Transformer Connection

The <...we...can...> do anything it likes however the question must be asked as to why the <...we...> would want to do this and where is the financial justification for so doing (rhetorical question - NNTR)? The best individual to ask is a Senior Electrical Engineer at the <...power plant...> in question. After all, no-one at an anonymous global internet Engineering forum can possibly see the installation other than this individual and the Original Poster.

Why wasn't this question asked of (a) similar individual(s) when <...visited another power plant...>?

Does the telephone service not extend to those locations?

In the second case you have the neutral immediatly available wich is a plus! and since they have the same power I dont see problems coming (but that depends on the connections that exist at the site.

...so the question to be asked is: Why, if it wasn't originally installed, does the <...power plant...> now need a neutral on the <...6.9 KV...>?

Like i told before, that depends on the installation that exists and what is to be fed...

..to which question the Original Poster needs to return.

No, you should not and cannot swap transformers. More information needs to be known by a qualified individual to say if this can be done. Since you do not know what information is missing, you appear to not be qualified.

The short answer is yes (if you really have to), but not without careful analysis first.

Each has their own advantages and disadvantages, both in protection schemes and cost (individual transformer and overall system cost) which will be different for each.

You cannot just change from one to the other without taking into account the overall system requirements and protection scheme along with all the relevant and local electrical codes and standards the site must comply with.

Anyway, changing to a Star/Star transformer is going to result in (at the very least)additional costs as you have to deal with the new Star point.

I suppose the 6.6kV switchboard has a tie feeder to 6.6kV Unit auxiliary switchboard, acting as back-up supply to the normal incomer from Unit Auxiliary Transformer.

The decision on Yd or Yy connection for the general service transformer is based on vector matching at the 6.6kV auxiliary switchboard when the normal and back-up supplies are momentarily paralleled for changeover.

All the above and more is part of sizing of general service transformer during Detailed Design phase of the power plant.

I think you got the answer.

Literally this would depend on the kind of task transformer need to be used.In so many applications,low current tapping is highly valued to avoid ripple of inrush current-- star/delta is prevailing wisdom to employ.If inrush is not an issue to you and many appliance has good protection system and can not tripped with current magnitude at any application star/star is widely convinced to take.

But please refer to you electrical expert as to why this would not interfere with available system or may it be more exigent to use this method at your plant now.They may probably have another issue connected to appliances and grid networks.