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

[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 !

 

[VJB]

I see two problems:
1. The dc link capacitance is too low
2. You seem to have a non-isolated ac or the two converters have different ground paths.

 

[ozmongoose]

Hello

I don' think the problem is the low DC link capacitance. The power modul has a built-in C in DC link of 235 uF. With two power modul, we have 470 uF. Then I added up a thoudsand uF more. I think it is quite enought.

The whole system has the same Earth signal. Power supply is taken from only 1 source.