The answer is no. A three-phase star- star or Yy0 transformer does not always demand tertiary delta winding. Let us explain the basic theory behind it.
Tertiary Delta Winding
Suppose, an asymmetrical fault occurs in the LV network of a transformer. Obviously, zero sequence currents start to flow though all three phase LV winding. Obviously, these currents produce zero-sequence flux in the core.
This zero-sequence flux induces current in each phase of the delta winding. In closed delta, we connect each phase in series. So, the induced currents start circulating in the delta winding. This circulating current produces an opposing flux. Therefore, it opposes the zero-sequence flux produced by the LV winding. As a result, the zero-sequence flux strongly reduces. Therefore, the zero-sequence flux does not increase freely as it would in the absence of the closed delta winding. Consequently, the effect of zero-sequence flux on the HV winding reduces.
Three Limb Core Type Transformer
Now, take an example of a core-type transformer. Here, we place low-voltage winding on the core. Then we place the HV winding above the LV winding. Both windings are star-connected and solidly grounded.
Now, suppose an asymmetrical fault occurs in the LV winding. As a result, zero-sequence current starts flowing through each limb of the LV windings. Therefore, their ampere-turns try to produce zero-sequence flux in the core. However, the flux does not have a normal low-reluctance return path through the three-limb core. Although the flux will try to find a return path outside the core. The flux may pass through the transformer tank and other structural parts. However, this path has a much higher reluctance than the normal magnetic path. Because the flux has to pass through the oil insulation and other non-magnetic regions.
This high reluctance, strongly restricts the zero-sequence flux. Therefore, the zero-sequence flux in the core is much smaller than the flux that would be produced if a low-reluctance return path were available. Therefore, the transformer core does not allow a significant zero-sequence flux to develop freely. This low zero-sequence flux induces low zero sequence current in the HV side.
Five Limb Core Type Transformer
Now consider a five-limb transformer. In a five-limb transformer, there are two additional outer limbs. These outer limbs provide a low reluctance magnetic return path. Therefore, during an asymmetrical LV fault, zero-sequence currents result zero-sequence flux more easily. So, the zero-sequence flux can link the windings more effectively. Consequently, the HV-side responses more significantly during an LV asymmetrical fault. This is where the delta tertiary winding becomes important.
Why a Tertiary Winding May Not Be Required in a Three-Limb Transformer
So, a star – star transformer does not essentially require a tertiary winding. In a three-limb core-type transformer, the magnetic circuit itself strongly restricts zero-sequence flux. Because it does not provide a low-reluctance internal return path for the zero sequence. Therefore, we design a three-limb star – star (YNyn0) transformer without a delta tertiary winding.
This is also consistent with the CEA transformer specification, which states that, in general, tertiary windings have been removed in three-limb transformers up to 200 MVA and 220 kV class, unless there is a special requirement such as loading the tertiary. CEA gives reliability and cost considerations as reasons for avoiding the tertiary where it is not specifically required.
Why Larger Transformers May Use Five-Limb Construction
Manufacturers generally prefer three-limb core construction. Because it can provide a relatively economical and compact magnetic construction. However, as the transformer rating and voltage class increase, transformer height increases. Particularly the manufacturer has to restrict the height for road and railway transportation.
Therefore, for large power transformers, manufacturers adopt five-limb core. Because in five limb construction the cross – section of the yoke becomes almost half. That is the reason for 315 MVA and above and for above 220KV the transformer demands a tertiary winding.