Stator Earth Fault Detection Scheme

[Dipeshrestha]

For our 14.9 MW hydropower project, we have two 9150 kVA, 0.85 pf, 6.6 kV (well theoretically??)identical salient pole generators connected directly to a bus bar. The bus bar is then connected to a delta-star configuration of three single phase 6.6/33 kV step up transformers with 33 kV side star connected with neutral solidly grounded. As suggested in multiple threads, we have a single NGR with rating 10 A for 10 secs connected to one generator at a time controlled by interlocked motor operated switch. For Earth Fault protection of bus bar we have 64B Bus Earth Fault Relay connected to the open delta winding of the Bus PT.
Now for the stator earth fault protection they have proposed restricted earth fault protection (64REF) taking inputs from two CBCTs of rating 50/1 A located at generator feeder side and neutral side (Residual Current Measurement). They say this is technically and economically perfect for multiple generators connected to the same bus. From my research, I found that this type of protection scheme is rare these days and are only used for small size generators where differential protection is not possible. We also have differential relay (87G).
I had proposed 27TN/59GN( Third harmonic Neutral Under/Over Voltage Protection) for 100% Stator Earth Fault Protection. But they are saying that voltage protection functions are reliable for single Generator-Transformer Unit Scheme not for bus bar connected generators as they precisely cannot locate the E/F location.
Now I'm in a dilemma between these two schemes. Voltage protection scheme is the one that i have seen in most of the SLDs of other projects with similar configuration (two generators connected to a bus bar). Will 64REF function be enough for the E/F protection for the size of our plant?
Could someone kindly clear my dilemma?? Thank you

 

[RRaghunath]

87G will be effective for Phase faults and cannot be of help with earth faults when the current is limited toa low value like 10A.
27TN/59GN also is ineffective in identifying which generator has fault.
Directional E/F protection with CBCT can be an option.
REF protection with low ratio CBCT on phase side is the effective approach I believe. High Impedance REF is preferable for its sensitivity and stability without compromising on speed.