In response to my question ID: 110766, I was advised:

'The supply of electric power to our houses from generating stations
is
mainly in the form of alternating current(a.c.). However the losses
experienced along the path of travel from the central power grid
station to the sub-stations and then on to the distributors are
phenomenal. This loss is dependent on the frequency of the a.c.
supply. Along the path there are transformers, transmission cables and
cores. The loss of energy in these parts depend directly on the
frequency irrespective of whether the voltage is being stepped down or
up.
 
Note: Static hysteresis loss is proportional to frequency. An equation
called Steinmetz equation can be employed to arrive at the fact that
60 Hz supply causes more dissipation of heat and energy than 50 Hz
systems. Hence it is not preferred by many countries. The losses being
proportional to the square of the frequency, is hence very high for 60
Hz systems.'

Now, Edison always reckoned that DC was the way to go but, unusually,
he lost this particular battle.

But, if the power losses from AC are indeed 'phenomenal', then for
environmental reasons should be not revert to DC? I understand that
all the technical limitations have been sorted and, indeed, DC is now
preferred for some distribution scenarios, such as under water.

The cost of converting the world is probably just the kick start that
is needed to get us out of recession.

Hi,

Welcome back to the subject of AC and DC power. I see that you have
quoted from my answer to that previous question (question id#110766).
I am happy to provide you an answer for this one too.

Of course DC is being used extensively these days in the form of HVDC
(High Voltage DC) lines. It is a very efficient system and is
preferred in underwater transmission as well as a back up supply
system (as well as a connector between two existing AC network
systems). However, its use for domestic supply as well as industrial
supply is limited mainly because of its High Cost. AC systems have
higher losses in the long run. These are due to; reactive elements'
losses, skin effects, harmonic losses and other losses typical to AC.
This is the reason why HVDC is used in some applications where losses
simply cannot be tolerated.

Now why is the cost of HVDC high? To understand this we have to go
back to the generating station.

In most of the cases (if not all) the prime mover (the driving source)
of the system, whatever it may be ( nuke, thermal, hydro...) runs what
is known as a turbo-alternator. In simple terms this is nothing but a
basic generator.
A generator produces AC output. Now why would one take all the trouble
to convert this to DC? Let us examine the costs involved. To make this
conversion possible, a unit known as a converter is necessary.
Following this, the DC voltage has to be stepped up ( I suspect that
the HVDC people anyways use a transformer for this purpose to step up
the AC and then convert it).

Terminal Converters are far more expensive when compared to simple
transformers. This single reason coupled with a few other reasons
which I will list below makes AC transmission a better choice.

Other Reasons: (and these technical reasons are yet to be hammered
out)

Reason #1:

Radio interference is a major disadvantage. A high band width is
required for trasmission purpose. In some cases PWM (Pulse Width
Modulation) is employed. As a result of interference, telephone lines
become susceptible.

Reason #2:
You have mentioned under-water usage. Yes, you are right. DC is
extensively used for under-water transmission. However it is now being
considered a menace as it interferes with compass readings and causes
deviations.

Reason #3:

In AC the concept of Neutral and 3-phase supply cannot be reproduced
at a lower cost using HVDC.

Reason #4: 
Stepping up or down the DC voltage ( from theory we know only of
choppers) is a problem and cannot be done as frequently as it is done
in AC.

Reason #5: 
Today most of the appliances are designed for AC. It would be
impractical to expect designers to remodel their systems to suit HVDC
terminals.

Reason #6:
HVDC causes harmonics which is responsible for the lower of the
quality of the power system.

Reason #7: 
Using HVDC means that one has to take into account the need for
filters, reactive power supply to maintain the power factor etc. etc.

...and many more reasons.

I hope this answers your question. I would be happy to answer any
further questions that you might have on the subject as this is my
speciality.

Thank you and have a good day.


Warmest Regards,
Shiv Reddy

Hi Shiv & Bobby

Good to hear from you both again.

Thank you for your very full and very informative answer.

I suppose therefore that, for once, Edison got it wrong and this is
evidenced by the world-wide selection of AC.

With regard to Bobby's comment, I recall that safety was an issue when
Edison and Tesla were arguing the toss and that, when Edison designed
the first electric chair, he opted for AC because he argued (contra to
Bobby's understanding) that it was actually more dangerous.

Also, someone (I'm not sure who) performed the public electrocution of
an elephant that was past its shelf life at the zoo. (I think the
execution took place at Coney Island.)

Fascinating stuff!

Thanks again.

Kindest regards

Bryan

I'm not posting this as an answer probonopublico, because this is from
my knowledge, and I don't have (and don't think I could find)
information to back me up.

Firstly, it was my understanding that the losses from AC were less
than DC (and hence we have individual transformers for our appliances
than one transformer at the power station into DC).  Also, I thought
that (and I'm not sure whether this is contrary to the other
researchers work, and I'm not saying that I am correct) a very high
voltage decreases the loss of voltage over power lines.  And I thought
that this conclusion stemmed from the fact that Power = Voltage
squared divided by Resistance.  Therefore, the higher the voltage, the
more power in the lines... Does that make sense?

Also, I thought that AC was more safe?

I'll stop, just in case I'm making a fool of myself!

bobby_d

Sometimes numbers are easier to follow than generalities. Suppose
Atlanta, Georgia is about to turn off the lights for 1/2 million
customers because a one gigawatt source needs to be shut down for
emergency repairs. Things are tough in Jacksonville because of the
extravagence of our mayor on our new DC power grid, so we will (for a
price) send you most of our power. (leaving most of our customers in
the dark) So Jacksonville connects 50 dc generators averaging 20,000
volts each in series to send one million volts to Atlanta. The current
for one gigawatt is 1000 amps.  In Atlanta they junked the 240 volts
ac (with a center tap) system, nearly everything runs on 200 volts dc,
which is lethal no more often than 120 volts ac used to be. Each
neighborhood has a series parallel arrangement of homes, so one
thousand volts can be applied. One  thousand neighborhoods in series =
one million volts. The toaster pops at Sally's house causing some of
Sally's lights to brighten. The voltage went from 200 volts to 222
volts. The central Atlantan computer spots the problem in one
millisecond and remotely turns a window unit almost a block away from
low cool to high cool. That drops the voltage to 198 volts but that is
close enough as something else will change in one second if not
sooner. I think it will work. The ten million remote switches in
Atlanta may even use less electricity than the 10,000 stepdown
tranformers lose on the present system. As long as there are no
grounds anywhere there is (no?) shock hazzard but boy do the sparks
fly for most any two grounds. Perhaps ground fault detctors can
disconnect a series parallel network with a ground, before a second
ground fault occurs somewhere. Power cubes to replace two penlight
cells will chop some pulses from the 200 volts dc then integrate at
less weight than the transformer presently used. There are more
(solvable?) problems, I think.  Since it is over 400 miles from
Jacksonville to Atlanta the optimum voltage is not much less than one
million volts.  Neil

It could get hairy trying to mow the lawn electrically in neighborhood
#1 on a dewy morning.