How to determine the earth fault protection?

Low Voltage (400V-440V) Earthing
This discussion applies to the low voltage side of a (6.5/.400 or 11/.400 or 33/.400 kV transformers). Since your neutral is solidly grounded, the earth fault level will be sufficiently high, to trip the over current (50/51) elements. You will not need any earth fault protection. If you are running long low (really long) voltage cables you may need to provide earth fault protection.

We never design low voltage systems where the earth fault is low and requires special earth-fault protection (unless it's an hazardous area). The best approach in a green field design is to re-locate your distribution transformers so that the length of low voltage can be reduced and earth fault level is sufficiently high.

However if I was forced to put in an earth fault relay (suppose if you have one remote customer), here's what I do:
* the minimum prospective earth fault current at the remote end.

* this is greater than 2 times your over current pick up (51), great don't put any earth fault element.

* it is less than (51), simply use 51N, and set it a 0.5 times the minimum prospective earth fault level.

The factor of 2 or 0.5 is assuming you have a 100% error in your calculations. If your calculations are correct, you can reduce this to say 1.2 and 0.8.

This approach is not exhaustive by any means; there are plenty of other approaches you can take. This is only one of the several approaches that you can take.

From experience, adding earth fault to low voltage systems inevitably causes spurious tripping (lots of it). This is mainly due to all single phase loads connected downstream. A 3-phase short-circuit or earth fault at your consumer end, must not trip your transformer low voltage circuit breaker (or fuse) under any circumstances. The customer circuit breaker must discriminate over the entire fault level domain for reliability of the system. So as a rule of thumb, I always avoid adding earth fault protection to low voltage feeders.

Medium Voltage Earthing
The medium voltage side of you distribution transformers (6.5/.400 or 11/.400 or 33/.400 kV transformers) will be grounded at the sub-transmission substations where you will have typically have larger transformers which convert sub-transmission voltage (132 or 66 or 33 kV or whatever) to distribution voltages (3.3 or 6.6 or 11 or 33kV).

I haven't come across distribution network (3.3 or 6.6 or 11 or 33kV) where the distribution voltage side neutral is solidly grounded (In the short time I've been in this industry). I suspect the earth fault level will be simply too high and pose significant hazard to public.