GT interactive question

Question for any or all of you:

I have a 9.9 kW system broken into three separate "systems" on three separate inverters. They backfeed a sub-panel, which back-feeds the house panel. The PV panels are 185w sharp and they go to A fronius inverter (3 strings of 8 = 24) a sunny boy (1 string of 11) and a Sharp SunVista (1 string of 19). Now, I am interested in installing an marine selector switch below the DC disconnect for the sunnyboy. When the grid works, i leave it (except to charge the batteries - see below).

So, when the grid goes down, i will disconnect from the grid and switch the sunnyboy to a sine wave inverter, which will have a charge controller and marine batteries. That, in-turn, will cause the panel to energize which will cause the two remaining inverters to begin operating. As they produce (over produce), the excess will go to a "dump", as an outlet.

I understand that there will be times when I will have to disconnect from the grid, to charge the batteries (for the long periods of time the grid doesn't go down)

Does this make sense to you?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: GT interactive question

    Welcome to the forum.

    I have moved this to its own thread because it is quite a separate situation from the other.

    To answer the question I have to say it does not make sense to me. There is a lack of detail in your description, but it sounds like a convoluted mess. You want to be able to switch some of the panels presently running a GT inverter to power a charge controller in the event of an outage, and then use that to recharge some batteries to power an off-grid inverter for emergency power?

    The biggest issue with this is: what is the array Voltage? Most GT inverters run very high array Voltage whereas charge controllers do not.

    The second issue is trying to use that inverter to energize the other GT inverters. This is called AC coupling, and is a giant nightmare on any system not actually designed to work that way. You would probably need to use a Xantrex XW for you back-up/sync inverter and they are not for the faint at heart.

    You will not be happy with the performance of those marine batteries.
  • yallerrab
    yallerrab Registered Users Posts: 4
    Re: GT interactive question

    Wow, thanks for creating a separate thread. Up until the point where you say "...that to recharge some batteries to power an off-grid inverter for emergency power?" you are right with me. What I want to do is have the "switched" panels (with the sine wave inverter) act as "the grid", which would then allow the grid tie inverters to operate. That would give me 5-6 kW/hr on a good day. The voltage coming from the panels themselves are in the hundreds (it's dark and I don't remember the voltage - but I remember it being high as well). I was a victim of the hurricane and, after 10 days of electrical roulette by the power company, I am trying to find a way to harness power from 43 panels. Again, thanks for the thread.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: GT interactive question

    In order to activate the GT inverters when there is no utility power present you need another stable Voltage/frequency source for them to sync to. This would be the off-grid inverter, and they only run from batteries. Usually in such a situation some panels are used to keep those batteries charged, hence the charge controller.

    So there's the first problem: GT array running typically around 300-400 Volts. Xantrex makes a very expensive (about $1,200) charge controller than can take this. Otherwise you have to reconfigure the array to a lower Voltage for a standard controller (still around $500 to $600). If you skip this step and just use batteries the right inverter can back-feed to the batteries and charge them while the GT inverters are running.

    Which brings up the second problem: the AC coupling. Normally GT inverters expect there is always a place for any and all power they can produce to go. If there aren't sufficient loads to take the power, it works back through the off-grid inverter and charges the battery. Once the battery is charged and the loads satisfied if there is still power from the GT inverters it has to go somewhere. What it usually does is continue to charge the batteries until they burn up. Not good.

    The right inverter can regulate this charging and "skew" its frequency when needed causing the GT inverters to drop out so that over-charging does not occur. The Xantrex XW is one that can do it, as well as the SMA Sunny Island (which is 120 VAC not 240 VAC). In fact SMA makes the best integrated system for this function, using the Sunny Island and Sunny Boy GT inverters (but that would involve changing your whole system). I think Magnum has a model now which can also regulate this way.
  • yallerrab
    yallerrab Registered Users Posts: 4
    Re: GT interactive question

    I'm not really into paying 1200 for a charge controller, on top of the inverter on top of the batteries. However, I am even less inclined to spend countless hours on gas lines, for my generator, while panicking people hoard gas. I am going to do my best to not get caught with my pants down again. So, now that I am done ranting......I was wondering, with the over-producing......if a "dump" could be used and, in turn, make the setup less costly?
    Also, I am not sure what you mean when you say I would have to change the whole system. Someone told me that the SMA GT and off Grid inverters are a good combination for this. The problem is that the SunnyBoy only has 11 panels, which is why I am trying to make that the string which powers up the off grid inverter.

    Thanks Again.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: GT interactive question

    The Sunny Boy would work fine with a Sunny Island. The other two would work, but not so easily. SMA has a comm system for integrating their GT inverters with the SI. Otherwise it has to rely on the frequency shifting trick to shut down GTI if too much power is available. It's the Sunny Island that is the key to success here.

    In other words there are several different ways to do it, but "easy" and "cheap" are not adjectives connected with the process.

    One point is that the GTI's will only produce additional power when the sun shines, which may not be any big help. Likewise no panels produce full power during storms; they need sunshine. As such the AC coupling of GT inverters to an OG inverter to keep them going when the grid is down is usually not worth chasing after. Simpler, since you need the OG inverter anyway, is to have a system for backing up the critical loads (refrigerator for instance) and some method of recharging the batteries without the grid. This can be re-configuring the array(s) for a standard charge controller or using a generator.

    If you could determine the exact array specifications you have it would be possible to work out some alternative arrangements for recharging. Again, though, without sunlight none of it works.
  • yallerrab
    yallerrab Registered Users Posts: 4
    Re: GT interactive question

    Thanks for your help. What kills me is that I HAD that setup. Xantrex with battery backup. The dolt who owned the place before me didn't set up the critical load panel and, it trickled to ground. On top of that, he worked for the company that installed it and they used metal conduit (which still leaks), which had too many wires pulled and a burr shorted out about 5 of them. See attached. His interior wiring made it such that I almost shut the whole system down. To continue to use that inverter, I would have had to pull new wire, disassemble the Xantrex, send it out etc..... After pricing it out, I went with the fronious (from 12 strings of 2 through a combiner box, to 3 strings of 8 (allowed me to use existing wire) - using the combiner box for the fuses only)) sold off the batteries and inverter. I still have the 2 charge controllers (morningstar - ts-m) and the outback metal "panel". Thanks again.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: GT interactive question

    Oooh! That's nasty!
    Just goes to show that no matter what equipment you get some half-wit of an installer can turn it into scrap in a hurry. And possibly burn your house down.
  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: GT interactive question
    yallerrab wrote: »
    I'm not really into paying 1200 for a charge controller, on top of the inverter on top of the batteries. However, I am even less inclined to spend countless hours on gas lines, for my generator, while panicking people hoard gas. I am going to do my best to not get caught with my pants down again. So, now that I am done ranting......I was wondering, with the over-producing......if a "dump" could be used and, in turn, make the setup less costly?
    Also, I am not sure what you mean when you say I would have to change the whole system. Someone told me that the SMA GT and off Grid inverters are a good combination for this. The problem is that the SunnyBoy only has 11 panels, which is why I am trying to make that the string which powers up the off grid inverter.

    Thanks Again.
    The short answer to your question is that (unfortunately) there is no cheap and/or easy way to get a GT PV system to keep producing energy during a grid outage. When running off grid, you run off batteries and the only role of the PV is to charge the batteries. If you have a lot of loads, you are going to need a lot of battery capacity, and any batteries made to start engines (high current for low duration) are going to succumb pretty quickly to the demands of powering AC loads (lower current for long duration). There's no way around it - producing your own power is very expensive. Utility companies can produce cheap power only through economies of scale.