Per-Unit Sequence Models

An ideal Y-Y transformer grounded via neutral impedances has per-unit sequence networks similar to a single-phase transformer when balanced positive or negative-sequence currents flow, negating neutral currents and voltage drops.

Zero-sequence currents from all three phases combine to form a neutral current, causing voltage drops across the neutral impedance and influencing the low-voltage winding voltage.

Practical Y-Y transformers include external impedances in their per-unit sequence networks. Each phase represents a core loss resistor in parallel with a magnetizing inductance.

These transformers have identical per-unit positive and negative-sequence impedances, while the zero-sequence network depends on neutral impedances.

Delta-delta transformers have identical positive and negative-sequence networks, with per-unit impedances independent of winding connections.

To model per-unit sequence for three-phase, three-winding transformers are created by connecting three comparable single-phase transformers using a common S-base and proportional voltage bases.

In the zero-sequence network, the high-voltage connection depends on the high-voltage windings' configuration.