some thyristor arrays from a ±350kVDC 1.2GW link in New Zealand:

I have stood in that very hall many years ago.
It has an eerie feeling of shear sheer power...


EDIT: My Typo 

[IanB]

I have stood in that very hall many years ago.
It has an eerie feeling of shear power...

I guess one could say it is cutting edge technology?