|
"Understanding
the Three-Phase Delta"
With
trilingual summary
In a three-phase circuit, the individual voltage phasors are diagrammed as three
arrows 120 degrees apart. This is shown in either a "wye" or start configuration
(the phasors joined at a common point in the shape of the letter "Y") or
"delta," in an equilateral triangle (the shape of the Greek letter "delta").
In an electrical distribution system, such connections typically consist of
three separate transformers. The common point or neutral of the wye connection
may or may not be brought out as a circuit conductor, grounded or not. Lacking
such a neutral point, the delta connection may take more varied forms.
Because no neutral exists to be grounded, the voltage-to-ground from any of the
three circuit conductors can vary widely, depending upon capacitive coupling
between conductors and earth. In a corner-grounded delta, one of those
connectors is grounded.
Another option, to serve mixed single- and three-phase loads, is the high leg
delta, in which one of the three transformers contains a center tap serving as a
neutral point for two single-phase load circuits connected between the tap and
the ends of that phase. The term "high leg" describes the voltage between the
tap and the third corner of the delta, which is 0.866 of the phase voltage
rather than the 0.50 fraction between the tap and the other two corners.
An open delta or V-connection uses only two transformers instead of three. All
three phase voltages still exist, 120 degrees apart. Presence of transformer
magnetizing current in only two of the three phases causes some voltage
unbalance. Also, the maximum load on each transformer is limited to 86.6% of its
capability in the closed (three transformers) delta.
Another way of using two transformers instead of three is to connect one of them
across the "high leg" but rated at only 86.6 percent of the other. The same
three-phase load can be served with the combined rating of the two transformers
slightly less than in the open delta.
In a similar circuit, the high leg transformer secondary is disconnected from
the tap point with its primary designed for 86.6% of the phase voltage. The
combined output forms a four-wire two-phase circuit. This "Scott connection" was
developed to adapt older two-phase apparatus to the newer three-phase supply.
Thus, the triangular delta connection in electrical systems, made in different
ways for different purposes, offers flexibility not available with the wye
connection.
From “Understanding
the Three-Phase Delta"
to be published in the
Electrical Apparatus January 2008 issue
. Visit our online webstore to
order
copies. © 2008
Barks Publications, Inc. All Rights Reserved.
|
