Rotor Earth Fault in Synchronous Motor During Regenerative Braking Mode

Timing or synchronous phasing perhaps. Are you sure you have the right circuit?

The possibility of winding over-voltage and conductor insulation breakdown because of it during regenerative breaking needs to be investigated.

When you checked everything out before running the motor was there an earth fault then? You did check everything out first, yes?

Is the "Grid" AC or DC?

If AC, I really don't understand how this could work, using the extremely limited information that you have supplied!!!

For the best help from CR4, you always need to give FULL information!! Err on the side of too much, rather than too little, is a great tip!!

In my opinion, the excitation[rotor winding] short-circuited behaves as an induction motor rotor windings.

At start from s=1[rpm=0] frequency 50 Hz and the induced voltage [emf] is approximate E*s=E. At close to synchronous speed s=~0 and emf=0.

In braking operation case the slip is more than 1.For -2000 rpm s=133.33%.Then the induced voltage will be 1.33 times the induced start voltage. In this case a weak insulation could be entirely damaged.

There are many details missing before any diagnosis is possible; things like the rating of the machine, synchronous or induction, AC or DC, how it is brought up to speed, is there a flywheel, if synchronous how is the field controlled, is it randomly reconnected to the grid, why is the speed falling as it reconnected to the grid, how does it pump power to the grid if its speed is below synchronous speed, etc., etc. If it's DC then some of these questions are irrelevant.