Transformer Impedence

Theoretically yes but practically it would be asking too much from a manufacturer to prvide something non-conventional, I would say rather stick to the transformer manufacturer's norms.

To reduce short circuit level try one of the following:

(a) Intertripping arrangement for feeder circuit breakers.

(b) Changing of overcurrent relays from Time grading to combined Current grading.

(c) Use of current limiting series reactors.

(d) Currently available CB's are of very high interrupting capacity consider minor alteration of your switchgear setup.

(e)Consider microprocessor based monitoring systems, some manufacturers such as ABB have commercialized these.

(f) change of BTD to cables etc.

Take into account that generators have high value of synchronous reactance, I mean don't get carried away by jargon, do your home work to assess if there is a real problem and if it is so what is the intensity of it.

When the problem is well defined chances are you will have simpler and more practical solutions in hand.

Asking a transformer Mfr to provide 12%z instead of 8%z is not so unconventional. The utility that employs me does it all the time in certain of its territories where stations are very close, load is concentrated, and fault currents are very high. They purchase power transformers from various vendors up to about 13%. It does work.

In my territory, your item (c) is also very common.

I think the OP's concern is not to have to replace other equipment that might be overdutied by the addition of this new unit. However, he would have to do an analysis of what each option would cost (replace a bunch of breakers, additional bus bracing, etc.)

A good short circuit & system study should show where the problem spots are, and then the alternatives could be evaluated.

Thanks for the comments.Can u Please let me know the manufacturer who has supplied the transformers with higher than normal %age impedance.

Waukesha Electric has done it for us. Also Pauwels and I believe ABB.