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Line Voltage from poll transformer being stepped down efficiency?

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Chris Cristini

Mon Oct 07 2013, 07:28PM

Registered Member #1749 Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497

I have been pondering an idea to build a large SMPS to lower line voltage from 100-250V down to 12-24V and running low voltage Lighting and appliances would that lower the KWH at the meter?

I have been reading articles on DC transmission VS AC Transmission and they are saying DC is more efficient. And what are your thoughts on UHVDC Transmission?

the_anomaly

Mon Oct 07 2013, 09:55PM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

It wont be less power because power in = power out, you are just lowering V and raising I.

I always thought AC was most efficient because the electrons move backwards and forwards only a small distance while in DC they travel round the circuit encountering heating losses due to distance. Again, I may be wrong on that point.

Edit: running LEDs in place of regular lighting might tip the efficiency scale, we usually get about 5W light power out for roughly 55W input power at work. IIRC the incandescent bulb is much less than that

...

Tue Oct 08 2013, 03:43AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Are your 2 questions related?

Using a SMPS to step down to 12-24v is (for most definitions of efficient) the most efficient way to convert 120v to anything other than 120v, and this is how almost all electronics these days do it (mostly because it is also usually the cheapest way to do it).

Some have argued that it would be better to put one single (large) SMPS supply in your house, and running 12v directly to things like lightbulbs and computers, saying that it would be more efficient because you could use a more advanced (ie, expensive) supply--however when you consider the cost of adding the additional wiring (which must contain roughly 100x as much copper than a comparable wire designed for 120v) you are usually better off just putting higher quality power supplies on each of your devices.

As to your second question, it can certainly be more efficient to use HVDC (key emphasis on the HV part), due to the fact that there are less corona losses for an equivalent DC voltage compared to a 60Hz system. It is much more expensive (due to the extreme costs of the inverters/etc at each end), and generally it is not cost effective except for the largest of links, and it is certainly not cost effective to rip out the existing lines and replace them with new HVDC links. However, this logic is only valid for extremely high power links (GW power levels), for lower energy levels you can economically use a low enough voltage to avoid the corona losses issue.

Chris Cristini

Tue Oct 08 2013, 12:49PM

Registered Member #1749 Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497

No there not related they are just running through my head. I figured as much about stepping the voltage down the wiring in your house would be so expensive the only gain I would see here is the fact that any individual could come in contact with these wires as the given voltage is at safe levels but make a short circuit and watch the smoke fill your house.

I only ask about the UHVDC Transmission because they are planning on using 800KV+ which is just insane.

EDIT: I may know how Tesla felt when Edison was trying to use DC.

Dr. Slack

Tue Oct 08 2013, 03:22PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Long distance DC transmission is more efficient as, for the same power, the peak voltage (read cost of insulation) is 1.414x less, especially important if the cable is buried or submarine. The rms current (can't be bothered to do the sums or look it up, but it's not varying, so it must be) (read heat losses or cost of conductor cross section) is less as well. The capacitive line charging doesn't up the VA either.

Now if the line is long enough, what you save in transmission costs will pay for the more expensive converter stations at the ends.

Using DC also frees you from requiring AC synchronisation at both ends, so easier recovery from outages if a generating 'island' gets disconnected from the network, and possible to import/export power across national borders where synchronisation is typically not possible.

Patrick

Tue Oct 08 2013, 04:39PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

there is a 300kv HV DC line in Japan, i think its used in a tunnel 1+km long with 2 or 3 wire "pipes" the outer metal tube is ground the inner is HV copper with sulfur hexa foloride gas in between. and the ends of the pipe were ball and sockets to allow for expansion and contraction if i remember right.

i cant remember this stations name though. might be worth studying their decisions though, it was for me years ago.

they are using this method for high power transmission to a station for AC conversion to the rest of Japans local grid.

In the US i know there is a special experimental HV transformer that has a huge rotating center winding and stationary outer winding, this rotating HV transformer is used to match the loaded-phases between two otherwise incompatible power regions.

once you make the decision to go HVDC for transmission, your stuck with at least two SMPS's at each end. I presume at GW power level SMPS's are way more expensive then even the most expensive AC transformers. Maybe that will change with quantity and advances in technology though.j

EEYORE

Thu Oct 10 2013, 09:55PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

This is an interesting topic as I work for an electrical utility...Sounds like you simply are after saving money on your electric bill? If so, it's not going to be worth your while trimming from your lighting needs.

Your AC and Hot water heater = your bill.

Coming up with a way to lessen the demand from those will = savings due to kWh charge. Since you probably aren't billed a penalty from poor power factor, your best bet at seeing any real savings would be to get an efficient AC and hot water heater and keep them running correctly.

Ash Small

Thu Oct 10 2013, 10:30PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

EEYORE wrote ...

This is an interesting topic as I work for an electrical utility...Sounds like you simply are after saving money on your electric bill? If so, it's not going to be worth your while trimming from your lighting needs.

Your AC and Hot water heater = your bill.

Coming up with a way to lessen the demand from those will = savings due to kWh charge. Since you probably aren't billed a penalty from poor power factor, your best bet at seeing any real savings would be to get an efficient AC and hot water heater and keep them running correctly.

I don't have AC, I have a log fire, and an oil fired 'range' for cooking, hot water, and central heating. I don't use a pump on the central heating, it circulates by convection.

I use electricity for the water pump that pumps the water up from underground, and for the UV sterilisation system, but have no water bill. I also use electricity for lighting and PC, TV, etc.

I have no control over 'installing a more efficient water pump, etc' as that's the landlord's job.

I could get a smaller TV, and a 7" tablet, to save electricity, otherwise the only area where savings are possible is lighting. I'm currently looking at possible designs for LED lighting systems with no resistors.

I realise I'm not 'Mr Average', but there are plenty of people in similar situations to me.

EEYORE

Thu Oct 10 2013, 10:59PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

First you'll need to calculate just how much your lighting usage is costing you...

Then you'll be able to better determine how much a possible solution is worth.

Ash Small

Fri Oct 11 2013, 12:26AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

EEYORE wrote ...

First you'll need to calculate just how much your lighting usage is costing you...

Then you'll be able to better determine how much a possible solution is worth.

Yes, but it is the only area where any saving is possible.

I do agree that your advice re AC and heating is relevant, but I don't use electricity for either.

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