DC Based vs AC Coupling

Hi All,

First Post and could use some thoughts.

I am in the planning process of implementing a 5KW Array with Battery Backup.

In general the grid connection in my area is good however, I want to have a solid battery option in the event that grid becomes less stable/available over time. It is important to me that I be able to run my system as a off grid system if that becomes necessary in the future. In the interim, I am happy to use the grid and run primarily a grid inter-tie system for a long as that is available.

I have been lurking on these forums for a few weeks and have read a lot about the Xantrex XW system. People on this forum seem quite happy that option.
I have heard some scuttlebutt that Xantrex has been purchased and may be focusing more on commercial solar and less on residential.

Alternatively, I have been reading about AC Coupling using SMA GT Inv. and Sunny Island, Santrex XW with Grid Tie and/or Magnum 4448pae with a GT Inverter.

At the moment, I am leaning towards AC Coupling. I have in mind using Enphase Inverters and the Magnum as a battery based inverter. The Magnum apparently has the ability to shut off the GT inverters when in an off grid situation and the battery bank is full. Less Elegant than the Sunny Island, but is that good enough?

In terms of the Enphase, I like the fact that each panel is a stand alone system without a single point of failure, and with individual monitoring/control.

In any event, I would appreciate some thoughts Pro/Con of running DC vs AC coupling and my design thoughts in general.

Thanks

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: DC Based vs AC Coupling

    First, the standard warnings...

    AC Coupling / Back Feeding an Off-Grid Sine Wave inverter with a Grid Tie / Utility Interactive Inverter is not a Listed/Approved function for any currently shipping Off-Grid Inverter (that I am aware of). And many Off-Grid inverters are not even Listed/Approved by UL/ETL/NRTL's/Etc. for home electrical use.

    So, any installation that you make in your home will be your responsibility to ensure that the installation is safe. If there are any "issues" -- Your insurance company may fight their liability arising from your claim.

    Next Question--Do you have any information on the Magnum 4448pae controller that shows it will charge a battery bank when AC Coupled (back fed) from a GT Inverter?

    Do you have plans for a backup charge limiting system in case your primary charge controller (Magnum shut-down fails)?

    How are you looking at the Magnum to control battery charging? To shut itself down and causing the GT inverter to shut down too? If yes--then you will lose your AC power from the Off-Grid Inverter during this period.

    If you are pretty confident in your technical skills--then it sounds like a fun project. If you would us others--perhaps you should not attempt this.

    Since the XW Hybrid System + XW DC MPPT Solar Charge controllers do the same thing with NRTL Listed Devices--It should have much less issues than the AC Coupled version.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jturbo68jturbo68 Registered Users Posts: 4
    Re: DC Based vs AC Coupling

    Bill,

    Thanks for the reply. My experience to date is in the DIY realm, and I am wanting to get to speed to do this project as well. I have reasonable experience with AC wiring from building and wiring several houses myself. Mentally, AC coupling makes sense to me as my system is likely to be attached to a grid 99% of the time. My personal skepticism about the state of the world leads me to want to be able to go off grid for self reliance reasons should that be needed and I want to have that option in place as this system goes in.

    It sounds like I am heading into a bit of a leading edge/bleeding edge area here in terms of looking at AC Coupling. That is ok for the moment as I am enjoying the figuring out part of this. That said, the XW is a known quantity compared to this.

    Magnum is developing some AC Coupling Applications and I am waiting for some information about doing this. Dont have it yet... well see if that materializes.

    My Understanding is that during a grid outage, the Magnum will disconnect from the Grid and continue to let the GT inverter develop power to supply house loads and charge the batteries. When Battery charge is full, the inverter will vary the island grid to keep the GT inverter off line until more charge is needed while supplying power to the island from the battery bank.


    My assumption is/was that the Magnum would keep from overcharging the batteries and grid tie inverter shutdown. If this is a bad assumption, then this design is not workable unless there is another device or design that act as a fail-safe. I understand that providing too much power to the island/batteries would be a big issue.

