We have 220 DC (UPS) distributions board supplying 220v/110v converters and 220/24v converters, only in the 110 dc board we get a frequent earth fault alarms in DCS (every 30 min not a permanent ) the problem that we can not localize which feeders are making the fault due to the big number (more than 100 solenoids valve are supplied ) and when we make the voltage monitoring all of them are indicating an unbalance value between + and earth - and earth , that mean during earth fault (milliseconds ) all the voltage in the boards are affected (logic: all the feeders connect to the same bus bars).
From the information we have 220 V DC (UPS) system supplying to one 220/110 & 220/24 V systems. And have 100 loads on 220/110 v and some on 220/24 V systems. The problem is getting earth fault in 220/110 v system for every 30 minutes. The reason could be one of the solenoid connected to the system getting over heated and causing earth fault. Checking the solenoids one by one and you can locate the culprit causing tripping. It may sound little tedious but you can get solution fast.
I'd borrow a design from electrical substations and float the DC system with no ground reference. Positive and negative DC are balanced between ground and a ground alarm is put onto the system so that if either leg is pulled to ground an alarm goes off and you can troubleshoot it without taking the entire system down. A good strategy for surviving system faults. Naval ships do the same thing with their power systems.
A transient ground sounds like either interference, or you have a solenoid being momentarily operated which has an internal fault to ground. Whenever you detect one of these transient faults the clue will be to review the operating log and identify which pieces of equipment operated or were called upon to operate during that time frame.