Electrical transmission is most efficient over long distances by using
very high voltage transmission lines. However, for small applications,
it may not be too easy to accomplish this task.  The background for
this question is that an engine or turbine powered electrical
generator is going to be used to generate three phase AC power which
is intended to run a motor of 100 or more horsepower, which is located
about a quarter of a mile away.  For this situation, it is desirable
to have a low power loss cable, without having heavy electrical wires,
which is what would be required for low voltage. I think the easiest
way to get a lightweight cable is to use much higher voltage to power
the cable.  (The motor and generator have to have this separation.)
The motor is likely to be about 460 volts three phase.  It likely will
also have a variable frequency drive associated with it. So, the
question is, what options are available, and what are the merits to
the options for transmitting the power to the motor. The options to be
reviewed are:

1.  Just connect heavy wires between the two and somehow deal with the
weight of the heavy cable.

2.  Somehow step up the voltage prior to putting it on the power
cable, and then convert it back to about 460 volts right before going
into the motor.  For this option, I need to know details about losses
from the associated transformers.  A good answer would include details
about new solid state power conditioning  equipment, which might take
the place of a transformer.

3. Use a generator that generates higher voltage power in the first
place, and then transform or somehow convert it to the lower voltage
close to the motor.  I do not know if such generators exist, and at
what voltage they run.

4. Anything else that might work in this situation.

A good answer would give lots of attention to the newer solid state
power conditioning equipment that might be able to do one or more the
tasks.  The overall weight and size of all equipment matters quite a
bit.

 A great answer will "generate" a tip.  Thanks.

I think used motors are available up to 5000 volts three phase for 100
horsepower, removed from decomissioned electrical power plants.
Typically these were well made and mantained skilfully. I see no good
reason 5000 volt generators (three phase alternator is the correct
terminology) would not be available at 110 horsepower. Some machines
can be used as either a motor or an alternator. I think you get about
a 20% voltage change if you Y connect at one end and delta connect at
the other end. Auto transformers at about 99% efficiency are available
for small voltage changes/any transformer can be wired as an auto
transformer, if the specs are reasonably close. Modest miss-match
between generator and motor and mechanical load will produce less loss
than 460 volts or tranformers. So you could use a 150 horsepower motor
if 100 horsepower is not available, Do make sure you get the RPM your
load requires, or reasomably close. 5000 volts (or there abouts) puts
your current per wire at about 60 amps instead of about 700 amps using
460 volts three phase.
 If you use 2 or 3 transformers at one end expect a 10% loss; 20% for
both ends, Very costly toroidal wound transformers are available at
better than 95% efficiency, I think.
 Most power conditioning systems available for 100 horse power are too
costly I think, and I suspect their performance is exaggerated,
especially if improperly sized to you needs.  Neil

Single transformers for three phase have 2 or three transformers
inside, so they may not be less costly than separate units. If you use
50,000 volts, you only need about 6 amps per wire, but the bigger
insulators may weigh more, and cost more than you saved in smaller
wire size and reduced wire losses. 100 to 1 transformers are likely
less available (compared to 10 or 20 to 1) more costly and slightly
less efficent, especially at 46,000 volts or more. I mentioned 46,000
volts as that is the voltage my utility uses in my neighborhood, so it
may be a standard value. 100 horsepower motors and alternators are
likely special order at much more than 5000 volts = 5 KV, but I am
guessing what is available.
 Weight over the 1/4 mile is zero if you beam the energy by microwave.
The USA government funded a 25 million dollar research effort about 20
years ago on beaming energy 23,000 miles from geo-stationary orbit.
They were very concerned with keeping weight as low as possible at the
satellite end, so you may get some good ideas if you can access the
detailed study. If you google SPS you will likely get more than you
wish. You can perhaps use about 60 replacement magnetrons for
microwave ovens to produce the 5.55 megahertz beam. The receiving end
is called a rectenna = antenna with diodes. Some contractor may have
built an antenna array of the size you need to send and receive the
perhaps 50 KVA. Reports of 50% effciency are likely not realistic, so
your turbine may jump to 200 KVA due to losses, so you should not go
this route unless the zero weight transmission line looks very
attractive. EPA, and FCC will be concerned about how much of the
microwave energy is absorbed by humans, so that could be a show
stopper. If you build one, I think you will be the world's first at
100 horsepower plus.  Neil