    Given that this type of configuration is less normal, I am guessing that this type of configuration would be hard to get through my counties/utilities permitting process?

    Is The Sunny Island the only 'certified system' for doing an AC Coupling type of arrangement?

    Thanks.

    Is anyone out there using AC Coupling with SI or others?
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: DC Based vs AC Coupling

    Sounds interesting... If the inverter takes the grid frequency outside the 60 +/- 0.5 Hz based on battery bank state of charge--The GT AC coupled inverters should drop out--Will be a neat option if the off-grid inverter does this.

    Placing the GT Inverters on the "protected side" of a transfer switch (output side of off-grid inverter) will be all that is required for a change in wiring (vs the normal GT connection straight to the house breaker panel).

    This type of battery charging (on/off) is very similar to operating a dump load--Not the optimum method of charging a battery bank. Probably will "cycle" the battery more than a XW type system with traditional MPPT DC battery charger. But better than not having access to all of those solar panels and no way to use them with the traditional GT systems.

    -Bill

    PS: The Sunny Island is the only off-grid AC power sharing network that has been designed for this purpose that I am aware of--But I am no expert in the field.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boBboB Solar Expert Posts: 995 ✭✭✭✭
    Re: DC Based vs AC Coupling
    jturbo68 wrote: »
    Bill,


    It sounds like I am heading into a bit of a leading edge/bleeding edge area here in terms of looking at AC Coupling. That is ok for the moment as I am enjoying the figuring out part of this. That said, the XW is a known quantity compared to this.

    Magnum is developing some AC Coupling Applications and I am waiting for some information about doing this. Dont have it yet... well see if that materializes.

    My Understanding is that during a grid outage, the Magnum will disconnect from the Grid and continue to let the GT inverter develop power to supply house loads and charge the batteries. When Battery charge is full, the inverter will vary the island grid to keep the GT inverter off line until more charge is needed while supplying power to the island from the battery bank.


    My assumption is/was that the Magnum would keep from overcharging the batteries and grid tie inverter shutdown. If this is a bad assumption, then this design is not workable unless there is another device or design that act as a fail-safe. I understand that providing too much power to the island/batteries would be a big issue.

    Given that this type of configuration is less normal, I am guessing that this type of configuration would be hard to get through my counties/utilities permitting process?

    Is The Sunny Island the only 'certified system' for doing an AC Coupling type of arrangement?

    Thanks.

    Is anyone out there using AC Coupling with SI or others?


    If Magnum is working on AC coupling improvements, then I haven't seen or heard anything publicly talked about for this. Last I heard, (1 yr ago) they hadn't tried this themselves but someone else in the field had tried it and it worked OK (except of course for the part where you have to dump extra energy when the batteries are full to keep the GT inverter from turning off.) That was not with the PAE version, but ~should~ work the same as non P units. Tony or Gary at Magnum may be the ones to talk with about this ? i.e. knowledge was very limited on this subject with the Magnums.

    People have been doing this with the OutBack FX inverters for years, but AFAIK, The Sunny Island is the only system that is made to work with their GT Sunny Boy and maybe Windy Boy inverters for this arrangement.

    I'd be interested to hear about any Magnum recent improvements with AC coupling.

    boB
  • jturbo68jturbo68 Registered Users Posts: 4
    Re: DC Based vs AC Coupling

    Right now, I am dealing with second hand information re the Magnum. I will see if I can get something from the horses mouth so to speak.

    Thx.
  • balee123balee123 Solar Expert Posts: 86 ✭✭
    Re: DC Based vs AC Coupling

    Here's a document, apparently from Magnum Energy

    http://www.wholesalesolar.com/pdf.folder/inverter%20pdf%20folder/MagnumACcoupling.pdf

    Note 3 seems especially appropo to what is discussed in this thread. The whole article is very interesting.

