[RE-wrenches] Inverter isolation issue

[Drake Chamberlin]

Hello Wrenches,

We are working on a system with three Sunny Boy inverters that feed 
into a system that is backed up by a generator.  The inverters are 
isolated from the generator by relays that open when the generator is 
in the warm up stage, before connecting to the electrical system.

What is happening is that the breakers, that connect the inverters to 
the grid, trip, .  What is apparently happening is that an inductive 
effect from the inverter's transformers is causing current rushs 
through the breakers when the relays open.  This only happens when 
the generator is in exercise mode, when the grid is still up and the 
inverters are operating.

My theory is that it is much like the ignition system of an 
engine.  (For the example I'm using a system that still uses 
points).  When the points open, the voltage rises, because current 
will try to continue flowing from an inductor. This raises the 
voltage to very high levels.  The resulting arc jumps the spark gap.

I think that with the inverters, the high voltage causes a current 
surge when the voltage reaches a potential where current will jump 
the gap in the contactor.

Has anyone experienced this?  Have you found a good solution?  We 
could retrofit relays on the DC side, but this would not be 
convenient.  If we use DC relays, is there a downside to 
that?   Could we use the AC contactor to connect the inverter output 
to a large capacitor to absorb the surge?  Do we need faster 
contactors with a bigger gap?  Would the voltages hurt the inverters?

Thanks in advance,

Drake Chamberlin
Athens Electric
OH License 44810
CO License 3773
NABCEP Certified PV Installer TM
Office - 740-448-7328
Mobile - 740-856-9648  
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