DC power transmission...

Hello Forum,

I know that electrical power travels AC and high voltage over long
distances. This serves to reduce ohmic losses. Also AC current is easily
generated using mechanical methods.

I have also heard about DC power transmission...does that offer particular
advantages over AC power transmission?

thanks,
fisico32

On 02/02/2011 06:23 PM, fisico32 wrote:
> Hello Forum,
>
> I know that electrical power travels AC and high voltage over long
> distances. This serves to reduce ohmic losses. Also AC current is easily
> generated using mechanical methods.
>
> I have also heard about DC power transmission...does that offer particular
> advantages over AC power transmission?

AC suffers from losses just because its AC.  The 'near field' losses go 
up proportionate with line length.  Then, when the transmission line 
length gets to be a considerable fraction of a wavelength, the losses go 
up _and_ the line starts to act like any long transmission line, with 
standing waves, impedance transformation effects, etc.  One wavelength 
at 60Hz is about 3000 miles, and the distance from the Bonneville Dam on 
the Oregon/Washington border to all those power consumers in Los Angeles 
is getting close to 1/4 of a wavelength.  Transmission line effects 
_really_ start kicking in between 1/8 and 1/4 of a wavelength.

Also, AC machines really like to be synchronized to one another, and the 
more generators you have on a grid the harder the job is -- even when 
you don't take all the phase-change effects of long lines into account. 
  So grids that are geographically separate tend to run independently 
(less so now, as we get smarter, but still...).

There's loss when you convert AC power to DC for a transmission line, 
there's loss when you convert back to DC, and DC transmission lines 
really, really, don't like being turned off suddenly.  For all that, 
long-distance transmission lines are often less expensive when they're DC.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:
> On 02/02/2011 06:23 PM, fisico32 wrote:
>> Hello Forum,
>>
>> I know that electrical power travels AC and high voltage over long
>> distances. This serves to reduce ohmic losses. Also AC current is easily
>> generated using mechanical methods.
>>
>> I have also heard about DC power transmission...does that offer
>> particular
>> advantages over AC power transmission?
>
> AC suffers from losses just because its AC. The 'near field' losses go
> up proportionate with line length. Then, when the transmission line
> length gets to be a considerable fraction of a wavelength, the losses go
> up _and_ the line starts to act like any long transmission line, with
> standing waves, impedance transformation effects, etc. One wavelength at
> 60Hz is about 3000 miles, and the distance from the Bonneville Dam on
> the Oregon/Washington border to all those power consumers in Los Angeles
> is getting close to 1/4 of a wavelength. Transmission line effects
> _really_ start kicking in between 1/8 and 1/4 of a wavelength.
>
> Also, AC machines really like to be synchronized to one another, and the
> more generators you have on a grid the harder the job is -- even when
> you don't take all the phase-change effects of long lines into account.
> So grids that are geographically separate tend to run independently
> (less so now, as we get smarter, but still...).
>
> There's loss when you convert AC power to DC for a transmission line,
> there's loss when you convert back to DC, and DC transmission lines
> really, really, don't like being turned off suddenly. For all that,
> long-distance transmission lines are often less expensive when they're DC.
>

Tonight's episode of Nova on "clean" energy stated that losses in 
the grid can approach 50%. Is that realistic? (I caught a couple 
of errors due to over simplification.)

Richard Owlett <rowlett@pcnetinc.com> wrote:
(snip)

> Tonight's episode of Nova on "clean" energy stated that losses in 
> the grid can approach 50%. Is that realistic? (I caught a couple 
> of errors due to over simplification.)

Well, to start you usually lose more than 50% on the conversion
from heat for a thermal source (coal, oil, gas, nuclear) to
electricity.

Otherwise, 50% would probably be my first guess.  If it is more
than that, then you work to reduce it, such as use larger wires
for the long distance transmission lines.  Now, optimize costs
of wire vs. power loss.  What is the result?

-- glen

Richard Owlett <rowlett@pcnetinc.com> wrote:
(snip)

> Tonight's episode of Nova on "clean" energy stated that losses in 
> the grid can approach 50%. Is that realistic? (I caught a couple 
> of errors due to over simplification.)

Well, to start you usually lose more than 50% on the conversion
from heat for a thermal source (coal, oil, gas, nuclear) to
electricity.

Otherwise, 50% would probably be my first guess.  If it is more
than that, then you work to reduce it, such as use larger wires
for the long distance transmission lines.  Now, optimize costs
of wire vs. power loss.  What is the result?

-- glen

On Wed, 02 Feb 2011 20:23:19 -0600, "fisico32"
<marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:

>Hello Forum,
>
>I know that electrical power travels AC and high voltage over long
>distances. This serves to reduce ohmic losses. Also AC current is easily
>generated using mechanical methods.
>
>I have also heard about DC power transmission...does that offer particular
>advantages over AC power transmission?
>
>thanks,
>fisico32

Hi,
  Have a look at:

http://en.wikipedia.org/wiki/War_of_Currents

[-Rick-]

what are exactly "near field" losses?

thanks

fisico32 <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:

>what are exactly "near field" losses?

That is where EM energy is transfered in a way that can't be
accurately described by photons traveling through space.
An ordinary transmitter works by near-field.  


Steve

Steve Pope <spope33@speedymail.org> wrote:

>fisico32 <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:
>
>>what are exactly "near field" losses?
>
>That is where EM energy is transfered in a way that can't be
>accurately described by photons traveling through space.
>An ordinary transmitter works by near-field.  

Oops!  I meant to say "an ordinary transformer" works by near field.

Almost all radio transmission is far-field.

Steve

thanks Steve....

now I am not clear on why an ordinary transmitter works with near
field....
A RF or laser transmitter emits radiation in the form of photons....
An antenna has both a radiation field and a reactive field....