How to Correctly Size an Earthing Reactor

I just joined a company involved in a power project made up of 8 gas turbines (25MW each) with 3 earthing reactors to its 3 MV bus bars. The MV bus is rated as follows; 11KV, 5000A(rated current), 31.5KA(short cct currrent). What other factors(data) are needed to size the earthing reactor correctly and how do I go about it? I'm not the key person in the firm but I would like to make useful contribution as we prepare the tecnical specification. Note that we are not connecting any other NER/NGR with the GSU transformers. I'll be glad to have useful tips or references from you guys.