Local Breaker for Large Transformer (20MVA)

Virtually none. The idea is to both protect the cable, and the transformer against a fault condition. Big transformers cost a lot and most faults occur on the cable side of things.

ARe we talking about the cable on the primary or the secondary side? The cable on the secondary side could be as long as you wish provided the primary protection and isolator operated for faults in the transformer or in the LV cable and the downstream LV distribution upstream of the next isolator.

Similarly, the cable on the primary could also be as long as you wish provided it is covered by a protectiv zone.

Now, 'voltage drop' in this long cable is another cat altogether.

I expect that the transformer is 33/ 0.44 KV step down one.

The basic requirement of Circuit Breakers in Electrical Distribution system is to cut off power supply before effecting any type of damage to the costly equipment and prevent fire and accidents .

The normal fault on cable is earth fault short circuit and the fault level depend on the current and voltage rating and length of the cable . The 20 MVA transformer have to be protected for over current earth fault and over temperature ,for this control cable have to be laid to the breaker from transformer.

Hence the maximum distance will depend on all these factors.