[RE-wrenches] Outback AC coupling Sunpower?

Wed Jul 31 12:51:53 PDT 2013

Hi,
That is correct...the XW does have a frequency shift (Batt V=Bulk) that 
causes grid-tie inverter to disconnect. Remember, the 
total kW of the grid-tie unit cannot exceed the rating of a single XW, 
even in a multi-unit system.
Eric
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From:
Ryan <ryan at kb1uas.com>
To:
re-wrenches at lists.re-wrenches.org
Date:
07/31/2013 11:16 AM
Subject:
Re: [RE-wrenches] Outback AC coupling Sunpower?
Sent by:
re-wrenches-bounces at lists.re-wrenches.org

Ray
You did not mention the XW, it frequency dithers so it is a pretty 
straight forward setup. It also comes in 4, 4.5 and 6KW

Ryan

On 7/31/2013 2:04 PM, Ray Walters wrote:
Hi Kelly;

I agree the Radian system is appealing.  Its basically uses a circuit 
breaker that can be controlled like a relay as well, and just disconnects 
the GT inverter.
I'm going to offer a brief table of what I've found for my choices:

Brand
Control Method
Pros 
Cons
SMA Sunny Island
Frequency shift to disconnect GT inverter
simplest wiring, Trusted GT industry player
most expensive, Freq shift can cause other problems
Outback Radian
Remote controlled Circuit breaker disconnects GT inverter
fairly simple wiring,  relay and breaker are combined, better control of 
battery charging
still expensive, not available in smaller than 8 kw model
Magnum
Diversion load controls for battery regulation  (soon to offer AC load 
control)
lower cost, proven technology, freq shift control offered as fail safe
most complicated wiring,  diversion loads not always reliable or available
Modsine Backup system
no AC coupling, just an old fashioned backup system
low cost
complicated, needs customer interaction, not as efficient, GT PV not used, 
dedicated PV modules only charge backup system
Generator
No AC coupling, generator runs backup loads
lowest cost,  backup power available for as long as outage (no batteries)
best for week long outages that only occur every few years
Not Renewable, fuel storage issues, doesn't switch fast enough to maintain 
computers, etc.

I'm still pricing out the options, but the SI and Radian seem a bit 
overkill for smaller systems.  Backup inverter power of just a couple 
thousand watts is all that most people need.  The Magnum is appealing, but 
I have quite a bit of experience with load diversion controls from  wind 
and Hydro systems, and I really feel that its overly complicated for the 
less than 1% of the time that the grid is out.  Keeping the battery from 
being damaged is what's important, but maintaining full 3 stage charging 
for a couple of days a year is not.  The batteries can be properly charged 
once the grid returns.  May be able to use Outback's ROCB with the Magnum? 
 That might get the best of both worlds.
If I was building the entire system to start, I'd just use a single 
Outback GVFX with its own PV array, and add GT inverters and array that 
were independent for the rest of the GT requirements.  This project 
however is retrofitting an existing GT system. 
I've done many backup systems in the days before Grid Tie, and they work 
fairly well.  Basically either the modules are not used much of the time, 
or the customer has a few circuits that are on the backup system (off 
grid) all the time.   They operate it just like an off grid system, the 
only difference is that when its time to run a generator, they can just 
switch to the grid.  These are not for your average customer, but can work 
well for someone that likes to save money and tinker.
Another consideration is that if the battery bank is large enough relative 
to the array, over charging becomes less of an issue; especially with 
flooded cells that can stand a bit of over charging ("equalization") every 
now and then.   However, I can see that the average small sealed battery 
bank hooked up to several KWs of PV could get ruined in a day.
I'll close with a quote from Joe Swartz in his 2012 Solar pro magazine 
article : 

"AC coupling is still the Wild West of renewable energy applications"

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer, 
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 7/30/2013 11:37 PM, Kelly Keilwitz, Whidbey Sun & Wind wrote:
Ray, 
The battery-based inverter in an AC-coupled system should not be 
grid-interactive. The grid passes through the BB inverter to the 
grid-direct inverter(s). The BB inverter is there to create a stable 
grid-like signal for the grid-direct inverters when the grid is not 
present, and to manage the batteries. Thus, the pass-through AC capability 
of the BB inverters normally (there is a complicated work-around) must be 
able to handle all of the PV AC output, when loads are low.
You probably are aware, but to make it clear in this thread: the system 
needs to have some method to protect the batteries from overcharging, as a 
result of the grid-direct inverter output when the grid is down and loads 
can't use all the solar energy available. With the Sunny Islands it's done 
by a signal that reduces output from the Sunny Boys without disconnecting 
them. With Outback and others it's done with a installer-designed relay 
that disconnects the grid-direct inverter when the battery voltage gets to 
a set point. We haven't done an AC-coupled system with the Radian, yet. 
Sounds like the remote controlled breaker is Outback's approved and listed 
way to do it. About time.

-Kelly 

Kelly Keilwitz, P.E. 
Principal
Whidbey Sun & Wind
Renewable Energy Systems
NABCEP PV Installation Professional
WA Electrical Administrator
kelly at whidbeysunwind.com
PH & FAX: 360.678.7131

On Jul 30, 2013, at 4:23 PM, Ray Walters <ray at solarray.com> wrote:

The Outback rep got me some good info, and I'm liking the Radian with the 
Remote controlled breaker to control the GT inverter when the grid is out. 
 I've heard of quite a few problems related to the Sunny Island frequency 
controlled system.
My question is can off grid inverters like the Magnum be AC coupled?  I'm 
trying to get the price down, and still handle the 240 vac input from the 
GT inverter.
I penciled out the Radian and I was topping $10k before installation. This 
customer is hoping for a solution under $10k, and closer to $5k if 
possible.  I think an elegant lower cost solution for Grid tie with 
battery back up is in order.  A Radian Lite?
For backup, it could even be mod sine, as it wouldn't be much worse than 
generator power or the average UPS setup.

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760

On 7/30/2013 2:59 PM, Ray Walters wrote:
Hi All;

I know AC coupled systems have been covered before, but I have a new 
issue:  positive grounding.  A customer with an existing 8kw Sunpower 
system wants to add backup.   He was put off by the high cost the Sunny 
Island, so I was steering him towards a single Outback VFX coupled to just 
one of his 3 inverters.  The issue is how will the VFX work with positive 
grounded inverters.  I'm assuming that since the two inverters will only 
be connected by AC, that the positive vs negative grounding of their 
separate DC systems will not matter.  Any experience with this particular 
scenario: Sunpower system AC coupled to an Outback?
Also, would I even need a GVFX, since the inverter would only be used 
during outages  (which might allow a generator to be used later as well) A 
VFX should still AC couple to the GT inverter during an outage, right?

Thanks as always,

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