Tertiary Winding in Transformers

1. Unbalanced loads and 
2. Harmonic distortions 

are major concerns in three-phase transformer systems affecting voltage stability and power quality. 

These concerns become particularly important in YNyn0 (Star-Star) linked transformers when unbalanced loading occurs or when a single-phase-to-ground fault exists. 

To mitigate these results a further winding called the tertiary winding is added.

This tertiary winding, which is commonly coupled in a delta configuration is essential for 

• System stability, 
• Zero-sequence current regulation and 
• Harmonic suppression.

Operation of Ynyn Transformer

The YNyn transformer arrangement is widely utilized in high-voltage applications for numerous reasons:

Zero Phase Shift

It is possible to design with no phase displacement across the primary & secondary sides allowing for simple system connectivity and synchronization.

Reduced Insulation Cost

When compared to a delta winding, the voltage per winding is only around 57.7% lower, considerably lowering insulation costs in the high-voltage applications.

Neutral Availability

Both the primary & secondary sides have accessible neutral points which allows for good grounding and the connecting of single-phase loads.

YNyn0 transformers (constructed with three single-phase units (or) a 5-limb core design) meet high impedance when unbalanced load flow across line and neutral. 

This occurs because such transformers provide a low-reluctance channel for unbalanced flux resulting in a high impedance for zero-sequence current flow.

As a result the transformer operates most efficiently under balanced load conditions.

Problems in YNyn Transformers without Tertiary Winding

When a load imbalance (or) a phase-to-earth fault develops, the neutral point of the transformer moves away from zero potential, resulting in neutral shift. 

This causes:

• Zero-sequence currents (3I₀) occur in all three phases at the same phase angle.
• There is no escape route for these zero-sequence currents inside the star-connected windings.
• Third-harmonic production occurs due to the transformer core’s nonlinear magnetization curve (B-H relationship), particularly near saturation.
• These harmonics particularly triplen harmonics (3rd, 9th, 15th, etc.), alter the output voltage waveform and spread through the network.

resulting in:

• Overheating of the transformer windings & core.
• Increased iron & copper losses.
• Thermal stresses on circuit components.
• Reduced overall system efficiency and dependability.

What is Tertiary Winding?

To address these problems a closed delta tertiary winding is fitted to the transformer. 

The tertiary winding provides a channel for circulating zero-sequence & harmonic currents, enhancing system performance and stability.

Functions of Tertiary Winding

The closed delta tertiary provides a circulating channel for zero-sequence currents (3I₀), reducing the transformer’s zero-sequence impedance.

Harmonic suppression captures third-harmonic as well as triplen harmonics in the delta loop preventing them from entering the primary or secondary circuit. This produces a cleaner voltage waveform for the load.

The tertiary winding prevents neutral point displacement by dispersing uneven load currents. This keeps the neutral potential near to zero and reduces voltage imbalance between phases.

The tertiary winding reduces fault severity and improves system protection during phase-to-earth faults by redistributing fault currents among all three phases.

The tertiary winding can power auxiliary equipment and meet voltage requirements for capacitor banks and other loads.

Advantages of Tertiary Winding in Transformer

1. Balances the primary winding under imbalanced load conditions.
2. Helps distribute and minimize fault currents during short circuits.
3. Provides an auxiliary supply with a varied voltage level.
4. The delta-connected tertiary stabilizes the system by reducing third-harmonic currents & preventing neutral displacement in star-star (Y-Y) connected transformers.

When can Tertiary Winding be Avoided?

In certain applications the tertiary winding can be eliminated if the system design assures that:

• The load imbalance falls within tolerable limits.
• The harmonic distortion remains below acceptable levels.
• In these conditions the extra cost & complexity of the tertiary winding can seem unnecessary.

Conclusion

The tertiary winding is an essential component in high-voltage YNyn transformer systems, allowing for effective control of 

• Zero-sequence currents, 
• Neutral shifts and 
• Harmonic distortion. 

By providing a channel for undesirable current components it improves transformer performance, assures voltage stability & protects against thermal & electrical stressors. 

While not necessarily required, its presence is an important design element for ensuring power quality & operational reliability in the modern electrical networks.