Y-Y CONNECTION IN THREE-PHASE SYSTEM

What are the principles behind the y-y configuration?

The most obvious way of transforming voltages and currents in a threephase electrical system is to operate each phase as a separate singlephase system. This requires a four-wire system comprised of three phase wires plus a common neutral wire that is shared among the three phases. Each phase is transformed through a set of primary and secondary windings connected phase-to neutral. This is commonly referred to as the Y-Y connection, as illustrated in Figure 9.1. The left-hand part of Figure 9.1 shows the physical winding connections as three separate twowinding transformers.

Both the primary and secondary windings of each of these transformers are connected between one phase, labeled A, B, and C, and the neutral, labeled N. The right-hand part of Figure 9.1 shows the winding connections as a vector diagram. The direction of the phase rotation is assumed to be A-B-C expressed in a counterclockwise direction. This means that when the vector diagram rotates in a counterclockwise direction on the page, a stationary observer sees A phase, followed by B phase, and followed by C phase in sequence. This counterclockwise convention will be followed throughout this section.

The term “Y-Y connection” should be obvious from the fact that the vector diagrams of the primary and secondary windings both resemble the letter Y. Each phase of the primary and secondary circuits is 120° electrical degrees out of phase with the other two phases. This is represented by angles of 120◦ between the legs of the primary Y and the secondary Y in the vector diagram. Each primary winding is magnetically linked to one secondary winding through a common core leg.

Sets of windings that are magnetically linked are drawn parallel to each other in the vector diagram. In the Y-Y connection, each primary and secondary winding is connected to a neutral point. The neutral point may or may not be brought out to an external physical connection and the neutral may or may not be grounded.

The transformer magnetizing currents are not purely sinusoidal, even if the exciting voltages are sinusoidal. The magnetizing currents have significant quantities of odd-harmonic components. If three identical transformers are connected to each phase and are excited by 60 Hz voltages of equal magnitude, the 60 Hz fundamental components of the exciting currents cancel out each other at the neutral. This is because the 60 Hz fundamental currents of A, B, and C phase are 120° out of phase with one another and the vector sum of these currents is zero. The third, ninth, fifteenth and other so-called zero-sequence harmonic currents are in phase with each other; therefore, these components do not cancel out each other at the neutral but add in phase with one another to produce a zero-sequence neutral current, provided there is a path for the neutral current to flow.

source:2010 IIEE Technical Manual