7 years ago
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. .
in a star configuration that they would have to be supplied from six pha se distribution or two of each of the secondary windings would have same primary winding exciting them and would therefore be functionally paralle l with no phase difference or voltage between them. 20 center tapped phase delta with the resultant high leg on one phase wou ld do what is needed. Either would give you Three phased power for eleva tor motors, air conditioning chiller or whatever the bigger Three phased load is. se, I have yet to see a situation were that is actually needed. Any Thre e 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 th at would run on either without modification. Even single phase home appl iances 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. s 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 br anch or feeder to each unit for the 240 volts. I have seen existing buil dings with Fourteen units with single phase supply but I have not seen an y 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.
40 volt Three phase then the designer should compare the cost of installa tion and operation of either two separate services to supply different vo ltages of Three phase or instal a boost transformer off of the 208 volt T hree 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 secon d meter each month. In the case of a larger load- I see no need (technical or economic( for a special 240 3 phase supply 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 s ide of the demarcation point it is the exclusive province of the Authorit y Having Jurisdiction (AHJ) over the enforcement of the locally adopted e lectrical code. The only exceptions I have encountered in almost Fifty y ears is with publicly owned utilities because some of those do the electr ical code enforcement.