[RE-wrenches] Outback AC coupling Sunpower?

Ray Walters ray at solarray.com
Wed Jul 31 11:04:58 PDT 2013


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 <mailto:kelly at whidbeysunwind.com>
> PH & FAX: 360.678.7131
>
>
>
> On Jul 30, 2013, at 4:23 PM, Ray Walters <ray at solarray.com 
> <mailto: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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