GSU Transformer - Specifying (generator side) voltage rating 1

[getshorty]

Have two 55 MW, 13.8 kV, hydro, synchronous generators and want to interconnect them to the utilities 115 kV transmission system. What should be the GSU transformer's lowside voltage rating? Some engineers say 13.2 kV, other's say 13.8 kV (the generators terminal voltage).

 

[sykimk]

According to IEEE C57.12.00, art.4.1.6, Secondary voltage of transformer shall not exceed 105% of rated voltage under full load condition, and 110% of rated voltage under no load condition.

In conjunction with these requirement, you shoud check the secondary voltage of GSU under various operating regions (leading and lagging) of generator connected to GSU as well as minimum and maximum operating voltage.

I think no one can say which voltage rating (e.g. 13.2kV or 13.8kV) is proper for your system because the selection of GSU parameters requires careful considerations to various operating conditions of generator and power grid, etc.

I recommend you to read IEEE C57.116 "IEEE Guide for Transformers Directly Connected to Generators"
I believe this guide will give you a solution.