why Generator transformer HV winding is STAR connected.

So you have a zero-sequence source to the transmission system. It's very difficult to detect ground faults if you don't have enough zero-sequence sources.

any other reason?

Donald,
I don't think the transmission system cares about detecting generator winding earth faults quite as much as the GenCo. TSOs can always insist on a delta/star step-up transformer.

1. For protection
2. because for High voltages the no. of turns required and also the insulation cost is less..

I believe that the most important reason for the star-connection of the HV side of a generator transformer is that when for non-symmetrical faults (1-phase and 2-phase short-circuits), the generator will not be involved in the "zero" sequence (current in the neutral/ground circuit).

An additional reason, albeit less important I think, is that a ground star-connection will ground any harmonics reaching the delta-connection of the LV side.

From manufacturer and practical points, it is a reasonable build up for the generator transformer. Beside the protection requirement mentioned above, STAR/DELTA is the most popular design for the generator step up transformer as from generator side (LV side), it needs high current and low voltage (good for generator output and its insulation requirement) and also good for the transformer size (phase current is divided by root 3); from high viltage side, it focuses on the high voltage and low current to reduce the losses on the transmission line, so the STAR build up can get the higher line-line voltage with lower phase-ground voltage to reduce the insulation requirement inside the transformer and further reduce the body size.

Bcz in Star Connection we use four wire three phase and 1 neutral wire when one phase is off then neutral wire is used as a third phase... For further details visit blog robohelp.webs

In star connection Phase voltage and line voltage are not equal where as phase current and line current are equal.when three phase current are same.at that time load on generator is symmetric whereas any change in phase current that result a fault current generate and that fault current is used for generator protection.that why generator is star connected for protection purposes 4 identification of fault current.