[RE-wrenches] AC Coupled Re-visited

Tue Oct 20 11:41:43 PDT 2009

Thanks Kent. And so I understand more clearly, in the case of 2 SIs as you
describe, each SI has it's own battery bank, and the solar would be split
between the two.

Considering the charging capacity of the SI, what would you say is the
largest battery bank size for each SI to insure an effective "C" value?

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
 <http://www.berkeleysolar.com/> www.berkeleysolar.com  

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From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent
Osterberg
Sent: Tuesday, October 20, 2009 11:10 AM
To: glenn.burt at glbcc.com; RE-wrenches
Subject: Re: [RE-wrenches] AC Coupled Re-visited

In off-grid mode the Sunny Island uses frequency shift power control to
reduce the output of the Sunny Boy inverters and thereby regulate the
battery voltage.  At some frequency shift (+1, or 2, or 3 Hz, I'm not sure)
the output of the SB is reduced by 100%.  It's proportional so that 1/2 as
much frequency shift gives a 50% reduction in the SB output.  To keep clocks
accurate, the Sunny Island later shifts the frequency a negative amount, but
the SB inverters ignore that.  For a grid backup system, a RS-485 cable is
required so the Sunny Island can activate (or deactivate) the frequency
shift power control capability of the SB.

You need to stack two SI inverters to get 240-vac, but that will allow you
to have a 10-kW of grid backup power with up to 12-kW of solar.  If you need
more, you can stack four of SI inverters.

Kent Osterberg
Blue Mountain Solar, Inc.

Glenn Burt wrote: 

Our installations of Sunny Islands (SI) with Sunny Boys (SB) have always
used the recommended RS-485 communications between all units involved. This
with reprogramming the SB's to be able to switch to off-grid mode per the SI
instructions allows a more integrated system.

We have had problems with the OB PSX-240 and stepping up the 120 to 240 for
the crit load panel (where the SB's connect) when the site has slightly high
AC voltages. The SB pushes the existing ACV higher, then it goes out of UL
spec & disconnects. Also the SB is now sensitive to imbalance on L1 & L2
because of the neutral sensing - we had a customer where the SB was
disconnecting due to this as well.

I thought the freq shifting was to allow other non-SMA inverters to be
controlled when batteries were full?

Where is the SMA rep on this list?

-Glenn

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kirpal
Khalsa
Sent: Tuesday, October 20, 2009 12:05 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] AC Coupled Re-visited

Greetings......it is my understanding that the Sunny Island coupled with
Sunny Boys is able to taper charge in an AC coupled system as the 2
inverters are able to communicate with each other and the Sunny Island
alters the frequency input of the Sunny Boys and lowers the total output of
the Sunny Boys to match the needs of the battery bank....this is SMA's
method of not using their "charger"....they simply alter the amount of
available AC input into the AC to DC converter present in the Sunny
Island.....This logic is what encouraged me to select SMA's for an AC
coupled design rather than mixing brands of battery-less and battery based
inverters.....in a mixed brand scenario there is no communication other than
an on/off command so no regulation is available.....My understanding may be
flawed--please correct me if so....
Another way for mixed brands to AC couple and provide some charge "control"
would be to have an AC dumpload on the AC input side of the battery based
inverter to suck up some of the excess power from the GT inverters so not as
much power is available for battery charging.....This dumpload would be
voltage based and in a mixed brand system would add more relays to the
mix...in many cases there may not be that many auxillary outputs available
to connect relays to.....
I would like to see more GT inverter companies make compatible battery based
inverters......One idea is for micro inverters to be paired with the battery
based inverters and when less power was needed to facilitate a taper charge
one solar panel at a time could be switched off......effectively providing a
smaller available charge current to help with the taper charging.....

-- 
Sunny Regards,
Kirpal Khalsa
NABCEP Certified Solar PV Installer
Renewable Energy Systems
www.oregonsolarworks.com
541-218-0201 m
541-592-3958 o

On Tue, Oct 20, 2009 at 8:34 AM, Mick Abraham <mick at abrahamsolar.com> wrote:

Hi, All~

On an AC coupled system as Jeff describes, the "battery charge circuitry" on
the battery based inverter is not even participating. 

A straight pure sine inverter...with no charger function built in...would
also "charge" the battery if AC coupled to a SunnyBoy with no grid
available. The "charge" is just the inverter's way of dealing with back EMF.

I agree that better control over that "recharge" is an important area; I
hope somebody is working on that. It's true that the "wild card recharge"
only occurs if grid goes away but as Jeff mentions, it only takes a few
times of crummy end of charge management to ruin a nice set of sealed
batteries.

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675

On Tue, Oct 20, 2009 at 9:21 AM, Jeff Yago <jryago at netscape.com> wrote:

We have completed several totally different AC coupled systems using
different inverters, due to large ground mounted arrays that had to be
located a great distance from inverter-battery-generator-grid BAS, which are
working just fine even with the mis-match of inverter brands.  The SunnyBoy
seems to not care what its connected to or how, as it just keeps doing what
it does and if a relay cuts off its connection to the grid when the battery
voltage goes high then it just waits and re-connects when the grid is back
or the battery voltage drops.

What I am bothered by is the need to custom design a power relay circuit on
each project which takes lots of fine-tuning of setpoints to get everything
to work correctly.  If you have not done one the problem is simple - when
you backfeed the AC output from a remote grid-tie inverter "through" the AC
side of a battery based inverter, everything works great and the solar AC
just passes straight through the sub-panel, back throught the battery
inverter, back into the grid.  However, when the grid is down and the
battery-inverter is no longer receiving (or sending) power from the grid,
for some reason I cannot begin to understand, any AC being fed from the
solar inverter goes straight into battery charging with absolutely no limit
on charge rate or charge limit, and if you do not add a relay to dis-connect
or shut-down the solar inverter you can quickly destroy a bank of AGM
batteries if there are no major system loads as it just keeps charging and
charging.

I am not an electronics engineer, but if the battery is being charged by the
battery charger built into the inverter, I just do not see why the same
battery charger suddenly has no clue that the battery is being overcharged
when its now receiving AC power from a different source.  I think with
larger and larger arrays being installed as module costs fall, higher DC
array string voltges to reduce wire costs, and more people worried about
grid reliability, there would be a good market niche for an inverter that
can properly charge a battery bank regardless of which way the AC power
comes into the charger section.  Whats the problem?

Jeff Yago

DTI Solar

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