How to know the reason to trip the switch gear of protection of MV Transfomer ?

There are many reasons may be 

- more overload

- damage or collapse isolation of load cable

- voltage drop

- damage "burning" one or all relays of the circuit breaker 

- If you not adjust the cutting AMP

- If you not adjust the allowance time of load increase 

- bad connection between the terminal of the circuit breaker from inside and outside  

I think you want to know how can you know the reason of the trip not what's the reason so if you to know how..... You can know by the measurement device built in the MV cabenit where any error happened will be recorded in the screen with error led. Example of errors (I°>I) (I°>>I) and you can know what's I° mean by the catalogue of the MV board.but if you want to know what's the reason so Mr Mohamed kamal answered perfectely

There can be many possibilities .

1)During overvoltage

2)During unbalanced loads

3)During zero sequence current circulation

3)Overheating of the winding

4)During  3 phase Faults