[RE-wrenches] AC coupled, grid-tied Sunny Island - contactor & bypass

[Kelly Keilwitz, Whidbey Sun & Wind]

Wrenches,
We are including 2 Sunny Island inverters into a previously battery- 
less grid tied design utilizing 20 kW of PV on Enphase micro inverters  
and 2 SMA Windy Boy 6000 inverters (10 kW XZERES 442 turbine). Our  
customer wishes to run his fuel generator as little as possible when  
the grid goes down. Nothing is in the power room yet, the house is  
under construction. Only the wind turbine is installed.

With a pass-through limit of 56A, the two Sunny Islands cannot conduct  
the combined RE production to the grid. There is not enough critical  
load demand to justify two more SI's, nor is there space for them. Our  
plan is to connect the micro-inverters (84A, peak) directly to the  
main service (grid) in normal mode, switching to the critical load  
panel (SI AC output) when the grid goes down. The Windy Boys (56A)  
would remain connected to the critical load panel.

I'm hoping that the frequency shifting control of the Windy Boys from  
the SI's will allow the PV micro inverters to connect and stay  
connected to the SI's while there is demand on the critical loads and  
the batteries are charging. When there is more PV output than load  
demand we expect that the SI frequency will rise to the point where  
the micros will go out of spec and disconnect, maybe in a nicely  
staggered pattern.

Questions:

1. Has anyone experience with this much grid-direct RE capacity and/or  
micro-inverters AC coupled to Sunny Islands? I'm a little scared by a  
previous post from Darryl wherein he found that the RE capacity from  
the GD inverters should be 1/2 that of the battery-based inverters in  
an AC coupled system. Please let me know if you are certain that this  
is not going to work!

2. Any experience with specifying a contactor/relay to switch the PV  
output? The contactor will be controlled by one of the SI relays.  
Switching from the main panel (grid) to the critical load panel will  
be during a grid outage, so doesn't need to be load-rated, but  
switching back after the grid comes back up could be load-break. Our  
current solution is to use two 80A contactors from Schneider  
(#LC1D80G7) with an intertie kit.

3. Any suggestions for an inverter bypass (service disconnect) for the  
two SI's? We're considering either a monster 2P2T 200A switch (100 A  
would do), or using a gen-transfer type panel for the critical loads.

Comments and informed opinions quite welcome.

Thanks,
-Kelly

Kelly Keilwitz, P.E.
Principal
Whidbey Sun & Wind
Renewable Energy Systems
kelly at whidbeysunwind.com
PH & FAX: 360.678.7131
NABCEP Certified PV Installer
WA Electrical Administrator