# Regarding AC distribution & DC distribution

J

#### Jayesh

Jan 1, 1970
0
Hi can anyone plz help me out in comparing between high freq AC
distribution line with the DC distribution line as nowadays DC is only
used esp. for spacecraft systems or others too such as fuel cell
vehicle. So i would like to know what are the disadvantages &
advantages of AC distribution over Dc distribution as research seems to
have stopped in this in round abt 1995.

Plz help me out

Thank you
jayesh

J

#### James Thompson

Jan 1, 1970
0
Jayesh said:
Hi can anyone plz help me out in comparing between high freq AC
distribution line with the DC distribution line as nowadays DC is only
used esp. for spacecraft systems or others too such as fuel cell
vehicle. So i would like to know what are the disadvantages &
advantages of AC distribution over Dc distribution as research seems to
have stopped in this in round abt 1995.

Plz help me out

Thank you
jayesh
Simply:
With dc, you cant use transformers and the voltage will decrease over
distance.
With ac, you can use transformers to step up the voltage and with higher
voltage on the lines they have to conduct less current, and this is why the
wigh tension wires are much smaller. Dc would require very big wires to
transfer the same power that AC high tension wires do. The power transfered
by the ac lines is the voltage x current. Think of it like a see saw, if
voltage raises - the current lowers but the power stays the same. In
essence that is why we use ac current over dc. Hope I didnt get you to
confused
JTT

J

#### Jayesh

Jan 1, 1970
0
thanks for your reply but i guess voltagexcurrent applies for Dc too
and nowadays we find Dc distribution only if you see in spacecraft so i
am very curios why so why not ac line

T

#### The Real Andy

Jan 1, 1970
0
Simply:
With dc, you cant use transformers and the voltage will decrease over
distance.
With ac, you can use transformers to step up the voltage and with higher
voltage on the lines they have to conduct less current, and this is why the
wigh tension wires are much smaller. Dc would require very big wires to
transfer the same power that AC high tension wires do. The power transfered
by the ac lines is the voltage x current. Think of it like a see saw, if
voltage raises - the current lowers but the power stays the same. In
essence that is why we use ac current over dc. Hope I didnt get you to
confused
JTT

I beleive HVDC trasmission is also popular these days. In fact, we
have one underwater DC transmission line here in AU:
http://tdworld.com/news/SEIMENS-PTD-Austalia/

T

#### Tim Williams

Jan 1, 1970
0
Jayesh said:
thanks for your reply but i guess voltagexcurrent applies for Dc too
and nowadays we find Dc distribution only if you see in spacecraft so i
am very curios why so why not ac line

Because you almost always use DC at the endpoint.

IIRC, big planes like 747s use 400Hz AC, higher than the line frequency used
everywhere else to save weight, but AC instead of DC to improve
transmission.

I don't know why you think something is exclusive to spacecraft.

And BTW, you will gain *much* when talking to people if you SPELL and
CAPITALIZE properly... this isn't some teen chat room.

Tim

L

#### linnix

Jan 1, 1970
0
Tim said:
Because you almost always use DC at the endpoint.

IIRC, big planes like 747s use 400Hz AC, higher than the line frequency used
everywhere else to save weight, but AC instead of DC to improve
transmission.

I don't know why you think something is exclusive to spacecraft.

Because the power is coming from low voltage DC solar panels anyway.
Real Spacecrafts are usually too small for high voltage transmissions.
USS enterprise is not real!!! We are not building spacecrafts bigger
than
a bug (volkswagen) yet.

E

#### engineer

Jan 1, 1970
0
I have doubts that AC v DC is really an issue for low power spacecraft.
As someone said, there is a transmission over long distance advantage
that AC has over DC.

One distinct advantage that DC power has over AC is both the reach
(radiated extent), and the change in electro-magnetic field intensity,
as it interacts with things in the envroniment. Some say that high
levels of low frequency emf (electro-magnetic fields) can cause
cancers. To date this hasn't been disproven.. or so I think... but it's
generally held that only ionizing radiation (wavelength on the order of
the width of a water molecule and shorter) is understood cleary to
cause cancers. If DC power distribution were better-done underground,
maybe the landscape would look nicer than how it's done today.

On small scales, RF and microwave power distibution systems could be
feasible.. lord know for what application, maybe a primary power
collection center with remote modules or capsule for some interstellar
expedition? Some lunatics got press time for proposing that we beam
microwaves down from space (from solar collectors).. but it's doubtful
that could happen for environmental reasons.

One thing that is useful about AC is that with electromagnetic
coupling, the power is moved without a continuous conductive
connection, which would have some advantage in a lightning strike.

One could come up with a longer list.. and argue this or that. In
systems that use batteries, voltages can be converted up or down by
various switching and storage techniques, more efficiently than other
means, such as "voltage regulators". It doesnt really make sense to
convert a battery (or solar) DC souce entirely into AC to in turn be
converted back to DC for use by DC circuits- taking the losses twice...
however I'd reserve some doubt, that it may be possible in some vehicle
that such AC power is more efficient for a some type of motor.

It seems in the realm of possibility that there may one day be an AC
battery, but not in the near future. Maybe technology would evolve to
that some mix of DC, AC, RF, Microwaves, and Light would comprise the
'power pipes' of the future.

narrow it down to one context that is useful to them, with one rule of
thumb to apply.

