Positive ground with batteries? [RE-wrenches]

[Christopher Freitas --- OutBack Power]

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Geoff -

All of the OutBack equipment can be used in a positively grounded system 
without "letting the smoke out" - but it is not specifically listed for 
doing this and there are significant NEC issues which can not be 
reconciled without us making a special "positive ground" version of the 
MX60.  So - yes it can (and has) been done with the OutBack products but 
it will not be NEC code compliant. 

The PV-GFP can be used with positive ground systems except the hook-up 
of it needs to be modified slightly.  It will work - but again its has 
not been listed for use on positive ground systems. 

The real issue I see is wiring up the system properly as a positive 
ground requires that the the circuit breakers be installed in the 
negative conductors and that the negative be color coded red while the 
positive is white...  which gets really confusing from my first hand 
experience.   Its also  very likely that miswiring  of the system would 
damage components as we did not include this when we did the failure 
mode analysis during the design. 

Because of all this I recommend doing systems with SunPower modules as a 
"microgrid" (also called a sunny island) configuration.  This allows the 
battery system to be wired up as normal with the negative grounded and 
the high voltage batteryless grid-tie system as positive ground. 

We have about 40 or so systems that I know of which are operating as 
microgrids which have been done with a variety of grid-tie inverters and 
sources of power (PV, wind, hydro, fuel cell...).  Several of these are 
quad inverter systems or larger.  The only issue has been how to 
regulate the battery voltage when the loads are small and the battery is 
full - on most we simply do DC diversion control - so the excess power 
is dumped.  This can be done with the AUX output control provided on 
each FX inverter or externally via C40 controllers (which many dealers 
have sitting around because they upgraded systems to MX60s). 

Another issue is how the system operates with the generator running or 
when the system is hit with large AC loads such as AC units or big 
pumps.  SMA offers a field installable software version which widens the 
operating windows (voltage and frequency) which solves this issue...  
but I don't think anyone else has this available.  If you are using an 
inverter/charger type generator or a DC output generator this is not 
probably going to be an issue. 

We do now offer an enclosed version of the X-240 which also has a 
cooling fan - called the PSX-240 - which is rated at 6000 watts and is 
ideal for use with the Sunny Island inverters  -   the OutBacks will be 
less expensive and will allow stacking of up to 10 inverters (36 kWAC) 
with a total transfer switch rating of 300 amps at 120/240 VAC for a 
total of 72 kWAC of transfer switch and pass through capacity. 

The idle power consumption of the Exeltech MX system I think is fairly 
high when you are talking about doing larger systems - we automatically 
reduce the  number of inverter running so the idle consumption of our 36 
kW inverter system is less than 60 watts DC.   The Exeltechs also do not 
offer battery charging capability either... 

>Darryl - This is not a residence - so no GFCI... but it is a grid tie...

>Chris F. hasn't chimed in yet, but I think that a positive ground on a 
>GVFX will let the smoke out (you weren't talking about an ungrounded 
>system were you?)

>I'd consider an Exeltech MX system if the OutBack can't go to positive 
>ground.

Christopher

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com  
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030  

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