Unbalanced Current in VFD Inverter Side Causing Earth Trip on One Pump Only—Why?

in one of our stations im facing problem with VFD where we have 5 VFD's  used to operate  5 pumps in one of them we used to have earth trip , after investigation we found that there is unbalace in the inverter side knowing that feeder side the power quality is stable  so there is some thing between transf. and thyristor . now we bypass the earth protection knowing that is protected by converter side earth protection and the system earth protection . what is the solution the question why it happens in thes vfd only while others didnt have this problem , when we ask the manufacturers support they attend and suggest the same . what are the conconsequences of bypassing in the inverter side . 

I think this problem is usual for most electrician installing VFDs. Here is a reference that discusses some of the possible reasons and the solutions they did: http://www.impomag.com/article/2016/12/solving-vfd-current-imbalance

thanks dear but still want to know why is trips with earth protection and since we are using it in pumping station so the load is same for all 5 pumps and all of them operated by vfd