January 2008 Electrical Apparatus

January 2008 Electrical Apparatus

This is a summary of the Electrical Apparatus January 2008 featured technical article,  by Richard L. Nailen, P.E.    

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.

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