[RE-wrenches] Frequency shift on AC coupled off grid resi system

Ray ray at solarray.com
Mon Nov 23 11:15:44 EST 2020 

My advice would be to use a DC charge controller, and forget the 
frequency control/ AC coupled goofiness all together.  This has been 
covered ad nauseam in the past on the list.  Its just not a good idea 
off grid.  I occasionally will design large systems with some AC coupled 
array, but only enough to match the daytime AC loads.  Newer AC coupled 
systems with better communication between the inverters work better, but 
if frequency shifting is your main control, its just never going to work 
very well.

Besides the issues you're having, you can add poor battery management 
and premature cell failures to the list, along with the unforgivable 
"Dark Start" when the batteries are discharged below the LBCO setting.

Ray Walters
Remote Solar
303 505-8760

On 11/23/20 8:57 AM, Mac Lewis wrote:
> Hi Chris,
>
> I have no direct experience trying to troubleshoot devices that can't 
> handle frequency shift, that seems hard to do. However, I might 
> utilize a relay approach to solve it.  You could use the relay 
> functions in the Sunny Island to start dropping strings and/or 
> inverters to keep the frequency shift from occurring in the first 
> place.  There is probably a more elegant way to do this, but it may 
> work.  I believe there is a way to keep frequency shift from happening 
> in one of the many setpoints on the Sunny Island.  I think this is a 
> serious disadvantage of frequency shift to control PV output because 
> so many devices are designed to work at a precise frequency.
>
> Good luck
>
> On Wed, Nov 18, 2020 at 8:09 AM Chris Anderson 
> <canderson at borregosolar.com <mailto:canderson at borregosolar.com>> wrote:
>
>     Wrenches community,
>
>     Hope folks are well out there. I am wondering if anyone has
>     experience addressing a frequency oscillation with a split phase
>     off grid Sunny Island system. I am working on an off grid home
>     project with 2 units that has just not worked right since
>     installation. The small number of 240 volt loads and most notably
>     the geothermal heat pump goes into a loud vibration as the
>     frequency shifts and does so for 20 seconds, then settles down
>     into normal operation. 45 seconds later, it does it again and just
>     repeats itself until the water in the storage tank is up to
>     temperature. The current draw doubles when it is vibrating and it
>     seems to drain the batteries faster than expected when left on.
>
>     Wondering if anyone out there has any thoughts or is experienced
>     off grid tech in New England interested in some troubleshooting
>     work to help resolve things.
>
>     Please email me directly if possible at the following address -
>     canderson at borregosolar.com <mailto:canderson at borregosolar.com>
>
>     Thanks and stay safe out there.
>     -- 
>
>     Best,
>
>     Borrego Solar <https://www.borregosolar.com/> 	
>
>     *Chris Anderson* | Senior Vice President
>
>     E: canderson at borregosolar.com <mailto:jpitera at borregosolar.com>
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> Mac Lewis
>
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> "Yo solo sé que no sé nada." -Sócrates
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