[RE-wrenches] Dual Fronius problems

[Allan Sindelar]

Jeff,
This is a theoretical opinion, not grounded in direct experience ;^) :
The well pump may indeed be causing a voltage dip due to its starting surge
- wire to AC pumps has been historically sized to permit up to a 35% voltage
drop at the pump at startup. Due to tight IEEE929 restrictions, that's well
outside the acceptable voltage range for the inverter. But that would be
likely to cause only a 5-minute line disturbance shutdown of the inverters,
per UL1741.

Undersized wire is more likely to cause a voltage rise, not drop. This would
be a separate issue. The wire resistance would cause the inverter's own
output voltage to rise to match the grid voltage plus the effects of wire
resistance, causing a voltage rise close to the additional voltage source,
the inverters themselves. Your observation that each works well by itself
supports this theory. Check the AC voltage at the inverters and at the mains
panel to see if this might be the case. Note also that some utilities raise
line voltage during times of peak load, such as summer daytime air
conditioning. Notice when the shutdowns occur, and if the line voltage is
higer at these times.

Allan at PosE

-----Original Message-----

I have installed many Fronius grid-tie systems having a single inverter, but
recently installed a dual 3K inverter system and are having all kinds of
problems with the inverters not connecting to grid due to out-of-range grid
voltage or cycle errors. If we shut down either inverter, the remaining
inverter usually works fine. The only thing we can identify is the
electrician may have under-sized the new feeder wiring as we had to run over
150 feet to the first breaker panel in a barn, which was another 150 or more
feet to the main panel and meter base. The sub-panel also serves a large
well pump that supplies several different buildings and this may be causing
some voltage sags. 

We are looking into up-sizing the feeders, but I am still wondering if there
are other issues or concerns when you have multiple inverters side-by-side
at the end of a long grid connection.

Thanks,

Jeff Yago