The future: DC mains

[Graham.]

I love the Tesla irony, but this is a non-starter isn't it?

[zall]

No, it is feasible, tho not necessarily the best approach.

[Fredxx]

It's been written by someone who doesn't seem to understand why we have AC into homes and thinks DC is all you need to charge a Tesla car battery. They don't seem capable of understanding Volts and Amps and that these need to be tailored to charge a battery.

With the advent of power electronics, 50/60Hz transformers could perhaps become redundant. The dielectric loss with burying AC cables is significant such that DC may also become the choice of underground power transmission within the UK.

[Cursitor Doom]

You're probably correct, but I hope I don't see it in my lifetime. As someone who's a bit careless and tends to get electrified more frequently than I would like, I find DC pretty scary. Nasty, zappy, unforgiving stuff it is.

[Fredxx]

I'm surprised at your fear. The war of the currents was specifically because AC was more dangerous but more practical for transmission:

[John Bryan]

Typical small world thinking, they have never been near any sort of manufacturing plant that use large motors.

[Colin Bignell]

It is an argument that raged at the start of the last century. However, when we developed a national grid, AC was chosen because of its advantages for long distance transmission. The exception being undersea cables, where dialectic loss is significant. However, the National Grid uses overhead lines for most HV transmission.

[Fredxx]

Very large motors used in rolling mills were often DC. Converting 3 phase into DC is pretty easy. It's been done for railways for a very long time.

The advantage of AC is the ability to change voltage for transmission, and then back down again for local use.

[Cursitor Doom]

You would not be so surprised if you had an unusually high skin resistance as I have. That makes AC the 'shock of choice' so to speak, for me. The downside however - there's *always* a downside - is I get zapped to hell and back in the Winter by static. Plus I blow senstive electronics to kingdom come at that time of year, too. You can't win either way.

[NY]

Why does having a high skin resistance make an AC shock better or worse than a DC one, if you equate RMS for AC with DC value?

I've never had a DC shock, because the largest battery I've ever encountered is a 12 V car battery, and the centre pin on a 20 V laptop charger is usually shrouded. though I've had a few AC ones due to my own stupidity.

On one occasion, many years ago, I'd turned an appliance off at its internal double-pole switch, which made most of it perfectly safe - until I happened to touch the live and neutral terminals on the switch where the mains cable connects to the switch. I still have a pair of "snake-fang" scars on the knuckle of one finger.

The other shock was something that only happens with modern Philips Hue bulbs which can be turned off internally (eg using a smartphone app) while mains power continues to be applied. I'd been changing all the GU10 light fittings in the bedroom ceiling. I'd been very good, turning off both the wall switch and the lighting-circuit MCB in the fuse box. Then my wife asked me to change a couple more. The bulbs were off, so the supply to the fittings was off. I didn't need to turn the switch off at the wall. Not true! I found out as I went to unscrew the fitting from the terminal block that connected it to the lighting circuit. That shock was the mildest of all, probably because the RCD tripped within its stated 30 msec.

I had a surprisingly strong shock off the TV aerial cable which was plugged into my old telly. I'd had the cable plugged into a USB tuner which was connected to my earthed PC, so the metal aerial plug was earthed - until I unplugged the cable with one hand while holding the metal PC case with the other. That was a nasty tingle. I spent a while trying to identify the source, because the aerial cable was plugged into the TV which in turn was connected by audio cables to the VCR and my hi-fi system. Which was the culprit? It turned out to be the TV which was applying about 150 V between aerial earth and mains earth, as measured by a high-impedance multimeter, although via a high resistance. With a resistor to simulate my arm-to-arm body resistance of about 100 k ohms, the voltage dropped to about 80 V - much less dangerous, but still enough to be felt, and enough to make me not want to repeat the experience to measure the on-load voltage for real, hence the resistor to simulate my body.

