[RE-wrenches] Head Spinning..(Con) fused.

William Miller william at millersolar.com
Sun Nov 14 21:35:50 PST 2010


Eric:

Thanks for the research.  I confess that I commented on an inverter system 
I knew little about.  My thanks to Darryl for keeping me honest, and for 
the kind words.  I had assumed that all modern inverters are certified to 
prevent back feed, but this needs to be checked for each new inverter.

I'm not sure the language below is the same as an independent verification 
that back feed will not occur.  You may need to ask your AHJ what they will 
accept.

The limit on allowable back current is the fuse sizing specified by the 
module manufacturer.  This relates to how much fault current the internal 
module wiring can handle before it fails (and possibly catches fire).

Humbly,

William Miller





At 07:42 PM 11/14/2010, you wrote:
>Thanks William and Daryl,
>
>Here is Auroras backfeed stipulation from the installation manual:
>
>"Installation and Operator’s Manual Page 98 of 108
>
>(PVI-3.0/3.6/4.2-OUTD-x-US Rev: 2.1)
>
>8.5 Input Source Backfeed Current
>
>PVI-4.2(3.6, 3.0)-OUTD-x-US Grid Tied Inverters are provided with two 
>separate and
>consecutive power stages:
>
>• Booster Stage (DC-DC converter) connected to DC Input Terminals.
>• Inverter Stage (DC-AC converter) connected to AC Output Grid Terminals.
>
>The Booster Stage is provided with forward diodes that allows the current 
>(power)
>flow only from DC Input terminals towards Inverter (Output) Stage.
>In case of any fault on Inverter Stage, these diodes avoid any back-feed 
>current
>phenomena towards input terminals.
>
>In case of fault of the forward diode, the corresponding booster MOSfet goes
>immediately and permanently in short circuit state and it avoids any current
>propagation form output to input terminals.
>
>Abnormal Fault tests conducted during UL1741 qualification show also that 
>these type
>of faults produce the opening of internal grid disconnect relays and cause 
>the external
>AC CB protection devices to trip, preventing any power flow from the grid.
>
>In conclusion for PVI-4.2(3.6, 3.0)-OUTD-x-US-y models it is not possible 
>to have
>any single fault responsible of Input source back-feed current flow. For 
>these models
>the Back-feed current into DC Source is negligible."
>
>According to this, the inverter will not backfeed even under most fault 
>conditions. My biggest concern is the four independent strings shorting 
>together somehow. I guess technically 4 strings of 8A for a total of 32A 
>doesn't exceed the ampacity of 10AWG XHHW-2....until you derate it for 
>roof temps.
>
>So whats the consensus?
>
>--
>Eric Thomas
>Solar Epiphany LLC
>(206) 919-3014
><http://www.solarepiphany.com>www.solarepiphany.com
>
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Please note new e-mail address and domain:

William Miller
Miller Solar
Voice :805-438-5600
email: william at millersolar.com
http://millersolar.com
License No. C-10-773985
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