Detection of Ground Fault on Ungrounded Systems.

If the Generator (11KV ?) is star connected output, then you have a Neutral point that can be earthed and provide a grounding (?).

In general, generator is Y connected to reduce voltage stress in windings. Therefore, genenator star piont should be grounded to provide earth fault protection between the zone generator to Busbar. You didnot mention the generator rating vis-a-vis faultlevel. Theoratically, as proposed by you, during occurance of earth fault, there will be a dip in voltage and that will be reflected in secondary winding of PT. If you connect voltage sensitive relay, the same may be used to trip the 11Kv switchgear;but you must consider the fault withstand capacity of the PT. The size of PT will be large and uneconomical.

The purpose of NGR ( Netral Grounding Resister) or NGT ( Neutral Grounding Transformer) is to restrict earth fault current to the minimum but above the normal leakage current so as to protect the generator winding and core from damages that may result due to arc energy if allowed to flow earth fault current without restrictions. Even in this restricted condition , to detect the earth fault we install some sophisticated protection schemes. If we avoid earthing and operate as un-grounded, all the misfortunes and losses that are listed for an ungrounded system will accur. Therefore NGR or or NGT is must depending on capacity of Generating source and detection of the E/F follows this necessity.