What is Tertiary Winding? What is Three Winding Transformer?
In some high rating transformer, one winding in addition to its primary and secondary winding is used. This additional winding, apart from primary and secondary windings, is known as 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
Tertiary winding is provided in 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
In star-star transformer comprising three single units or a single unit with 5 limb core offers high impedance to the flow of unbalanced load between the line and neutral. This is because, in both of these transformers, there is very low reluctance return path of unbalanced flux.
If any transformer has N turns in winding and reluctance of the magnetic path is RL, then,
Where, I and Φ are current and flux in the transformer.
Now, from equation (1) and (2), it can be rewritten as,
From this above mathematical expression it is found that, impedance is inversely proportional to reluctance. The impedance offered by the return path of unbalanced load current is very high where very low reluctance return path is provided for unbalanced flux.
In other words, very high impedance to the flow of unbalanced current in 3 phase system is offered between line and neutral. Any unbalanced current in three phase system can be divided into three sets of components likewise positive sequence, negative sequence and zero sequence components. The zero sequence current is actually co-phasial current in three lines. If value of co-phasial current in each line is Io, then total current flows through the neutral of secondary side of transformer is In = 3.Io. This current cannot be balanced by primary current as the zero sequence current cannot flow through the isolated neutral star connected primary. Hence the said current in the secondary side set up a magnetic flux in the core. As we discussed earlier in this chapter, low reluctance path is available for the zero sequence flux in a bank of single phase units and in the 5 limb core consequently; the impedance offered to the zero sequence current is very high. The delta connected tertiary winding of transformer permits the circulation of zero sequence current in it. This circulating current in this delta winding balances the zero sequence component of unbalance load, hence prevents unnecessary development of unbalance zero sequence flux in the transformer core. In few words it can be said that, placement of tertiary winding in star-star-neutral transformer considerably reduces the zero sequence impedance of transformer.
Rating of Tertiary Winding of Transformer
Rating of tertiary winding of transformer depends upon its use. If it has to supply additional load, its winding cross – section and design philosophy is decided as per load, and three phase dead short circuit on its terminal with power flow from both sides of HV and MV.
In case it is to be provided for stabilizing purpose only, its cross-section and design has to be decided from thermal and mechanical consideration for the short duration fault currents during various fault conditions single line to ground fault being the most onerous.