American electrics

On Tue, 21 Jun 2005 01:23:11 +0100,it is alleged that "Dave Plowman (News)" spake thusly in uk.d-i-y:

Good thought, ouch. 400 Hz is the NU tone frequency before BT started using silly announcements, I would _not_ want that in the substation next door.

Except for the cost of the converters at each end of the line, which is astronomic compared with cost of transformers.

HVDC is used to cross synchronisation areas. It is also advantageous in cables where the DC stresses the insulation less than AC. But the cost of the end station plant and any line switches means it's used only when absolutely essential. Generally it's much cheaper to make sychronisation zones as big as possible. Great Britain is all one zone. I haven't seen a recent map of Europe, but prior to the fall of the Berlin wall, continental Europe was 2 zones, the West controlled by Switzerland, and the Eastern European countries controlled from Moscow (although the USSR itself was multiple zones -- too big for one zone). The East and West zones weren't linked -- cost of the converters made it non-viable. GB and the West were/are linked across the English Channel on DC links, through which we buy the output of the French nuclear power plats they build along their north coast, due to the politics involved building them 25 miles further north in the UK.

I don't think anyone cares about transformer size anything like to the extent you seem to.

Well 120V doesn't go very far before the regulation has gone to pot.

I think it's a non-starter.

Cost. It may not be much per item, but add all those small cost savings up and it comes to 1 with a long string of 0s on the end. Savings on all domestic transformer operated gear, from wall warts to stereos to microwaves. Savings on all inductor fluorescent lights. Savingsd on reservoir capacitors everywhere. Savings on motor caps and pf correction caps. Etc.

If we were starting from scratch now, we could save a nice little pile with 400Hz - or if the sync zone would not be big enough to cover UK, maybe less, but high than 50.

Question: in this day of rf comms anad accurate time standards, why can we not use one central standard to sync gens all across the country, or anywhere as large as is wanted. It may have not been that way in 1900, but accurate time standards are fairly trivial now.

It is now, because of all the 50 and 60Hz kit. Theres too much of it that wont be replaced any decade soon.

And its not really practical to make everything 50/400 compatible for the next half a century before switchover.

The noise issue would not (need to) be a problem. Appliances would have a noise rating and probably legal requirements, just as they have various other requirements and ratings.

NT

Wall warts are purchased in bulk for figures like 30p each, so they must already cost less than this to manufacture, including the transformer. Maybe you could knock 5p off the price of a wall wart?

These are all going electronic anyway.

It's not a time standard problem, it's a transmission line problem. The National Grid is a large transmission line, and different parts of it will be at slightly different phase angles due to transmission delays. Now that might not be an insurmountable problem if it was just a long 1-dimensional line, but it's a complex 2-dimensional mesh, and the problem rapidly becomes unsolvable as the mesh size grows or the frequency increases. I don't know what the wave propagation velocity is on the National Grid, but best case it's the speed of light, so you have a 1/4 wavelength phase shift in around 1000 miles. At 400Hz, you shrink the max length of a synch zone to just 1/8th size, which is 1/64th of the area, so you need

I don't think that's the only issue. Another one which just occurs to me would be skin depth effect on large high current conductors; not insurmountable, but you'd have to change the conductors to multiple strands or flat tapes at 400Hz for smaller conductor sizes than is required at 50Hz, which would make cables and terminations more expensive.

All this to save 5p off the price of a wall wart? I have my doubts...

So lets say we cut from 30p to 20p each at the factory gates, or =A32 to =A31.33 on the retail shelf.

Now, how many warts have you bought in total? Lets take a vague guess at 10. Times 60 million people in UK gives us: factory gates savings of =A360 million and consumer savings of =A3400 million.

And thats just for warts, which are definitely not the biggest saving.

not microwaves. We might save =A35-10 retail per nuke. Say =A35, and say a history of 5 nukes owned per person.

Get the picture?

OK, I'll rephrase the question: why can all gens not be synced by a common time standard instead of by the neighbouring mains waveform?

skin effect at 400Hz??

NT

OK, I'll rephrase the question: why can all gens not be synced by a common time standard instead of by the neighbouring mains waveform?

