Three Winding Transformer 2

[jasl]

I would like to have some information about usage of three winding power transformers. At my work I come across a 140 MVA, 220/34.5kV, 50 Hz . The primary is star connected with its neutral solidly ground. The secondary too is star with neutral earthed through resistor.The tertiary winding is delta connected. Let me add a note that there are huge VSDs in the system and I feel that this has to do with the harmonics. Could the experts throw some light on this subject.
What could be the possible reasons for selecting such a three winding transformer?

 

[cuky2000]

This is a collection from personal experience and different sources

Three winding transformer is choose for several reasons. Few of those reasons are:
a- Limit the short circuit on the secondary side by splitting the transformer in two parts. Keep in mind the coupling between the secondary winding have large impedance.
b- Cost. One three winding unit are cheaper than two transformer of same capacity.
c-
IEEE transformer committee says:

“Delta-connected (buried) tertiary windings are generally buried either under the
transformer cover, or two terminals are brought-out for closing the delta corner and
making external ground connections, when used in Y-Y or auto-connected transformers
for any of the following reasons:

a- To protect the transformer and system from excessive third harmonic potentials.
b- To prevent telephone influence due to third- harmonic currents in the lines and ground
c- To stabilize the neutral of the fundamental frequency voltages.
These types of tertiary winding arrangements are generally specified as " Used for
Harmonic suppression or used as Stabilizing Winding”.

See the enclose info for additional reference from Westinghouse T&D Handbook