How to eliminate harmonics on DC link in 4 quadrad inverter ?

[ozmongoose]

Hi all friends !
I got a problem with my experiment. A DFIM is controlled from rotor side by a bach to back VSC. DC link voltage is at 700V. Capaccitor on DC link: 1470uF, Reactor at AC input of Power Unit : 4.8 mH. The whole control system is programmed by myself with dSpace.
Problems: If only the PWM three phase bosst rectifier works, having 700V on DC link, this voltage is very stable, with low harmonics. When I start the second inverter to control the DFIM, then the DC link voltage starts oscilating, with FFT analysis, I can see the harmonics 2nd, 5th 7th up to 2Khz 4 Khz.. Sampling time for control system is 250uF and PWM at 8Khz.
Can any one tell me how to design a filter for Dc link to cut off the high frequency harmonics??
I have heart that we can use two small epsilon Capacitor (10uF-450V) and connect each pole of DC link to Earth. I have tried it and having trip due to the protection. It seems to be something like a shortcircuited between N(Neutral) and PE(Protective Earth). I change the connections to N, and having an Overcurrent fault when ENALBE the retifier.

Thank you very much for your helps

Mongoose !

 

[wrong100]

ozmongoose

I will bite what is a dfim and vsc.

If I understand, I think you have a frequency response
that is not in tune with your input source resistance
and fc. This can cause
many, even if "stable" problems, (for emi, etc).

Suggest retune for longer time, or buffer, but the buffer
gets bad, 10,000uf at .3r that should seperate the loop from the input.

Good luck I hope this helped.