Will DC power distribution be back soon? 1

[Skogsgurra]

We all(?) remeber when Edison and Westinghouse tried to introduce DC and AC as their respective standard systems for power distribution more than 100 years ago.

We also know that AC won - for obvious reasons. And we all(?) still think that the best guy won. Don't we?

But, time and technology has changed. We have more and more electronic devices and more and more switched power supplies. We also get more and more variable speed drives and the micros already are mostly electronic and the computers all use internal DC and and...

All these devices use rectifiers to produce the DC link power. And the rectifiers distort mains voltage and load the Neutral unexpectedly much and and and... They also have losses. And the DC link capacitors are bulky and costly.

Local DC distribution - within a block or within a house - would make things a lot easier. No harmonics, no hot Neutrals no bulky capacitors. Integral VSDs would be a natural thing and not a special motor. So, there is a lot going for DC again. And there will probably be a time when we regret that AC did win. DC will simply be the better solution for local distribution.

Of course, there are problems as well, but I think that it is about time that we took the AC/DC thing under some consideration.

Comments? Anyone? Are there DC islands already? Work in progress?

 

[busbar]

 
What are the AC- versus DC-system capacities on the US space shuttles and {proposed} International Space Station?

 

[jraef]

Busbar,
In response to your earlier post "... and if given the chance, tour a converter station. [Unfortunately, I understand both ends of the ±500kV 3100MW Pacific DC Intertie are no longer open for walk-in visits.]"

I was fortunate enough to get to work breifly at the Dalles Oregon converter station for the Pacific Intertie back in 1996. I will never forget the sight of those huge rectifiers and all the rounded dischage tubes with the big spheres on the ends. I was reminded of old silent movies about the future where everything had an Art-Deco look to it. It was one of the most amazing electrical installations I have ever seen. I wish I had taken pictures or even bought postcards that were in the lobby. Everything there is closed up as tight as a drum now with security cameras, armed guards and electic fences.

"Venditori de oleum-vipera non vigere excordis populi"

 

[GOTWW]

With all done and said, I have dual wired 110Vac and 12/24vdc-#8AWG wired my house 12 years ago. Basically, the dc is only used now to power the miniature loads that drive the remote telephones and an array of inefficient DC incandescent lights that come in handy when the battery system is too low to support an inverter or when the inverter is in the sleep mode. And the monolifiic featureless DC super insulated fridge. The initial advantage of the better efficiency DC motors is rapidly out advantaged by the flexibility of the AC-multi-speed motors and the rapid improvements in efficiency. Sure, maybe my maytag has an integrated DC drive or at least a VSD-repulsion unit that is currently an object of class-action lawsuit & recall. But rather, the DC is very inflexible. The new electronicals are now 9Vdc or whatever the latest momentum of the cheapest to implement. This requires regulation, or a DC to DC converters, typically manufactured in china, and impossible to repair.

 

[mc5w]

Kollmorgen has a servo drive rack that uses a single rectifier to power 2 to several servo drives such as on robots and multiple axis machine tools. This allows regenerated energy from one drive to be stored in the supply capacitor and used later or to use a single dynamic braking resistor that really has a continuous rating. Too bad that their motor repairs stink.

GOTWW, your 12/24 volt DC system is something that I would have wanted in my house during the blackout. I ran some 12 volt fluorescent drop lights and my shortwave radio off of my car using extension cords. You could also replace your 12 volt bulbs with DC input fluorescent ballasts from Bodine which would also allow you to upgrade to a 24/48 volt system. SquareD's QO circuit breakers up to 70 amps are rated for that voltage. Bodine has 2 types of DC input ballasts, one type for backing up line voltage ballasts off of central office power and NOT continuous duty rated and the other type that is continuous duty rated and intended for recreational vehicles and off grid buildings.

By the way, many of the subways in NYC and Boston use 25 Hertz power. Only some of them use DC.

Mike Cole, mc5w@earthlink.net

 

[Skogsgurra]

Thanks to all for a great many interesting responses to my humble question.

My starting point was this: Most lamps, computers, kitchen and other houshold machinery use some kind of inverter technology today. Variable speed is known to save energy in ventilation, HVAC and refrigerators and it is almost a necessity in washing machines already.

So there are between twenty and hundred local rectifiers and DC links in a normal family house. Power range is between ten watts (small computers and lamps) up to one or two thousand watts (ventilation).

All these local rectifiers and DC links have costs, volume and losses associated with them. They also produce harmonics, which can not be dealt with cost-effectively in many small units.

By having one rectifier unit at each entry and distributing DC (I am thinking around 325 V in Europe, the voltage you get when rectifying 230 V), the rectifier can be built with good harmonics mitigation, lower total cost and a lot better efficiency than the twenty - hundred local rectifiers used today.

Existing AC devices with switching power supplies can be used directly on the DC distribution (not necessary to replace all equipment), but those with a transformer between mains and rectifier have to go. Heating will probably also need some consideration since the normal switches are built to break AC - they will not be able to break DC reliably. So there will be problems. But there will be some benefits in the long run and every change has its problems - I know, we changed from left side driving to right side driving more than thirty years ago. It wasn't that difficult and we all benefit from it today.

Sorry, that last part was a bit off topic. More comments to this?

 

[lengould]

s: Agreed completely with your rational regarding residential. Difficulty will be getting any sort of standard agreed to.

a) 325 - 330 V should work cheaply at central rectifier / distribution for most of the world.

b) using fuses for circuit protection should eliminate high cost of DC breakers, let that market develop if it wants. Alternatively put current limiting electronics in front of branch circuit breakers to allow present standard AC units to serve.

c) A single ring running throughout the home should have sufficient capacity for most smaller loads, then dedicated fused circuits feeding central air ventilation motor, laundry machines, ?refrigerators, etc.

d) Can't see any other problems, shouldn't cost much more than present systems given widely accepted standards on plugs and rated amps.

How much efficiency gain do you think we'd see from it as far as eg. present AC/DC wall warts having their transformers connected 24/7? How much of that will be lost in the central rectifying unit?

What are the odds it could be designed from the outset to carry hi-speed communications to every outlet point as well?

What loads are left which still must absolutely have AC branches (aside from legacy stuff)?


Pechez les vaches.

 

[GOTWW]

I had an ancient E Prof., Robert E. Now?, accustomed to gauging the "potential" of wiring using the back of his hand, until, crawling under a house in China with 400Vdc wiring. This was an experience that he obviously never forgot, and made damn sure we never forgot either.

 

[GOTWW]

It was somehow related to him shooting a hole through the roof of a house with a shotgun, while eating duck, as I recall.