[RE-wrenches] AC Coupled Re-visited

Tue Oct 20 13:39:49 PDT 2009

Kirpal,

You’ve got it summed up pretty well, although I would add that you need to
use RS485 cards and cabling between the SBs and the SI Master to get
regulated charging – the comm. allows the SI to send a handshake to the SB,
telling the SB to go into “off grid” mode and accept wider voltage and
frequency swings whenever the grid is not present.   Kent sums up this
control mechanism very succinctly below.

The biggest challenge when mixing brands is dealing with the 300 second
timeout after grid disturbance (what I call 5 minute PWM) make for less than
ideal system operation.  There needs to be a better way to regulate power
than kicking the string inverter offline.

Imagine, say, a 7kW array pumping energy into the home on a beautiful sunny
day.  The grid goes down, but the battery based inverter (BB) picks up the
slack and the grid-tied batteryless inverter (GT) stays online.  Home is
drawing less power than the array’s output (~6kW worth), and the batteries
are full.  Battery voltage hits the regulation voltage, and the BB inverter
(or the voltage controlled switch) activates a relay to open the circuit to
the GT inverter, effecting a blackout on that circuit.  Suddenly the BB
inverter and the battery bank is hit with the full 6kW of load.  The
batteries are a little undersized because that was the easiest portion of
the system to cut costs on, and they’re a little old and dried out from
years of sitting in an uncooled garage, so their voltage sags under the
load.  The voltage controlled switch senses the drop, and closes the relay
to the GT.  However, the GT has to stay offline for another 299 seconds and
the battery voltage continues to fall…..

I’ve been worried about just this scenario for some time, especially as
systems age.  Therefore, I’ve been suggesting to anyone interested in using
mixed brands of inverters (or those who don’t want to use RS485
communications with SB/SI combos) that they strongly consider installing a
diversion load and controller capable of absorbing at least the majority of
the expected surplus energy.  A blackout relay can be used as a secondary
control mechanism.  I think that this is going to provide the most reliable
operation, ultimately.

To answer the question about number of inverters, the SMA 5048U supports
parallel operation of up to 4 inverters on a single phase system, up to 4
inverters on a split-phase system, or three inverters on a 3-phase system --
all connected to one battery bank.  The PV can come through any number of SB
inverters, but they would all be connected to the same protected load panel
served by the SI.  This is one "cluster."  The rule of thumb John Berdner
passed along years ago was up to 2kW of PV for every 1kW of SI.  If you can
break the load into multiple clusters, the system can be any size -- you
just run multiple clusters side by side independently.  If the loads can't
be separated, then you need to look at using the Multi-cluster box, but
that's not yet Listed or even technically "available" in the US.  But if the
job is *really *that big, you're playing in a whole 'nother ballpark.
Definitely not the job to take on for your first battery based project.

Phil Undercuffler

Director, Battery-based and Off-grid

Distribution Sales Group

*Conergy*

*Our World Is Full of Energy*

1730 Camino Carlos Rey Suite 103

Santa Fe, NM  87507

p.undercuffler at conergy.us

Direct | 505.216.3841

Toll Free | 888.396.6611 x4841

Fax | 505.473.3830

www.conergy.us

Phil Undercuffler

On Tue, Oct 20, 2009 at 1:38 PM,
<re-wrenches-request at lists.re-wrenches.org>wrote:

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>   1. Re: AC Coupled Re-visited (Walt Ratterman)
>
>
> ---------- Forwarded message ----------
> From: "Walt Ratterman" <wratterman at sunenergypower.com>
> To: <glenn.burt at glbcc.com>, "'RE-wrenches'" <
> re-wrenches at lists.re-wrenches.org>
> Date: Tue, 20 Oct 2009 12:37:04 -0700
> Subject: Re: [RE-wrenches] AC Coupled Re-visited
>
> Glenn:
>
>
>
> This is correct on the battery bank separation.
>
>
>
> You don’t split out the battery banks until you reach a “cluster”.  For
> example, one three phase setup is a cluster.  If you have two, three-phase
> setups – you have two clusters – and two battery banks.
>
>
>
> And…I think if you check through the charging parameters, you will find
> that the ratio of 10 to 1 as you have below is very conservative for PVkw
> versus battery Ah.  We have gone with 10KW PV vs 1800 Ah with no problem.  I
> think maybe what they may mean is that the 10 to 1 ratio is for what you
> intend to CHARGE in the battery bank.  So – if you are planning on a 50% DOD
> every day,  then maybe the 10 to 1 would work well, for that 50%.
>
>
>
> Walt
>
>
>
> *From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
> re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Glenn Burt
> *Sent:* Tuesday, October 20, 2009 12:14 PM
> *To:* 'RE-wrenches'
> *Subject:* Re: [RE-wrenches] AC Coupled Re-visited
>
>
>
> SMA recently told me that the recommended Ah of the battery bank is related
> to the amount of PV and connected inverters.
>
> They recommended a 500Ah bank with a 5kW PV SB, and a 600Ah bank with a 6kW
> PV SB.
>
> 100Ah to 10,000Ah are the supported capacities.
>
>
>
> I don’t believe you need to have a separate battery bank with two SI’s. I
> have been told they can share one. One SI is programmed as the master & of
> course they are data commed together so they can talk.
>
>
>
> -Glenn
>
>
>
> *From:* re-wrenches-bounces at lists.re-wrenches.org [mailto:
> re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of *Mark Frye
> *Sent:* Tuesday, October 20, 2009 2:42 PM
> *To:* 'RE-wrenches'
> *Subject:* Re: [RE-wrenches] AC Coupled Re-visited
>
>
>
> 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
> www.berkeleysolar.com
>
>
>
>
>  ------------------------------
>
> *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<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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