An ideal Y-Y transformer, grounded through neutral impedances, displays per-unit sequence networks akin to those of a single-phase ideal transformer when subjected to balanced positive- or negative-sequence currents. These currents do not produce neutral currents, and their associated voltage drops.
Zero-sequence currents, which are identical in magnitude and phase, generate a neutral current, resulting in voltage drops across the neutral impedance and the low-voltage winding. If the transformer's neutral is ungrounded, zero-sequence currents cannot flow to the ground but can still circulate within the windings.
Practical Y-Y transformers' per-unit sequence networks incorporate external impedances. Their shunt branches represent a balanced-Y impedance load, with each phase corresponding to a core loss resistor parallel to the magnetizing inductance. These transformers possess identical per-unit positive- and negative-sequence impedances, whereas the neutral impedances influence the zero-sequence network.
Delta-delta transformers feature per-unit sequence networks with identical positive- and negative-sequence impedances. However, different winding connections can influence these impedances in practical transformers. Per-unit sequence models of three-phase, three-winding transformers can be constructed by connecting three identical single-phase transformers, using a common S-base for terminals and proportional voltage bases.
In the general zero-sequence network, the configuration of the high-voltage windings dictates the high-voltage connection. Per-unit negative-sequence network impedances always match the positive-sequence network impedances.
