- Tertiary Winding Definition: The tertiary winding in a transformer is an additional winding besides the primary and secondary windings, creating a three-winding transformer.
- Advantages of Tertiary Winding: It reduces unbalance in the primary winding, redistributes fault current, and supplies auxiliary loads.
- Delta Connection: The tertiary winding’s delta connection helps limit fault current during short circuits.
- Stabilization Role: In star-star transformers, the tertiary winding stabilizes the system by allowing the circulation of zero-sequence currents.
- Rating and Design: The design of the tertiary winding depends on its use, requiring considerations for load capacity or short-duration fault currents.
What is Tertiary Winding? What is Three Winding Transformer?
Some high-rating transformers add a third winding besides the primary and secondary. That extra winding is the Tertiary winding of transformer. Because of this third winding, the transformer is called three winding transformer or 3 winding transformer.
Advantages of Using Tertiary Winding in Transformer
A tertiary winding is fitted on an electrical power transformer to meet one or more of the following requirements:
- It reduces the unbalancing in the primary due to unbalancing in three phase load.
- It redistributes the flow of fault current.
- Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.
- As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.
Stabilization by Tertiary Winding of Transformer
Star-star transformers made from three single-phase units, or from one five-limb core, present a high impedance to unbalanced load current between line and neutral. Those cores give a low-reluctance return path for unbalanced flux.
If any transformer has N turns and the magnetic-path reluctance is RL, then
I and Φ are current and flux in the transformer.
Equations (1) and (2) then combine to
That expression shows impedance is inversely proportional to reluctance. Where a very low-reluctance return path exists for unbalanced flux, the impedance to unbalanced load current is very high.
In other words, the path between line and neutral presents a very high impedance to unbalanced current in a three-phase system. Any unbalanced current in a three phase system splits into positive-sequence, negative-sequence and zero-sequence components. Zero-sequence current is the in-phase current in the three lines. If that co-phasial current in each line is Io, the total current in the secondary neutral of the transformer is In = 3.Io. Primary current cannot balance that current, because zero-sequence current cannot flow through an isolated-neutral star primary. The secondary current therefore sets up a magnetic flux in the core. As already noted, a bank of single-phase units and a 5-limb core both give a low-reluctance path for zero-sequence flux, so the impedance to zero-sequence current is very high. The delta-connected tertiary winding lets zero-sequence current circulate. That circulating current balances the zero-sequence part of the unbalanced load and stops extra zero-sequence flux building up in the core. In short, a tertiary winding on a star-star-neutral transformer cuts the zero-sequence impedance of transformer.
Rating of Tertiary Winding of Transformer
Rating of the tertiary winding of transformer depends on its duty. If it must supply extra load, winding cross-section and design follow that load plus a three-phase dead short-circuit at its terminals with power infeed from both HV and MV.
If it is fitted only for stabilising, cross-section and design follow thermal and mechanical limits for short-duration fault current. Among those faults, a single line-to-ground fault is usually the most severe.