A

Jan 1, 1970
0
Jayesh said:
Hi can anyone plz help me out in comparing between high freq AC
distribution line with the DC distribution line as nowadays DC is only
used esp. for spacecraft systems or others too such as fuel cell
vehicle. So i would like to know what are the disadvantages &
advantages of AC distribution over Dc distribution as research seems to
have stopped in this in round abt 1995.

There is some very basic stuff (relating to tramways) at:
http://www.bathtram.org/tfb/tT42.htm

T

#### Tim Williams

Jan 1, 1970
0
engineer said:
Maybe technology would evolve to
that some mix of DC, AC, RF, Microwaves, and Light would comprise the
'power pipes' of the future.

"Paging Commander Laforge, deck 13 junction B7 for plasma conduit repair"

Tim

T

#### Terry Given

Jan 1, 1970
0
The said:
I beleive HVDC trasmission is also popular these days. In fact, we
have one underwater DC transmission line here in AU:
http://tdworld.com/news/SEIMENS-PTD-Austalia/

DC transmission is used to counter transmission-line effects in AC power
distribution systems. Multiple AC generators need to be precisely
synchronised to prevent circuilating currents; when transmission lines
become an appreciable fraction of a wavelength (1000km or so), this
becomes very, very difficult. So HV rectifiers create HVDC, its shipped
long distances as DC, then a giant inverter generates AC again.

Megavolt DC links are common, and the power electronics is pretty neat too.

Cheers
Terry

S

#### Stanislaw Flatto

Jan 1, 1970
0
Jayesh said:
Hi can anyone plz help me out in comparing between high freq AC
distribution line with the DC distribution line as nowadays DC is only
used esp. for spacecraft systems or others too such as fuel cell
vehicle. So i would like to know what are the disadvantages &
advantages of AC distribution over Dc distribution as research seems to
have stopped in this in round abt 1995.

Plz help me out

Thank you
jayesh
No it did NOT.
The ~100 years of usage of both styles produced a lot of practical
solutions like:

TV syncronisation over whole countries or continents to line frequency
easly later replaced by (new, better, improved) electronic cirquits.

Hydraulic power generators in Sweden having their power converted to HV
DC for 2000Km transfer to the "needy" inhabited and industralised
region, reconverted to AC for "normal" distribution. On this length of
line the magnetic fields in 3-phase AC made the dual change cheaper.

Heart patients in High Intensity rooms are connected to measuring
instruments insulated from mains cirquitry by RF coils so NNNNOOOO
galvanic contact exist for ANY amount of leakage.

And so on and on.
Create a definition of requirements for power distribution and some used
before solution would pop up.

Have fun

Stanislaw

D

#### Don Bowey

Jan 1, 1970
0
thanks for your reply but i guess voltagexcurrent applies for Dc too
and nowadays we find Dc distribution only if you see in spacecraft so i
am very curios why so why not ac line

There are some high voltage DC transmission systems.

Go back to google and do a better job.

D

#### Don Lancaster

Jan 1, 1970
0
The said:
I beleive HVDC trasmission is also popular these days. In fact, we
have one underwater DC transmission line here in AU:
http://tdworld.com/news/SEIMENS-PTD-Austalia/

Twice as much power can be sent over a HVDC line for a given breakdown
voltage.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

Please visit my GURU's LAIR web site at http://www.tinaja.com

G

#### Genome

Jan 1, 1970
0
Jayesh said:
thanks for your reply but i guess voltagexcurrent applies for Dc too
and nowadays we find Dc distribution only if you see in spacecraft so i
am very curios why so why not ac line

AC transmission implies AC generation.

This may well make your spacecraft spin in the opposite direction. You might
compensate for the spin up to power the targeting systems but compensating
for the load dump when you fire makes aiming your X-ray laser a bit
difficult.

DNA

P

#### Pooh Bear

Jan 1, 1970
0
Jayesh said:
thanks for your reply but i guess voltagexcurrent applies for Dc too
and nowadays we find Dc distribution only if you see in spacecraft so i
am very curios why so why not ac line

Probably weight ! Eliminating transformers saves weight so useful for
spacecraft. 400Hz AC is typically used in aircraft ( also saves weight compared
to 50/60 Hz ).

Graham

P

#### Pooh Bear

Jan 1, 1970
0
linnix said:
Because the power is coming from low voltage DC solar panels anyway.
Real Spacecrafts are usually too small for high voltage transmissions.
USS enterprise is not real!!! We are not building spacecrafts bigger
than
a bug (volkswagen) yet.

Doesn't the shuttle qualify as a spacecraft ?

Graham

T

#### Terry Given

Jan 1, 1970
0
The said:
I would love to see the power electronics in the newer outfits. Pretty
amazing stuff i would imagine.

lots of LASCRs in series to form a unidirectional switch, 4 of these per
half bridge. all driven directly by fibre optics (Light Activated SCR)

the (LA)SCR failure mode is short-circuit, so each switch has far more
LASCRs than are strictly necessary. I'm unsure as to the exact number,
but ISTR its about 2x as many LASCRs as required, so half can fail and
the unit keeps operating. this means PM can be done when convenient,
rather than when an LASCR fails.

Cheers
Terry