Parallel load circuits trips inverter input protection.
I have two 5kW inverters in parallel which have been working ok for a couple of months.
This week my installer wired up an EPS circuit but has now disconnected this at the inverter as it was constantly tripping the RCBOs for both inverters when power returned.
While on site, calls were made to Sunsynk who updated both inverters to the latest firmware and advised not to use the ATS 230V signal for the earth bond contactor but to use the dry contact on pins 5&6. This was done after changing to a N/C contactor but the problem persisted.
My installer has gone away and is trying to get answers from Sunsynk so I have today made my own checks.
I have found that if the master inverter Load only is connected then the EPS works without tripping anything.
As soon as the slave inverter Load is referenced to the master the problem returns. I checked this by removing all EPS wiring and connecting master and slave Load neutrals together.
This is the sequence of events:
1) Isolate the inverter AC input to simulate a grid outage. This immediately:
a) Isolates the “Grid” from the DC/AC inverter and “Load”.
b) Drives the contactor creating an earth neutral bond for the Load circuit
2) Restore AC input to the inverters:
a) Grid remains internally isolated
b) Using an oscilloscope, the phase difference between grid and load was observed to go from 80deg to 0 deg in approx. 20 seconds. Both inverters load and grid outputs end up in phase.
3) 75 seconds after power restore, the contactor drops breaking the earth neutral bond
4) 90 seconds after power restore, the inverter internal contactor is heard connecting the Grid which instantly trips both RCBOs for each inverter.
Why would connecting Load terminals between inverters cause this problem?
Also the 15 second delay between the earth bond being broken and the Grid reconnecting (and providing the TN-C-S bond) is concerning. There does need to be a delay but not 15 seconds. It almost sounds like someone entered 15 instead of 1.5 in the firmware. I can confirm the timing is identical for both the ATS 230V signal and the dry contact.
My supply to the Grid of the inverter is via an underground cable. My installer says this must be protected from earth leakage so he fitted protection both ends. One device at the supply end and two (one for each inverter) at the inverter end.
From my testing I have found that the supply end protection (feeding both inverters) does not nuisance trip and removing the 2 earth leakage devices the inverter end "fixes" the problem.
Still waiting for my supplier to come back to me as he needs to agree and sign this off.
.