in vibration frequency analysis of 5 MW ,996 RPM high amplitude of 100 HZ frequency observed,what may be the reason?.

Mehdi,
Based on a simple calculation, it looks like your torque ripple is the 6th multiple of your mechanical frequency.
This might be due to electromagnetic torque ripple generated from the interaction of the B-EMF and current harmonics.
I can try and help you with that, but first can you tell me what is the number of slots and poles in the machine.
Also is it a Wound Rotor Synchronous Machine or a different machine?

Dear Nir,
number of rotor bar are:93
it is wound rotor induction motor.
pole number is :6

Interesting.
In order to diagnose if the torque ripple is due to the electromagnetic field I would first measure the all 3 phase voltages and currents.
Specifically, please measure your phase currents and voltages and make sure that you have a balanced 3 phase set by making sure that all are 120 deg shifted from one another and that there magnitude is similar.
Another test you should do while looking at the 3 phase quantities, is make sure that there is no dc-bias in any of your voltages and/or currents.
Also, make sure that the waveforms are indeed pseudo-sinusoidal and don't contain substantial non-fundamental harmonics.
We can got from there.

Hello Mehdi (Nir).

100 Hz on a 50 Hz motor is core vibration due to magnetostriction predominantly. Any electromagnetic core will vibrate at twice supply frequency.
If the problem has only started recently - and the 100 Hz component was not there before (at least with any significance) - then check the motor stator core, frame, holding-down bolts etc.
Also - check where the 100 Hz vibration is worst: Is it on the bearing housings (where vibration measurements are normally made), or more on the frame of the motor - near the stator core?
I would guess that it is more on the frame near the stator - if it is this root cause.

I hope this helps.
Take care.
Robert.