"our protectionsystem is ok." It doesn't sound like it to me.

Why were the fuses uprated? Is affecting just one transformer?

Could be something as simple as excessive inrush current or an incorrectly sized fuse (current or tripping curve or both.

Do you have lots of air conditioning that is set to turn on at the same time?

Hire a qualified engineer before someone guesses what the problem is, fails, and causes a calamity which hurts or kills a lot pf people.

You are saying that you have 4 x 2500 kVA transformers ==> 10 MVA which requires a 175A fuse to supply the 4 transformers.

If your demand is 3200 to 3600 kVA, is it from the 4 transformers or just from 2 units and the rest is supplying some other sections?

4000 kVA means 3200 kW and 2 trans will be 5000 kVA == 4000 kW (70A max)

To Blow the 150 A fuse, there must be other loads or a significant surge, OR there is a faulty connection somwhere. High Tension (33 KV) is delicate on junctions being properly done and professionally done. It is easy to make a slopy connection with bad insulation that can leak to ground.

If you know your load has not increased, De-energize the system and check the corona shields for failure at the terminations and inspect your surge arrestors for fault(s) if equipped.

If there is not any visible damage, make sure all loads and surge arrestors are disconnected then perform a meggar, hypot, or AC HV test of the system.

If no faults are identified, re-energize the system with all loads disconnected then allowing a significant amount of time between closures, energize each load one at a time until the fuse opens/blows.

Whichever load opens/blows the fuse then must be analyzed/tested to locate the fault.