It's a grounded input system?
May you really measure phase voltages?
I think it's unground and choosed to measure line voltages.
As error is constant may be a common-mode voltage issue.
May use a battery bank connected to VFD DCbus.
By contactor or diode and separate charging.
Total battery voltage need to be above max nominal DCbus, so keep them "isolated from VFD" and supply energy to VFD at grid undervoltage only.
Inverter need to be power-on over night?
In the morning any solar inverter start at some solar panels voltage and limit output current itself, no need for other parts.
What kind of PM motor is? Interior or surface magnet?
What kind of rotor alignment method was set for open-loop?
Such high current difference indicate an incorrect VFD setup or may be some error/missing in firmware control routine in open-loop.
There are VFDs that allow to add power modules to increase output power, but use same DCbus and sure, all module receive same control signals.
Separate VFDs may work in parallel if was designed for that and use a dedicated communication link for synch, load sharing and response to...
As grid inverter it's an ac power device connected in parallel with grid, from control section may choose power flow direction.
In inverter control section (firmware and circuits) may implement active and reactive power control level and direction.
After control section make all calculation...
Most probably it's a grid-tie inverter with so called "reverse power limiting device" that send signal (RS485/Modbus) to inverter to reduce it's power to avoid reverse power when local consumption decrease and/or PV panel production increase.
Communication between device and inverter may be lost...
VFD synchro is related to speed synchonization of attached motors in a stable ratio (1:1 or other) but can't be made by DC bus.
Common DC bus is used mainly in multi-axis motors machines to obtain some energy recovery as at a time some motors may be in braking (regenerative mode) same in...
Needed start torque being very low may use RL series circuits to limit current and short them after.
A smaller VFD with synchronous transfer feature may be an option.
No diodes issue.
As Mark said, may be Middlebrook instability - when output impedance of input filter (or just power supply if no filter) is (or become at some frequency value/range) greater than input impedance of rectifier/converter.
Such oscillations after diode commutation are "normal" for...