Laws of physics - there are stupendous forces involved in trying to drag an alternator out of synchronism once its locked in. I'd have to dig my old student days books out of the loft to find the calcs but one of the examples we studied on a 250MVA unit slipping by a factor of 0.1 astounded the whole of our group. The synch restoring torque was massive

I think because there is the unavoidable problem of phase shift along the transmission cables. The wavelength of 50Hz is about 6000Km, at which the phase would have shifted a full 360 degrees. Sounds no problem, but that represents a 10 degree sync difference only every 167Km or 104 miles. That might cause a problem on the UK's grid.

You mean that if generator "A" and generator "C" are each 100 miles away from generator "B", are all synchronised together in *absolute* time by an external reference and all supply the same line, the waveform supplied by "A" and "C" will be 10 degrees out of phase with that supplied by "B" as measured from "B".

A=====B=====C

Sounds nasty.

So am I right in thinking that the way it is actually done is (effectively) to have one generator start first, and for each of the others to synchronise with the received waveform at their own locations? In other words the generators are, in absolute terms, out of phase with each other but due to wavelength / propagation delay / whatever are for all practical purposes synchronised.

I can see how this would work for a "bus" topology, but not for anything involving either rings or a mesh as the length of two or more paths from generator to generator will be different and hence received waveforms from each one will be different. How is the grid/supergrid actually organised in this country? How do they do it in the US where distances are vastly greater?

Learn something new every day on this ng :-)

Hwyl!

M.

I expect there is no true electricity supply sync across the US. It's simply too big as you say.

Even for the phone network there are occasional clock slips where a bit is lost or gained.

I probably have the numbers widely out, but all transmission lines have a line wavelength, which is the line length over which the phase lags by 360 degrees. It means that if a sinewave is started at point A, then there is a delay before before it starts at point B, etc. An effective phase shift.

Apparently the re-start after New York's great Blackout (some years ago now) was very difficult.

Not my field Martin, so I have no idea how the UK grid works, nor if there are difficulties with interconnections.

Be interesting to know though.............

With a mesh, the UK is fairly narrow, so the phase shifts east-west would be small compared to north-south, I guess.

In principle I imagine one could have no end of phase shift if, and only if, something... ha, I cant explain it. But I guess its apparent theres no limit to how much phase shift could be accomodated within a network, but it would depend very much on its layout and current flows. You'd have to be careful what you connected to what via what, but in principle one could presumably operate a grid with a ripple shaped phase shift, like a ripple in water, with the phase shift adding upto several whole cycles.

Someone can now explain why this is total twaddle.

NT

I always assumed that 50hertz came about as a doubling of 25 hertz to reduce flicker and improve transformer efficiency.

Also, 60 Cycles Per Second is the logical extension of 60 seconds in a minute, 60 minutes in a hour.

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He even arranged for the first electric chair to use his competitors AC power. This marketing plan backfired when it took forever to kill the criminal.

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Edison developed the 3 wire dual voltage configuration for his DC system. This was minus 100V, Plus 100V, & Neutral at 0Volts. This provided two voltages for different uses and saved more than 25% of the wire of two separate 100V circuits (neutral size could be reduced).

The US AC folks copied this system for residential and small commercial end distribution. It was trivial to implement with transformers and allowed an existing DC facilities to be changed to AC with only the replacement of some motors. Universal wound motors such as in vacuum cleaners and hand drills are happy with either AC or DC.

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UK Ten 100watt lamps at 240V equal 4.166 amps on circuit rated at 6 amps with 1mm wire.

US Five 100 watt lamps at 120V equals 4.166 amps on 15amp rated circuit with 14gauge (2.08mm) wire.

In this example the US system has a massively greater safety margin.

The proof is that you keep saying "for a given load" and the loads are NOT the same, we have many more circuits in typical dwelling.

Simply a system with more smaller circuits each with an equal or greater degree of safety margin.

As an example a modest 3 bedroom suburban home normally has a 200amp

They are a homeowner/shoddy contractor item and are frowned upon. We also would never consider it on a 30/32amp circuit.

And some that would not cause a fire at 120V will burst into flames at

Why snip the explanation?

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The North American power grid consists of four major synchronous interconnect regions - Western, Eastern, Texas, and Quebec. Remove SPAMX from email address

Sort of like 12 inches to a foot, three feet to a yard, 22 yards to a chain, 8 chains to a furlong, Bush installing democracy in the Arab world in an afternoon (whether they wanted it or not).... that type of thing.

A logical progression......

..... and New York. On good days at least.

Not so - it's the current that causes a fire. If low voltage was safer from the fire point of view, cars would never suffer electrical fires.