Some transformers are designed to work best with current flow only in one direction. When we talk to transformer manufactures sometimes this causes PV people to get all flustered. Which side is the primary and which the secondary when current could go either direction? It gets complicated.
One almost wants to drop the terms 'primary' and 'secondary' and instead specify:
1) which side has the voltage adjust taps which change core flux
2) which side gets energized first
3) which way does power flow
In a normal transformer, the 1) is the primary side, 2) is the primary side, and 3) power flows from primary to secondary.
However in my lab I have a transformer where the voltage adjust taps are on the secondary side (and don't change core flux; the core is just slightly oversized)
For solar use, 1) gets connected to the utility, 2) gets connected to the utility, but 3) is in the 'reverse' direction.
To the OP: what voltage are the inverters operating at and what voltage is the utility supplying? Is the utility supply balanced?
-Jon