[RE-wrenches] Outback AC coupling Sunpower?

Wed Jul 31 13:16:49 PDT 2013

Eric,

Can you expand on your "total kW of the grid-tie unit cannot exceed the rating of a single XW, even in a multi-unit system" comment?

I assume that with multiple XW (or other battery-based) inverters, since the AC is all coupled together, there is no way to prevent a pass-through AC relay in one of the inverters from being potentially overloaded. Since no two relays will have exactly the same resistance, and since the power will always take the path of least resistance, it could cause the amperage of the least resistive relay to exceed its rating.

Is this correct, and is this what you are referring to?

If so, I assume that you could use multiple XW inverters, as long as they had separate GT inverters, with separate critical loads panels, installed. They would essentially be separate back-up systems, each running different loads, but all connected to the grid in the same Main panel.

Brian Teitelbaum
AEE Solar

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Eric.Bentsen at schneider-electric.com
Sent: Wednesday, July 31, 2013 12:52 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Outback AC coupling Sunpower?

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<mailto:ryan at kb1uas.com>>

To:

re-wrenches at lists.re-wrenches.org<mailto: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<mailto: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
[cid:image002.jpg at 01CE8DEF.74D6FDF0]

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<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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