A transformer that has one of its windings connected to one of these circuits, as a dedicated transformer, is a converter transformer, or rectifier transformer. IEC standards refer to these transformers as converter transformers, while IEEE standards refer to these transformers as rectifier transformers. Because it is IEEE practice to refer to these transformers as rectifier transformers, that same term is used throughout this discussion. Transformers connected to circuits with a variety of loads, but which may contain some electronic circuits that produce harmonics, are not considered to be rectifier transformers. However, they may have harmonic heating effects similar to rectifier transformers. Those transformers are covered under IEEE Recommended Practice for Establishing Transformer Capability when Supplying Non-Sinusoidal Load Currents, ANSI/IEEE C57.110.
One of the major advantages of the Δ-Y connection is that it provides harmonic suppression. Recall that the magnetizing current must contain odd harmonics for the induced voltages to be sinusoidal and the third harmonic is the dominant harmonic component. In a three-phase system the third harmonic currents of all three phases are in phase with each other because they are zerosequence currents. In the Y-Y connection, the only path for third harmonic current is through the neutral. In the Δ-Y connection, however, the third harmonic currents, being equal in amplitude and in phase with each other, are able to circulate around the path formed by the Δ-connected winding. The same thing is true for the other zero-sequence harmonics.
If the magnetizing current waveforms of all three phases have approximately the same shape and magnitude, and if the phase angles of the 60 Hz fundamental frequencies are 120°apart, then the third harmonic currents circulate only within the transformer bank and do not emerge as physical currents at the terminals of the windings. This causes these currents to be suppressed in the primary and secondary circuits.
Another important advantage of the Δ-Y connection is that it provides ground current isolation between the primary and secondary circuits. Assuming that the neutral of the Y-connected secondary circuit is grounded, a load connected phase-to-neutral or a phase-to-ground fault produces two equal and opposite currents in two phases in the primary circuit without any neutral ground current in the primary circuit. Therefore, in contrast with the Y-Y connection, phase-to-ground faults or current unbalance in the secondary circuit will not affect ground protective relaying applied to the primary circuit.
This feature enables proper coordination of protective devices and is a very important design consideration. A transformer-connected Δ-Y with the neutral of the Y grounded is sometimes referred to as a grounding bank, because it provides a local source of ground current at the secondary that is isolated from the primary circuit.