about having STAR/STAR 110/11kv transformer for substation which is going to provide uninterrupted power supply to a SWRO plant at Chennai. What's the advantages and disadvantages of having STAR/STAR 110/11kv transformer .

Transformer magnetizing currents are not purely sinusoidal, even if the exciting voltages are sinusoidal. The magnetizing currents have significant quantities of odd-harmonic components. If three identical transformers are connected to each phase and are excited by 60 Hz voltages of equal magnitude, the 60 Hz fundamental components of the exciting currents cancel out each other at the neutral. This is because the 60 Hz fundamental currents of A, B, and C phase are 120° out of phase with one another and the vector sum of these currents is zero.
The third, ninth, fifteenth and other so-called zero-sequence harmonic currents are in phase with each other; therefore, these components do not cancel out each other at the neutral but add in phase with one another to produce a zero-sequence neutral current, provided there is a path for the neutral current to flow.
Due to the nonlinear shape of the B-H curve, odd-harmonic magnetizing currents are required to support sinusoidal induced voltages. If some of the magnetizing current harmonics are not present, then the induced voltages cannot be sinusoidal.

No disadvantage of what so ever,but today that is the industry pratice along with high resisatnce grounding of LV neutral.But when three phase bank is used or 5 limbed 3 phase core is used, stabilising delta winding shall be added.Earlier days for such an applicatioin delta/star was the norm.to make LG fault on the secondary as a LL fault on primary and also to avoid harmonics from secondary load entering in to grid.

Esmail is making an important point about Yy transformers. If you choose that vector group, it is advisable to consider a three-winding design with a delta tertiary winding to balance the 3rd harmonic excitation current caused by the core. The vector group would be something like YNd11yn (or YNd1yn) and the tertiary is typically buried inside the tank (no external bushings). The tertiary acts as a harmonic stabilizing winding.

If you choose to stay with the two-winding design to help lower the cost, it is essential that the neutral point be connected to a zero sequence return path (grounded return shield wires) to allow the zero-seq 3rd harmonic current to return to the source that energizes the transformer (probably the 110 kV system). An example would be YNyn. If the 3rd harmonic excitation is not permitted to return to the source by not grounding the neutral, the output voltage of the transformer (11 kV) will likely contain high amounts of 3rd harmonic voltage distortion.

The above observation is not applicable for 3 phase core type Transformer as the tank of
the transformer can be considered as closed delta winding which serves the purpose of Tertiary winding.Hence 2 winding transformer YNyn can be used provided it is core type.
In fact we have large no. of YNyn Transformers in our grid.

In this connection one important point i wish to add regarding protection. To prevent spurious
tripping of HV circuit breaker, the o/c & e/f relay at the HV side should be Directional, facing
your switchyard. Also you may provide Restricted E/F relay at the HV end to detect internal fault.

Only catch. In Chennai your 110 kv grid is unearthed with delta /star transformers. Better check with other industries how their experiencew ith Yy units.

Thank you Mr Ramchandran for the catch.I did not know. Why they are running the system as un-earthed?
Could you please enlighten us further?

The disadvantage of star/star transformer is -
secondary side earth fault is reflected on to primary side and can cause the 110kV OHL protections to maloperate, under certain conditions.
However, in your case, I expect the 11kV neutral will be resistance grounding limiting the earth fault currents to 300/400A. Thus, the above is unlikely to be an issue.