    Note 1: The Magnum inverters referred to in this paper are
    listed to UL1741 as standalone inverters, not as utilityinteractive
    inverters. Therefore, as an inverter, the output of
    the inverter should not be connected directly to the utility
    power distribution circuits. These inverters can parallel (or
    AC couple) to the AC wiring circuits only when the utility
    power is connected to the input of the inverter and the
    inverter is in Standby mode (charging with AC passing thru).
    Note 2: The maximum power rating of the renewable
    energy source must be no greater than 90% of the
    continuous power rating of the Magnum inverter.
    Note 3: The MS-PAE Series ( ≥ Rev 4.1) inverter/chargers
    have the AC Coupled Support mode. This mode, when
    activated, causes the inverter output frequency to change
    to 60.6Hz. This mode is enabled using an optional remote
    that allows the Battery Type to be set to “Custom”, and
    activates when the battery voltage increases 2-volts (24-
    volt units) or 4-volts (for 48v units) above the Absorb
    Voltage setting. The frequency returns to 60.0Hz when the
    battery voltage falls 2-volts (24-volt units) or 4-volts (for
    48v units) below the Absorb Voltage setting.
    Note 4: The battery bank voltage can be managed by installing a
    diversion controller and load capable of absorbing the majority of
    the expected surplus energy, using: 1. DC diversion controller
    and DC resistance loads; 2. AC diversion using AC resistance
    loads driven by DC controlled relays; or 3. Magnum’s AC Load
    Diversion Controller. Now in development, Magnum’s AC Load
    Diversion Controller will allow customers the ability to maximize
    the use of onsite generated renewable power and provide
    multiple-stage battery charging with temperature compensation
    using AC loads. For more information on the release of this AC
    Load Diversion Controller, continue to monitor our website at
    www.magnumenergy.com.
  • solarvicsolarvic Solar Expert Posts: 1,060 ✭✭✭✭
    Re: DC Based vs AC Coupling

    I called magnum about 3 months ago and they sent me the same copy but I couldn, reattach it to put on here. I plan on doing this with the pre PAE model. :Dsolarvic:D
  • balee123balee123 Solar Expert Posts: 86 ✭✭
    Re: DC Based vs AC Coupling

    Solarvic,

    Is there any reason you decided to go with a Magnum set up instead of XW inverter/charger set up? If so, can you walk us through your rationale?
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: DC Based vs AC Coupling

    The sunny island + sunny boy is the only system that can take the batteries through an absorb charging phase without micro-cycling the battery. Here's a paper describing the problem with using the XW or Magnum methods of ac coupling: http://currentgeneration.co.nz/site/current/files/Partial%20AC-coupling%20in%20Minigrids.pdf

    During the absorb phase the charger is meant to keep the battery at a specific voltage for a length of time. So what happens with the XW, magnum method is:
    - Battery hit's absorb voltage
    - inverter increases frequency
    - GTI disconnects
    - battery voltage drops, while the GTI's internal timer runs- how long this is, is dependent on the type of GTI (not sure about US, but in UK this is about 5 minutes). During this period all the loads are being driven from the battery.
    - at the same time inverter resets the frequency and the GTI can reconnect as soon as its timer runs out
    - GTI reconnects and starts charging again
    - Repeats the above many many times throughout the absorb process.

    The same thing happens after absorb when the battery goes to float.

    If you do want to use Magnum, XW or outback (or any other bi-directional inverter/charger) then it seems like disabling the frequency control and using a diversion load controller across the battery bank will give you a much better absorb charging phase.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: DC Based vs AC Coupling

    How about this idea:
    Attachment not found.

    If you are going to use enphase for the GT inverters you can split up their outputs into seperate circuits and have them switched by seperate voltage controlled relays.

    This is like the charge control circuits in the Ananda (or APT) Power Centers, circa 1990ish. They typically had three mercury relays which were driven by seperate voltage controlled circuit boards. Each was set to a different voltage so when the battery voltage was low, all sections of the array were connected to the batteries. As the voltage rose the first would turn off at the lowest set point. The second would be set for the highest voltage but if the available current was low enough it might just keep the voltage somewhat elevated instead of turning off all together.

