[RE-wrenches] SI/SB MicroGrid - Recovery From Low Battery Shut Off

Sun Jun 7 21:17:10 PDT 2015

Hi William,

These same concerns apply to any AC coupled inverter system.  The reason
that it doesn't apply to DC coupled charge controllers is that they are in
direct electrical contact with the battery bank, if a battery bank is there
and within the acceptable battery voltage range,  they should boot up and
start charging the batteries when the sun is around.  In an AC coupled
system, the battery based inverter is responsible for "tricking" the solar
inverter into thinking that there is a grid there.  If the battery based
inverter shuts down on low battery (or on any other fault) there is no
"grid" to export solar power and so the grid-tied inverter could stay off.
This can cause a circular problem, no solar charging because there is no
micro-grid, no micro-grid because the batteries are low etc etc).

In a DC coupled system, the battery based inverter can shut down the loads
while the charge controllers are bringing the battery state of charge up.

The real issue is that in an AC coupled system, your charging system and
your loads are on the same AC bus.  There is no way to isolate solar input
bus from the load bus.  This is why its a good idea to have load-shedding
contactors in AC coupled systems.  This gives the system control over some
of the heavy loads at least.

On Sun, Jun 7, 2015 at 9:08 PM, William Dorsett <wmdorsett at sbcglobal.net>
wrote:

> Do these same concerns apply to XW? There is a minimum battery (I think
> 100Ah) required.
>
>
>
> Bill Dorsett
>
>
>
>
>
> *From:* RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On
> Behalf Of *Drake
> *Sent:* Sunday, June 07, 2015 11:16 AM
> *To:* RE-wrenches
> *Subject:* Re: [RE-wrenches] SI/SB MicroGrid - Recovery From Low Battery
> Shut Off
>
>
>
> That is what we did for our home system. The main impetus was to keep the
> batteries charged without having to charge from the grid using the on-board
> battery chargers. Grid charging uses a lot of utility power.
>
> This system has stacked Outback inverters. We keep the backup inverters
> turned off at the Mate, but with the instrumentation running. If we did
> have a low battery situation, the voltage would rise enough to start the
> system back up from the 80 W of DC coupled PV.
>
> Another way to recover from low battery shutdown would be to program the
> shut down parameters below battery voltage when the sun came back out.
> Also, it is likely that if the system were completely turned off at the DC
> breaker, the battery voltage would rise enough to start the inverter(s)
> when solar resources improved.
>
> At 05:51 PM 6/6/2015, you wrote:
>
> Years ago, when I was researching SB/SI microgrid solutions for a Ted
> Turner high-end off grid rich man's lodge, John Berdner (at that time the
> head of SMA-America) had made me the same recommendation: that a small
> amount of PV be wired as a DC charge-controller-based input. This was a
> final fail-safe approach, in case the array was insufficient and the gennie
> didn't start. It would bring the batteries to a minimal voltage level, such
> that the SB/SI system would return to operation once the batteries
> recovered.
> Allan
>
> *Allan Sindelar*
> allan at sindelarsolar.com
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder (Retired), Positive Energy, Inc.
> *505 780-2738 <505%20780-2738> cell*
>
>
> On 6/6/2015 8:23 AM, Mac Lewis wrote:
>
> Hi Mark,
>
> The Sunny Island has various battery protection modes and will shutdown
> based on user programmable state of charge levels. Level 2 battery
> protection will start the Sunny Island every 2 hours in an attempt to get
> charge into the batteries.  You should set this level up at times when
> energy input is likely.  See page 113-114 here:
> http://files.sma.de/dl/15216/SI4548-6048-US-BE-en-20.pdf  If it doesn't
> get any charging to the batteries for 10 minutes, it will shut down and
> wait again.
>
> This is an advantage of having at least some DC coupled solar input.
>
>
>
> On Sat, Jun 6, 2015 at 7:39 AM, Mark Frye <markf at berkeleysolar.com >
> wrote:
>
> Hi Guys,
>
> Just wondering about how a SI/SB microgrid works. If all the PV is
> connected to SB inverters, what happens if the loads manage to drain the
> batteries to their low voltage shutdown? Wouldn't the SB stop providing the
> AC sync so then the SB wouldn't have anything to sync to even if they had
> power available from the PV?
>
> How is this handled? It seems to me you would always want to have some
> amount of PV DC coupled.
>
> Thanks,
>
> Mark Frye
>
> Berkeley Solar Electric Systems
>
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>
>
> Mac Lewis
>
> "Yo solo sé que no sé nada." -Sócrates
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-- 

Mac Lewis

*"Yo solo sé que no sé nada." -Sócrates*
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