[RE-wrenches] Convert Grid Tie SW4048 to Backup System; Add New Inverter, AC Coupled

Tue Nov 22 09:47:54 PST 2011

Allan,

Thanks for reprinting the discussion. The retired engineer and his SW4048 are still both alive and doing well. Aside from the Trace SW4048 grid-tie low dc-to-ac efficiency (~65%), it is still one of the finest inverters ever made. In case I outlast my 13 year old SW4048, what new inverter has the same rugged durability but higher grid-tie efficiency?

Joel Davidson
  ----- Original Message ----- 
  From: Allan Sindelar 
  To: re-wrenches at lists.re-wrenches.org 
  Sent: Tuesday, November 22, 2011 8:07 AM
  Subject: Re: [RE-wrenches] Convert Grid Tie SW4048 to Backup System; Add New Inverter, AC Coupled

  Nathan,
  I neither stated nor implied that the SW series could be AC-coupled, because I don't know, friend. I have never had reason to try this. The issue was discussed on this forum several years ago, and I have found two key posts from David Katz of AEE about this topic in my email Wrenches archives. David only addresses compatibility with early Sunny Boys. The SW is true modified sine waveform, so whether is would be capable of providing a reference signal to which a modern grid-tie inverter could latch would be subject to several variables, and would likely vary among inverter brands and models. Here's what David wrote on 4/15/2005, excerpted:

    I have connected Sunny Boy inverters directly to the output of Xantrex SW inverters and Outback inverters. It works well. The Sunny Boy operates when it sees the SW or Outback output.  Sometimes it shuts off when a large load, like a pump is applied to the circuit, because it looks like a grid failure. If the SB shuts off, it waits 5 minutes to turn on.  If there is no AC load on the system and the SB is supplying power, you will charge the batteries connected to the SW.  You need to put a voltage controlled switch to turn of the AC between the Sunny Boy and the sub panel if the battery voltage gets too high.

    The SW is bi-directional. if you put ac into the output, it charges the battery. The AC in terminal is connected directly to the output through a relay. The inverter is quite amazing. If you connect the output of the SB and the SW to the same sub panel and turn off all loads you will see it charge the battery. If you turn on loads to the point you are drawing more than the SB is supplying, it will begin to draw power from the battery and supply the excess that the SB is not supplying. I use an outback for backup power on my grid connected house this way. As long as the SW in turned on, there is a sine wave present at the AC output. This makes the SB think the grid is present. I hope this answers your question. If not let me know and I will try to draw a diagram. 
    By the way, if you connect the AC input of the SW or (FX) to a breaker in the main panel in a grid connected system it will become your connection from the main panel to the sub panel (120VAC) and the inverter will turn on whenever the grid fails and keep the SB running. If you are using a 240 V SB, you will need an autotransformer or a second SW. 
    David 

  You mention the GTI below, but have not indicated whether the SW you are considering is or isn't a GTI-equipped model. If it is 4.10/GTI-equipped, I'd suspect it's incompatible with AC-coupling. When one of these is not selling (such as at night), has no loads on it but is grid-connected, it is continually drifting between buy and sell by an amp or two. Note that I have only observed this on the SW's meter display, which is a pretty crude measurement. But I would expect that this would interfere with any GT inverter's latching function, given the mandatory 5-minute waiting period.

  This is all based on observing one system that we installed around 2002 for a well-known retired movie star. The system remains fully operational today. We had an SW5548 (for most loads) and an SW 4048 (for background loads with surge, like pumps) that were not series-stacked, in a GTWB system, small by today's standards at 4.4kW of PV. Both inverters had GTIs, but only one was set to sell, in order for the two SWs not to argue with each other and to minimize their inherent float inefficiency previously noted. 

  I'd guess your chances of AC-coupling would be better with a pre-GTI unit. Others here may have had more experience with this question. 

  Just to stir the pot some more, here is an email from Lloyd Gomm, then at Xantrex, about the GTI/listing issue from 11/17/2003:

    Joel Davidson Wrote: 
      November 15, 2003 one of our customers (maybe more) with a line-tie SW in Southern California Edison territory got a notice from Xantrex dated October 22, 2003 about a required firmware update. Anyone else get this notice? 
    Joel, this firmware update is necessary for SW inverters tested / listed by UL.  This batch of SW's were produced by Xantrex between Q4 2000 and Q4 2001 and, to be clear, were de-listed by UL in November 2001. 

    This firmware update is NOT required for ETL or CSA marked SW's tested to UL 1741 under the NRTL program.  Everything produced after Q4 2001 are CSA marked, GTI compatible SW's.  Everything produced before Q4 2000 was listed by ETL and is unaffected.  
    We have been working with SCE, SDG&E, and PG&E to minimize impact on home owners and dealers.  The firmware update is easy to accomplish and we are compensating dealers to do the work ( 2hrs per SW @ $65 per hour plus a $50 per site allowance).  The compensation is paid by check within 2 weeks of receipt of our form.  We have contacted all dealers who have customers with an inverter that requires the update. 

    Joel Davidson Wrote: 
      This particular customer is a retired aerospace engineer whose SW4048 was working fine, but still had to get a GTI and now has to get some firmware. 
    Joel, if this customer has a GTI he does not require the firmware update.  Please contact me so we can update his information.  This will allow us to close the loop with his utility. 

    If anyone requires any further information on this program please contact me off list. 
    Lloyd Gomm 
    Director, Marketing 
  Allan

  Allan Sindelar
  Allan at positiveenergysolar.com
  NABCEP Certified Photovoltaic Installer
  NABCEP Certified Technical Sales Professional
  New Mexico EE98J Journeyman Electrician
  Positive Energy, Inc.
  3201 Calle Marie
  Santa Fe, New Mexico 87507
  505 424-1112
  www.positiveenergysolar.com 

  On 11/22/2011 8:10 AM, Nathan Jones wrote: 
Bill,
I believe Allans post indirectly answered my question.  While he did not specifically state the SW could be used in an AC coupled arrangement, I believe it was implied. Since the listing issue, as I understand it, was due to failure to drop out of sell mode in a certain set of circumstances and never had anything to do with the behavior of the inverter in any of its "incoming AC only" settings as far as disconnecting there should not be any question as to its suitability for AC coupling there. The only question would be how they would react and manage backfed AC from a GTI in event of a grid failure. That question is unknown to me and has not been specifically answered yet. Any one have an answer to that?
Nathan Jones
Power Source Solar
Springfield, Mo
R
417-827-0738

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