    If you were using the XW you could use the built in AUX to drive one relay for a "bulk" PV circuit and let the frequency shift handle the "Finish Array".

    Don't forget that you also have high battery voltage disconnect settings on both Magnums and XWs as a failsafe incase things get out of hand.

    Alex Aragon
  • solarvicsolarvic Solar Expert Posts: 1,060 ✭✭✭✭
    Re: DC Based vs AC Coupling

    I bought 2 xantrex inverters to grid tie with . Before I had the money to finish they were not allowed to be gridtied. So When I got enough money to buy the GTI interfaces they are no longer available. So I supose you would say I have a grudge against xantrex since I cant use them for GTI. Yes 2 unused inverters. Everytime I tryed to sell them here someone always tells negative things about them.so they are just setting in the closet. I bought gridtie fronius inverters and like them. When I decided to build a backup offgrid inverter system I remembered how xantrex abandoned me. The engineers at Magnum were very helpful and actually talk to you on the phone. I like Magnum because it has a 240 vac output and is cheaper, takes up less room and I havn,t heard of any problems with them. It can furnish the surge to start the 3/4 hp 240 ac pump even when The washer and dryer and geospring water heater was all on at the same time.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: DC Based vs AC Coupling
    How about this idea:
    Attachment not found.

    If you are going to use enphase for the GT inverters you can split up their outputs into seperate circuits and have them switched by seperate voltage controlled relays.

    I would look very carefully at the inverter warranty fine print before doing anything like this.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: DC Based vs AC Coupling
    ggunn wrote: »
    I would look very carefully at the inverter warranty fine print before doing anything like this.

    What? Do you mean AC coupling? Aside from backfeeding the PV power through the AC load panel and dissabling the enphase inverters by frequency shifting everything else is operating as designed.;)

    Yea, I would definately check out the AC coupling / warranty issues.
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: DC Based vs AC Coupling

    Here is a link to a thread in the enphase energy community (forum) which talks a lot about AC coupling.
    http://community.enphaseenergy.com/enphase_energy_community/topics/grid_tied_battery_backup?utm_medium=widget&utm_source=widget_enphase_energy_community&from_gsfn=true
  • SolaRevolutionSolaRevolution Solar Expert Posts: 407 ✭✭✭✭✭✭
    Re: DC Based vs AC Coupling
    Don't forget that you also have high battery voltage disconnect settings on both Magnums and XWs as a failsafe incase things get out of hand.
    Just to clarify my concern for the need for a "failsafe"

    NEC 690.72(B)(1)
    In effect: A PV system with a diversion charge controller as the sole means of battery charge regulation shall be equiped with a second independant means to prevent overcharging.

    I was thinking about the High Voltage Cut Out setting (HVCO or HVD) as a secondary protection against over charging the batteries through an AC coupled system. The wording of 690.72(B)(1) does not directly address AC coupling but I believe the intent is to protect against things going wrong with the connection to the diversion load. I was carrying this over to the regulation of the AC coupled non-battery inverters.

    Alex
  • mike95490mike95490 Solar Expert Posts: 9,358 ✭✭✭✭✭
    Re: DC Based vs AC Coupling
    A PV system with a diversion charge controller as the sole means

    Diversion charge controllers are seldom used for PV, PWM or MPPT is much more common, and does not need the failsafe
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: DC Based vs AC Coupling
    mike90045 wrote: »
    Diversion charge controllers are seldom used for PV, PWM or MPPT is much more common, and does not need the failsafe

    This is a case where the controller is connected to the battery that is being recharged in an unregulated fashion by backfeeding the OG inverter from the GT unit(s). Under such circumstances you have to be able to divert the unregulated charge current to another load or else the batteries fry. Hence the reg for back-up controller - and my reluctance over the whole idea.
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