[RE-wrenches] AC Coupled (Again)

Fri May 29 00:18:06 PDT 2009

Jeff,

I’m assuming that you mean that you are putting a 4kW Sunny Boy out at the array, not a Sunny Island as you wrote.

The easy, albeit expensive, fix to the 120-240V problem is to use two Sunny Islands, instead of an XW. You could also use two OutBack VFX3648’s.

If you are going to use the Xantrex XW6048, you don’t also need to use the Sunny Island. It’s an “either-or” not a “both”.

You can use the XW in league with a Sunny Boy (or other grid-intertie inverter) as an “AC Coupled” system, just like the Sunny Island/Sunny Boy combo. The only difference (other than that the Sunny Island is only 120VAC) is that the XW will not communicate with the Sunny Boy so you will have to place a power relay in the AC line to the Sunny Boy, and control that relay with the AUX output of the XW. When the grid is down, and the batteries are full, the AUX output of the XW cuts the AC power to the Sunny Boy, shutting it down, and preventing an overcharge on the battery bank. Set the AUX relay to operate at the battery’s bulk voltage setting.

Put the Sunny Boy out at the array, and run it’s 240VAC output into the sub-panel that is fed by the XW (with that power relay inline). When the grid is up, the Sunny Boy’s power production is fed into the sub, then through the XW’s transfer switch, and then into the main panel, and out to the grid. This gives you the full efficiency of the Sunny Boy, without any losses from the XW and battery. No transformers needed.

The generator is a bit more problematic. You will need a second power relay in the Sunny Boy’s AC line that is only held closed when the grid is up, but is open when the generator is providing the power. Power that relay from the grid only to accomplish this. You will need a relay with a 120VAC or 240VAC coil. Connect the Sunny Boy to the NO contacts on the relay to act as a fail-safe – if something were to happen to the AC connection to the relay, the relay will fail open.

Using a single Sunny Island instead of the XW is not a great idea. First you will need a transfer switch between the main panel and the Sunny Island, that switches between the grid and the generator. Then you will need two transformers - one to step the Sunny Island up to 240VAC to go into the sub-panel, and another to step the grid/generator output of the transfer switch down to 120VAC to go into the Sunny Island. In this case you will have to run a com line out to the Sunny Boy, but you won’t need the overcharge relay. Be aware, however, that this means that the PV’s power production has to always run through both transformers, with the attendant transformer losses. It also means that the connection to the grid is at 120VAC, so bigger wire and a larger single-pole breaker is needed. You could eliminate one of the transformer’s losses by installing it between the generator and the transfer switch, to step the generator down to 120V, but then you are really getting out there in left field.

BTW – Xantrex has a White Paper out explaining how to use the XW with Xantrex’s GT inverters in an AC Coupled system. The XW’s AUX output can shut down GT’s without a separate relay. You might want to consider this combo. Contact me off-list as the White Paper doesn’t appear to be on the Xantrex website.

Cheers!

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 Jeff Yago
Sent: Thursday, May 28, 2009 6:55 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] AC Coupled (Again)

Although we have completed tons of grid-tie systems, and off-grid battery systems, this will be our first AC coupled system and there are still a few things we need to resolve.  We will be using a 4 kW Sunny Island on a ground-mounted array about 150 feet from home.  We are considering using a Xantrex XW6048 since we have both a grid connection and a backup generator and thought this would be easier since everything will be 120/240 VAC except Sunny Island.  We will use a 120/240 Outback transformer to match the Sunny Island to the 240 volt hardware.

Here are the issues that we are having a problem:

1.  Is this inverter match up the best since we have the 240 VAC grid, 240 VAC generator, and a 240 VAC well pump on the back up panel? My concern with the Sunny Island is it may require added issues with trying to connect to two sources of AC power.

2.  I know the Sunny Island "communicates" with the Sunny Boy by altering ithe grid frequency it sees, but I am concerned this type of control may not shut down the SunnyBoy when it sees power from the generator and will still need a isolation relay.  Is this a valid concern?

3.  Looks like we will be in the 35 amp @ 120 VAC from the Sunny Island back-feeding the backup power panel.  Should we have the "relay" to dis-connect the SunnyBoy when the grid is down and the battery is full, or to avoid backfeed into the generator when the grid is down, but what "relay"?  Is there a specific model or size most of you are using, or do we need a contactor instead of a "relay" ?  What relay?

4.  Do we need to run any control wiring out to the ground mounted array and remote SunnyBoy to "signal" when we are dis-connecting it from the system during the above conditions?  I have installed some inverter brands that were factory programmed to shut down when they exceed so many of the same type outage condition "errors"  thinking there was some kind of system problem.

Thanks for the help,

Jeff Yago

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