DC mains (was: Are 3A plug fuses really necessary?)

[charles]

which is why all the long distance feeders in Russia use DC?

[tabbypurr]

DC now wins for long distance high power links. It loses badly for household use.

Household 12v or 5v dc rings might yet emerge.

NT

[charles]

I remember installing a 12v ring in my wife's school lab and that mmust have been 40 years ago.

[John Rumm]

For national power grids DC is not really practical, but for specific applications like long distance under sea links you get significant efficiency gains.

[Chris J Dixon]

On that scale things are rather different.

IIRC they built a very long AC line which managed to have the wrong relationship with the wavelength, and the far end was a node at close to 0 power.

Chris

[Harry Bloomfield]

charles pretended :

There are bound to be some exceptions for special reasons, but generally DC has limited value in transmission networks.

[newshound]

There were still some Ward Leonard sets supplying the travelling cranes on Magnox power stations within the last ten years. Dungeness A and Sizewell A iirc.

[Scott]

Could you describe the foot operated switch. Was it a part of the socket or a separate in-line switch? (I remember conical shaped bars sticking out but nobody ever told me they could be operated by foot),

[MrCheerful]

Recently it has been getting a resurgence, China has put it into the "largest in the world " Three Gorges power plant transmission system.

[Vir Campestris]

The main advantage of AC is that it's really easy to build a no-moving-parts voltage convertor. AKA a transformer.

Andy

[Bob Eager]

Part of the socket. As I recall, the socket was a rectangular box about

4-5 inches wide, about 2-3 inches high, and about 1.5-2 inches deep. Some pretty solid material. One 15 amp round pin receptacle. There was a slight bulge in the top edge, near one end (the receptacle was near the other) and a button sticking out, tilted forward. The button was about 0.75 inches in diameter (perhaps a bit less) and protruded from the housing by up to 0.5 inch. It had quite a long travel, and was a push- push operation (push once for on, push again for off). There may or may not have been some kind of mechanical indicator.

I've tried to find a picture, but with no success.

[Johnny B Good]

Hmn... I wonder what a quarter wavelength at 50Hz is? Answer: 1500Km without taking velocity factor into account. For 60Hz it would equate to just under 1250Km. However, for properly terminated transmission lines, that shouldn't really matter (other than for the transmission losses over such a long route).

The losses in an extremely high voltage DC line over that sort of distance are far less than an AC equivalent which is why High voltage DC is used on such interlinks (especially on undersea interlinks where the dielectric losses in the insulation materials become significant even at frequencies as low as 60 and 50 Hz).

[The Natural Philosopher]

Not just that. There is an issue with very long AC grids to do with phase which is why the USA is split into more than one.

Essentially AC arriving by different length paths tends to cause out of phase currents to flow increasing losses dramatically.

Which is why HV DC is preferred even over land.

It's practically mandatory under the sea for capacitative loss reasons.

In both cases losses are incurred moving 'wattless' current round through resistive wires, but in the first case it's not driving a large 'capacitor to earth' under the sea, it's driving the reactive load of a grid that is very large with different paths which makes out of phase current inevitable.

[tabbypurr]

That's why AC ruled for so long. But now that silicon can handle HV downcon version, dc wins for long lines. DC stays at max voltage all the time, wher eas ac is ever varying between zero and max voltage, so the dc line carries a lot more power over a given line.

NT

[The Natural Philosopher]

Bollocks.

The limits on power handling are down to resistive heating which is down to the AVERAGE RMS current being carried. And so is the power.

That is, given a resistive load, the ratio of the power taken by the the load to the power lost in the wires is the same.

[Clive Page]

It persisted at least until 1965. When I bought a kettle in a Cambridge shop in that year they surprised me by asking whether I had 200 or 240 volt mains. Fortunately I was in a modern part of town; I think the last bits of 200 volts were converted in the next year or two.

I can remember my grandmother's house being on DC in the 1950s - my father showed me that one could put wires from the mains into a bowl of water and get hydrogen from one and oxygen from the other. The DC to AC conversion process was done quite efficiently, as far as I know, as many people got brand new domestic appliances free of charge. Of course most people didn't have many appliances in those days.

[tabbypurr]

That's a bit confused. Power = voltage x current of course...

Every line is voltage limited. DC can use that max V all the time, ac doesn't.

Every line is rms current limited, that current is the same for ac as for dc.

So dc transmits more power over a line than ac. Any line.

NT

[damduck-egg]

Does skin effect come into consideration to any degree? ISTR that as DC uses the whole conductor so a thicker cable can be used to the maximum whereas with AC you have to use multiple smaller conductors spaced apart.

While DC -DC conversion has got relatively easier quenching the arc on DC switchgear at very high voltages is still a technical feat that is very expensive to overcome. I believe on the point to point links that are already in use the routine switching is done on the AC side of the converters.

G.Harman

[The Natural Philosopher]

Its hardly an issue. In general every line is current limited, and you will use whatever is the standard voltage.

You might say that AC lines can use higher voltages because the chances of arcing becoming persistent are non existent with AC.

So that's where your thinking is bollocks. At best peak to RMS voltag is

41% more. which is not a huge difference even if v9ltage were a limiting factor, which it aint.,

So far so good...

But that is bollocks.

Not at 50Hz no.

No, that's not so. AC overhead lines typically have to be aluminium wound on a high tensile steel core rope. The compromise is mechanical strength for suspension. HVDC are usually underground or undersea, and they dint meed quite the same physical strength, and will in any vcase be armoured on the outside.

Yup. When I visited the very first UK undersea cable back in the 60s they said 'we can draw an arc for half an hour off that cable once we switch it off'

US cable= BIG capacitor, then...

I suspect its more complicated than that.

IIRC they now use power MOSFETS or similar to do the inverter switching and rectification, so there us a chance to simply switch those off.

[harry]

You are in Lala Land.