Or, in Britain, Crompton, Kelvin and Hopkinson v Ferranti, Mordley and Thompson.
Possibly something to do with 40Hz being about the minimum frequency at which flicker was not noticeable to most people with early incandescent lamps. There being a limited number of manufacturers in the world making generating equipment was probably also a factor.
In Britain, in 1930, there were 15 different AC frequencies in use, from
25Hz to 100Hz, but 81% of all generation was at 50Hz. Standardisation cost £17.5 million.
y millivolt/milliamp world, we don't have to contend with forces sufficient to rip a machine's internals to pieces as you do with power engineering, s o the need for nice, smooth 3 phase supplies simply doesn't arise in the fi rst place!
I think you may be right. A recently qualified young electrician told me, i n regards to electrical safety, that "even just a few megaohms can kill you ." I shuddered at the time, but feel much less alarmed now in the light of your remarks.
Agreed, and certainly not serving 200,000 homes unless it caused a trip in one of the much higher voltage lines, which I would have thought was unlikely.
It went quiet because a breaker did operate, but only once there was enough ionisation from the fire to allow an arc between phases or from phase to ground. Perhaps a few hundred amps? Which is a few megawatts, maybe 1000 homes?
Apart from skin effect, 50-60 Hz keeps the generators simple: two poles per phase at 3000 rpm or occasionally 4 poles at 1500 for 50 Hz. You can't speed up big (500-650 MW) the generators significantly because you are close to the strength limits of suitable steel. You have conversion losses if you try to increase the frequencies with electronics. The losses in DC converters at each end of a DC link (like the interconnectors under the English Channel are offset by reductions in transmission loss.
On Sunday 07 July 2013 19:15 Andrew Mawson wrote in uk.d-i-y:
Dunno - the Robertsbridge overhead feeders are 33kV (it says so on some of the warning labels) and they are wooden poles, lines fairly close, low down.
In days of yore as the national grid was being created, the were auto trnasformers between areas of different local voltages as a temporary thing. The electricity boards were responsible for collecting up all appliances and converting to the new voltage. Vast job even back then. Pre and post war.
There would be no save for higher frequency in fact there would be increased iron or magnetic losses. Power is realated to voltage and in-phase component of the current only.
The reason aircraft run on high frequency is that electric motors/transformers can be made much smaller and lighter because more power can be transmitted through any magnetic circuit by increasing the frequency. However they are far less efficient. Cable sizes are unaffected.
50 Hz is not the optimum frequency either. The Yanks have it right, 60Hz is optimum.
If anything, we may go to DC in the future. Makes a lot more sense these days. I remeber helping to remove DC stuff back in the 60's. And put some in. It was the induction motor that did it. Mostly it was in places that got their initial electricity from coal mine/factory power houses. We will have come full circle.
s face it: even 230V is well into the lethal zone so going higher isn't goi ng to be any more dangerous. Plus 800 is still low enough not to arc or fla shover - given remotely adequate insulation. And just think of the saving i n terms of conductor! Given the price of copper (notwithstanding it's slipp ed back lately) I'd have thought pushing up the supply voltages would be ob vious thing to do from the pov of conserving natural resources.
690V (400V to neutral) three-phase is used in some countries, suitably-woun d motors can be used at 400V delta or 690V star, so easing standardisation, maybe we should consider it.
Actually there's still quite a bit of 66kV, it's been established in North- East England from the time of the 'Newcastle-on-Tyne Electric Supply Co' wh o built a regional grid between the wars, also for underground cables in Lo ndon where 33kV isn't enough and 132kV cables would be too expensive.
Nope. Definitely a couple of hundred thousand. I should, however, point out that this was back in the late 70s, so it's possible they've reduced the voltages these lines are permitted to carry since then.
certainly up to 1962, Cambridge town was supplied wth 200v mains. Sellers of electrical goods needed to supply the right ones for use there. My 200v coffee pot survived about 1 year on 240v elsewhere, my soldering iron - quite a bit longer.
I found doing a bit of software development work on high power HF amplifiers / transmitters was quite enlightening in that respect... It certainly changes ones TTL/5V view of the world, when dealing with 10kV step up transformers!
Not entirely sure, but when they replaced the main fuse and housing here some years ago, I'm sure I remember seeing four cores (three phases + neutral, with the sheath as earth). It could well be that they brought
3-phase to every house on the road when they were built in the 30s and only connected one phase at each. If I were to need 3-phase, that'd be a useful cost saving!
This house (built 1911) has all 3 phases incoming - I assume so proper balance could be obtained empirically. When they built a new estate of 25 bungalows, just round the corner, some 30 years ago - they were all on the same phase. How things change.
Most of those are the 11 kV local distribution. Some maybe 33 kV feeds to local area substations. Look at the insulators, around here the 11 Kv ones are small, brown with one flange. The 33 kV ones are larger glass and have two flanges.
Yes there is *a lot* of energy available on an 11 kV line. Wouldn't be surprised if the potential short circuit current is measured in the low kA, so with 11 kV driving it you have 10's of megawatts available. Put it all in a small space and you get a very big bang...
Plenty of videos of substation fires, arc overs etc on YouTube.
There's still a bit of kit up at the Institute of Astronomy which runs at 200V. Apparently the electricity board threw up their hands in horror at the complexity of replacing it, and just fitted a suitable transformer.
When I worked for an electricity board, it was more usually done surreptitiously, in one case by cutting small holes through the wall and driving six inch nails through the neighbour's meter tails, in order to keep the electricity bill down.
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