Regarding AC distribution & DC distribution

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

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

I beleive HVDC trasmission is also popular these days. In fact, we have one underwater DC transmission line here in AU:

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

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.

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.

Advantage and disadvantage depends on the context.. most people like to narrow it down to one context that is useful to them, with one rule of thumb to apply.

There is some very basic stuff (relating to tramways) at:

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

:-)

Tim

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

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 Slack user from Ulladulla.

There are some high voltage DC transmission systems.

Go back to google and do a better job.

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

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

Doesn't the shuttle qualify as a spacecraft ?

Graham

Indeed. I notice that the Itaipu HVDC link uses +- 600kV !

Graham

I would love to see the power electronics in the newer outfits. Pretty amazing stuff i would imagine.

you should see the really high voltage stuff. MVDC links are fairly common.

Cheers Terry

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