I knew a woman at university who could feel voltages as low as 1.5 V by touching the two terminals with finger and thumb. She said she could pick up batteries one by one from a pile, and sort the charged ones from the flat ones by touch. Most of us have to test with out tongues to feel voltages that low.

[John Rumm]

For high power transmission (>= 1GW) ISTR the cost crossover point is something like 430 miles. The DC system has less parasitic losses on the wire (and needs fewer of them since it's not 3 phase), but the conversion gear is massively more expensive than a transformer.

[zall]

I didn't read the original and stupidly assumed that they were talking about the main long distance grid, not what is used at the consumer level.

[Brian Gaff]

Well, back in the day of course we had both in different parts of the country. The drawbacks with DC were corrosion, variable voltages and no way to send over long distances as you could not use a transformer on DC. Obviously a lot of these things can be designed around nowadays, but why not simply go to 400hz AC as they use on some military equipment. The transformers are more efficient, after all look at the tiny ones inswitch mode power supplies. In the end though, there simply is too much investment in AC. Assuming we want to move power over very long distances and can design the kit to convert it, I believe DC does have some advantages, but are they really enough to make it worthwhile doing? Brian

[The Natural Philosopher]

Mostly wrong. AC allows easy voltage conversion so that long overhead cables can be high voltage, and so use less aluminium and steel. It could have as easily been high voltage DC if they had had semiconductor technology capable of transforming it.

Undersea cable are usually DC not because of dielectric losses, but because of *resistive* losses incurred by the cable capacitance necessitating large out of phase currents that deliver no useful power, just heat up the cable cores.

In short what you want for transmission is high voltage. The AC- or DC- ness is purely a pragmatic way to get costs down, depending on the technology available and its cost.

A further issue with AC is that in a network of multiple parallel paths

- like a continental grid - power via pone route may arrive out of phase with anther route, due to path length differences. This again looks like 'out of phase' current and just heats up the wires.

[Tim Streater]

The point about DC is that you can't let go, if you inadvertently grasp a live wire. With AC you can.

[jon]

I grew up with DC Mains and with all my early electrical experiments I used rotary transformers. I obtained a variety of ex-gov/military bits and pieces and was able to convert mains voltage 250v to 28vDC, 12vDC and

400vDC.

[Fredxx]

It is said the opposite is true, With DC you get a single shock, and single muscle contraction. With AC your muscles contract 100 times a second.

It is also said that AC is 5x more likely to lead to death but I don't know how that is determined in terms of volts AC/DC/rms/peak etc.

[charles]

Electric chairs ran/run off AC. It';s obviously more dangerous.

[Martin Brown]

I agree it seems to have been written by someone clueless.

Japan has an interesting and long standing DC grid interconnect.

This is because Tokyo adopted German AEG kit on 50Hz (and then later UK kit) whilst Osaka installed USA GE kit on 60Hz. Accordingly all their domestic appliances have to run on either frequency.

Unfortunately the DC interconnects were not able to move enough power to the 50Hz grid after Fukoshima went down so it all got a bit messy.

International DC interconnects at high voltage may become more common for undersea and underground cables for the reasons that you give. I can't see them ever replacing the supergrid pylons any time soon though.

[Joe]

Another point is the use of AC between countries is difficult because synchronisation of the countries' frequency and phase are required at the point of interconnection. In Europe, this would require all countries to be synchronised to France, because otherwise the French would pick up their marbles and go home.

Given the distances, no loop could be formed, the continental grid would need to be a tree. Over thousands of miles, even 50Hz phase differences become significant.

And picking one make of car ("It is an irony and an anachronism that our AC (alternating current) grid power distribution, first championed by Nicola Tesla over 130 years ago is completely unsuited to the job of charging Elon Musk's eponymous Tesla motor car!") to standardise DC distribution to the home is beyond ludicrous.

"...100 times a second huffing and puffing of AC power into the strong steady flow of DC, which is what batteries crave." Honestly.