The last time I looked at variable speed, decades ago DC motors were
used. Likely there are ways with 3 phase motors, about which I haven't
a clue. If the fastest speed is only about twice the slowest speed
your best bet may be to vary the speed of your turbine. The motor
speeds will vary proportionally. Well built motors will also tolerate
some faster than design, so three to one may be cheaper by over rating
the components. Neil

Thank you neilzero-ga for your insight so far.  The zero weight is not
essential, thus I agree that we should forget the microwaver route. 
As for controlling AC motor speed, commercially available variable
speed drives (VFD's) are available, which control AC motor speed by
changing the AC frequency.

Oceanmark-ga

Again, I did wait for researchers.

Smallish (150kVA) three phase transformers would be appropriate and
reasonably cheap for voltage step up though availability is a little
limited. At 75% load you?d expect efficiency in the high 90s. You?d
probably want 440 to 2kV as a suitable compromise. Variable frequency
would snooker this idea though. Plus it would still be a lot cheaper
over only a quarter of a mile to use heavy cable, especially since
higher voltage rated cable will not be vastly cheaper than heavy 440.
Depends in part if you intend overhead or buried.

Electronic voltage up / down conversion would work out much more
expensive and less efficient. Also less reliable.

Couldn?t (easily) find gennies of your size with higher than 440 volts
but it?s not inconceivable that they exist.

Best

Thanks again guzzi, this is good advice, and it is very valuable
information. Perhaps someone will still fully answer this one, or at
least add more comments.


Oceanmark-ga

Hi oceanmark. Surprised that no researcher grabbed your two generously
priced questions so we can continue if you wish.

The near constant load is a big plus. You?ll be able to spec a genny
at it?s maximum efficiency. Unless it is at odds with specific
requirements, a diesel or gas (not petrol) genny is likely to be your
best route. They are of course still thermally inefficient but such
are the rules of thermodynamics.

Regarding the driven motor, variable frequency drives abound, but
they?ll be a bit pricey at your power. If you need it though...... But
for variable speed, you might also consider a DC motor incorporating
speed control. Not as backward as you might think -- lot of heavy
movers use DC. There are also multi-wound AC motors designed for a
degree of speed switching but you?d probably have to get a custom job
for that. Starting torque considerations may apply, depending on your
application.

This is, I appreciate, a first order evaluation which you are embarked
upon, but perhaps you are now nibbling at the edges. If there is more
I (or others) can do, such as researching equipment, keep posting.
BTW, country?

Hi Guzzi.  Thanks again.  The country is USA.  This device may end up
being used elsewhere, but for now, California is the main
consideration.  Emissions of the generator need to be considered too,
which is why a turbine device might be good to consider.  I think that
combinded cycle turbines are available, but I really am not too sure. 
Any insight here as to manufacturers would be appreciated.  The single
stage turbines are not too efficient, and I have no use for the waste
heat they generate.  The piston types you suggest may be OK too, if
the emissions can be kept in check.

Oceanmark-ga

Hi again oceanmark. Efficiency is the dominant criterion -- over cost,
size, reliability etc, but pollution is up there too. I?m vaguely
thinking triple expansion. This is going to need a lot of hunting.
I?ll keep looking and if I find useful links I?ll post. Perhaps though
(since it seems no one wants to unload your pocket) perhaps post a
dead simple question, like ?What generator system (150 kVA), using
what fuel, is most thermally efficient, regardless of initial cost??

Best

Hi oceanmark -- couple of pages.

This is historical but might provide some insights.
http://www.navweaps.com/index_tech/tech-077.htm

Some of the links in this may help.
http://solstice.crest.org/discussion/gasification/200110/msg00012.html

Best