From: dhky@shaw.ca
On 17/06/2015 8:59 AM, Tom Horne wrote:
> 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.
You can have 3 single phase transformers with center tapped secondaries
connected on the primary to the HV side (delta preferred) and secondary
center taps connected to ground so that there would be 120V on each
pair of adjacent legs but 120/240 between opposite legs. This idea has
been used for many years for rectifier supplies
Technically speaking it is a 6 phase star (the center tapped single
phase setup (edison) can be considered as a 2 phase star (although it is
called this only in countries that don't use it.
The idea of doing this fordistributioncan have advantages- at higher
distribution voltages as the clearances can be reduced.
In some areas this is worth while- chopping big holes through heritage
trees is not a good idea. .
>
>
> 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.
I live in an area where air conditioning loads are light and the supply
to the transformers on the street is 7200V line to neutral- only one
feed from the nearby 3 phase line is taken down this street- hence
120/240 single phase supply to each home.
There are rural lines that are single phase. 13-15 KV line to neutral,
ground return (actually a grounded neutral wire is also used) is
common. You are right in that generally new construction will have
120/208 in the units- as the load density is sufficient to warrant it.
>
>
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