From: hornetd@gmail.com
On Thursday, February 14, 2008 at 1:16:11 PM UTC-5, phil-new...@ipal.net wrote:
> On Thu, 14 Feb 2008 03:44:25 GMT Don Kelly wrote:
>
> | The "6 phase " scheme wouldn't cost any more in terms of the transformers
> | and could be done using a bank of 3 single phase pole pigs. However, it
> | would appear that there would be additional costs involved with no net
gain
> | as well as a more complex system. Where possible one should avoid adding
> | complexity (KISS). This doesn't mean that it can't be done but may mean
> | that it shouldn't be done.
>
> So what would you have instead, keeping to the requirement of genuine
> 240 volts at the single phase circuits?
>
>
> | There has been some playing around in the past on the basis that 3 phase
is
> | good so maybe 6 or 12 phase is better- tain't so except in some special
> | cases.
> | Where 6 phase has been used is in some urban distribution where there is a
> | savings in terms of space necessary at a given line to neutral voltage
> | because the line to line voltage between adjacent phases is then the same
as
> | to neutral. Whether this is still being done is something that I don't
know.
> | It also has been used for rectifier supplies in order to reduce harmonics
> | and ripple.
>
> I've wondered if any of those 6-wire transmission lines might be phased
> this way, even if the loads end up being split at the far end.
>
>
> | Your scheme of 480Y and single phase transformers is far better.
>
> So that's the way you would go? What if the utility says they don't want
> to have the loss of an extra set of transformers with MV -> 480 -> 120/240
> and insist on no more than one transformer per MV -> meter path?
>
> --
> |---------------------------------------/----------------------------------|
> | Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
> | first name lower case at ipal.net / spamtrap-2008-02-14-1211@ipal.net |
> |------------------------------------/-------------------------------------|
I have been in the electrical craft since Nineteen Sixty Six and I have seen a
lot of different service voltages supplied to buildings. First I am not an
Electrical Engineer although I have always been the go to guy on theory
questions that the trainees
and apprentices come up with. So I'll just go ahead and admit that I don't
understand how you could get a six phase output out of a three phase input.
If it is a delta secondary it can only be corner or center of one phase
grounded as the code
specifically requires those as the only place the bonding jumper can be
connected. I have worked in manufacturing plants were the delta was
ungrounded but elaborate measures were taken to insure that any ground fault
was detected well before a second
such fault on a different phase had any chance to occur. You simply cannot
connect any delta system to ground on two separate phases or you have a dead
fault. I have yet to see any supply system that allows me to take half phase
loads from more than
one phase.
Someone has already pointed out that if you have six secondary windings in a
star configuration that they would have to be supplied from six phase
distribution or two of each of the secondary windings would have same primary
winding exciting them and
would therefore be functionally parallel with no phase difference or voltage
between them.
It would seen to me that either 208/120 wye connected supply or a 240/120
center tapped phase delta with the resultant high leg on one phase would do
what is needed. Either would give you Three phased power for elevator motors,
air conditioning chiller
or whatever the bigger Three phased load is.
As to actually insisting that individual units have 240 volt, Three phase, I
have yet to see a situation were that is actually needed. Any Three phase
heat pump, air conditioner, or other motor load that will run on 240 volt
three phase that I have had
to install could be re-tapped to run on 208 Three phase or the motors were
labelled as 240,208 Three phase that would run on either without
modification. Even single phase home appliances will run on either in most
cases. Why then would one need to
have actual 240 volts in single or Three phase all the way to the individual
unit. I have never actually seen Three phase power run to the individual
units of an apartment building.
If you actually must have 240 volts to each unit in the original posters small
apartment building; although no one has explained why that might be true; then
you will need either a single phase supply or a separate branch or feeder to
each unit for the
240 volts. I have seen existing buildings with Fourteen units with single
phase supply but I have not seen any new construction wired that way in the
last several decades.
If the elevator motor or some other relatively large load had to have 240 volt
Three phase then the designer should compare the cost of installation and
operation of either two separate services to supply different voltages of
Three phase or instal a
boost transformer off of the 208 volt Three phase to derive the needed 240
volt Three phase. Keep in mind that most utilities charge additional monthly
fees for a second service to the same premise even if it is only in the form
of a fee for reading a
second meter each month.
As for the utility insisting on anything, they must supply any service that is
in the regulatory tariffs maintained by the state regulatory body. Since the
derived system transformers that would produce the 120/240 volts from the
480/277 are not part of
the distribution network they are beyond the utilities power to control. Like
anything else on the load side of the demarcation point it is the exclusive
province of the Authority Having Jurisdiction (AHJ) over the enforcement of
the locally adopted
electrical code. The only exceptions I have encountered in almost Fifty years
is with publicly owned utilities because some of those do the electrical code
enforcement.
--
Tom Horne
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
|
