Transformer Tertiary Winding Application

A tertiary winding is an additional winding, typically having fewer turns than the primary or secondary windings, present in some transformers. Its primary purposes are:

• Stabilizing the neutral point, especially in three-phase transformers.
• Providing a path for zero-sequence currents and third harmonic currents.
• Connecting auxiliary power loads.

Tertiary Winding in Star-Star Transformers

The Star-Star (Y-Y) transformer connection is a scenario where a tertiary winding is particularly beneficial, especially when the neutral points are ungrounded or provide limited grounding.

In a standard ungrounded Star-Star connection:

• Third harmonic voltages can cause the neutral point to oscillate or become unstable.
• Zero-sequence currents lack a return path, potentially leading to overvoltages and operational issues.

By connecting a third, delta-connected tertiary winding:

• It provides a closed path for third harmonic currents, preventing them from circulating in the line or causing neutral instability.
• It allows zero-sequence currents to circulate within the delta winding, effectively grounding the neutral point for zero-sequence components and stabilizing it.
• This ensures a more stable system voltage and reduces stress on the insulation.

While tertiary windings can be used in other configurations (like providing auxiliary power), their role in neutral stabilization and harmonic suppression is most critical in Star-Star connected systems without robust grounding.

Therefore, the tertiary winding finds its primary use in enhancing the stability and harmonic performance of Star-Star connected transformers.