[BC] AC Voltage Drop

[PeterH]

On Jun 29, 2009, at 4:13 AM, RichardBJohnson at comcast.net wrote:

> Delta is nice because it provides good balance. However, you need  
> to use a transformer to generate 120/240, which puts your entire LV  
> panel on one incoming leg.

The most popular form of Æ is 120/240.

If a three-phase transformer is installed, the capacity of the A-C  
phase, which is that which supplies 120/240 loads, and the capacities  
of the A-B and B-C phases, which are those which supply only three- 
phase loads (or customer-installed and owned "dry-type" single-phase  
distribution transformers) are the same.

In the more usual case, a larger singe-phase transformer will be  
installed for the A-C phases, and one smaller transformers will be  
installed for the A-B phases, or two smaller transformers will be  
installed for the A-B and B-C phases. The former is called "open Æ",  
the latter is called "closed Æ".

Let's say your installed three-phase load is 50 KVA, and your  
installed single-phase loads are 100 KVA.

This could be accommodated by a 150 KVA 120/240 transformer plus two  
50 KVA 240 transformers.

One advantage of Æ is there can be no "zero-sequence" currents in an  
ungrounded system. There will always be "zero-sequence" currents in a  
Y system.

Æ is used for most transmission and subtransmission systems as Æ  
requires only three conductors (Y requires four conductors), plus Æ  
can still deliver three-phase power in the presence of a temporary  
single line-to-ground fault (any line-to-ground fault in a Y system  
results in loss of power).

Y is occasionally seen in older subtransmission systems where the  
original Æ system was "Y-ed up" to a higher subtransmission voltage.

The required insulators are the same, so a Æ may be converted to a Y  
simply by adding a fourth conductor, which is a neutral conductor,  
and which is normally at ground potential, but carries any "zero- 
sequence" currents (currents attributable to an imbalance in the  
three phases).

A 7,200 volt Æ system may, thereby, be converted into a 12.5 kV Y  
system quite economically, as the line-to-ground voltage in both  
cases is 7,200 volts..