If you have the necessary CTs, use a differential. Differential isn't particularly more expensive than just overcurrent but provides considerably better protection. No hard and fast requirements based on size, just a general consensus that bigger transformers have differential and smaller ones are less likely to have it. At one time we were using fuses on 16MVA transformers, now we use differential on any new substation transformer or any that we update the protection on.
There's no standard that I am aware of. The bigger decision is what to use as the primary protective device. If you want to use 87T, there must be something upstream to trip, not just fuses. If you're willing to spend the money to purchase a primary circuit breaker (or circuit switcher, etc), the additional cost of a differential relay above an overcurrent relay is not very much.
One consideration might be arc flash hazards. With high side fuse protection, you may have very high incident energy on the side between the transformer and the first low side protective device. Adding a high side breaker or circuit switcher and 87T relaying will reduce arc flash incident energy considerably.
For 87T protection; If your Xfmr has a primary CB, then all you need is a PR with 2 sets of current inputs and a set of secondary bushing CTs on the xfmr secondary. This can eliminate a secondary CB, and provide lower incident energy on the transformer secondary. I have done this on xfmrs less than 2MVA. Saves $$ as well.
Of course this assumes a non secondary selective scheme, and a fast (3-cycle) primary CB.
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
I think high voltage fuses tend to be limited by what you can find. What I've seen is up to 300 amps. Above that you need a breaker, or switcher, which at that point a differential is easy to install. Also arc-flash is an issue that a differential will help solve.
If you are buying a transformer, why not also buy it with CT's installed if you can.
Other thing to think about is what happens if a transformer develops a fault. Would you have it rewound, or replace it with a new one. If the answer is to scrap it, then it does not matter if it is slightly damaged, or a smoking mass. If the intent is to repair it, then the fastest protection (differential) is a given.
