Thanks, jraef ,Skogsgurra
ABB suggested to place a Common mode choke in the VSD panel, they will provided technical clarification in next week.
We still dont know the exact reason...
There are 4 set 4.16KV VSD (ABB ACS1000) in my subtation.
Since the convter transfomer is oil type, it had been installed on the ground outside substation. The cable length between converter transformer and VSD is about 90m.
But we had been informed by the ABB that the maximum length should be...