Sell voltage set point- Outback FM80

jkeener
jkeener Registered Users Posts: 19 ✭
in PV Installers Forum - NEC, Wiring, Installation #1
Does anyone care to share their sell voltage settings or recommend them?

My system:

Outback GVFX3524, Flexmax 80, 2300w grid (1700w avg), 4x 6volt 232ah @ 24v.

I have the absorb/bulk voltage at 28.8v.
I have played with the sell voltage between 25v and 29v.

At 25v, I am selling about 1000w back to the grid.

At 29v I am obviously selling very little because the batteries rarely get over the 28.8v, based on the charge set points.

Is there an ideal voltage that I can set as the sell point but still maintain a fully charged 24v system?

Attached is my battery charging specs.

Attachment not found.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Sell voltage set point- Outback FM80

    You've got a few problems.
    Number 1: your 'SELL' Voltage should not be below full battery Voltage. If it is, the inverter will try to sell to grid when the batteries aren't fully charged. As such the batteries will not fully charged because they aren't being allowed to rise and hold at Absorb Voltage.
    Number 2: your battery bank is too small to work for grid tie. You need a minimum of 100 Amp hours per kW of inverter, or at least 350 Amp hours here. This reduces the AC ripple effect and the repetitious ON/OFF fluctuations in sell.
    Number 3: if you want to sell power from a 3.5 kW inverter you ought to have enough array on it to fully power the inverter. That means slightly more than 3.5 kW of array; about 4550 Watts. Of course if you have that much the one FM80 won't be able to handle it all (good to about 2400 Watts per unit) and that battery bank really doesn't need that much array.

    With a hybrid system the idea is to charge the batteries up, and then keep them "Floating" (around 27.6) all the time so that they don't really do anything but 'filter' between the PV and the inverter.

    I don't think you're going to get satisfactory results without bringing the array and battery bank in line with the inverter capacity.
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  • jkeener
    jkeener Registered Users Posts: 19 ✭
    #3
    Re: Sell voltage set point- Outback FM80

    The inverter only powers a sub panel with 5 critical loads, which is about 500-600w. So I have an excess of +/- 1000w even when the batteries are full. The idea is to "sell" the excess back to the grid, which in turn will be used by loads not on the sub panel.

    As far as float, Outback recommends there be NO float time when selling.

    I think a sell voltage point above the float voltage would work?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Sell voltage set point- Outback FM80

    No Float in the traditional off-grid sense. But the batteries still 'float' in that they aren't supposed to do any work in supplying loads.

    Raising the 'Sell' Voltage will be good for the batteries, but the system still isn't going to perform well because it isn't balanced right.

    When you us a GT system there is no "power to loads/excess to grid" there is only power out of the inverter. The sub-panel determines what are the critical loads that will be supplied by the inverter should the power go down. Otherwise the connection to the main panel will take everything the inverter puts out and it will go either to household needs or if they are lower than inverter output to the grid.

    You probably should have the smaller 2524 inverter: http://www.solar-electric.com/outback-power-grid-interactive-sinewave-inverter-gtfx2524.html
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  • jkeener
    jkeener Registered Users Posts: 19 ✭
    #5
    Re: Sell voltage set point- Outback FM80

    OK...now I am lost-

    "When you us a GT system there is no "power to loads/excess to grid" there is only power out of the inverter."

    Although I understand the concept of "there is only power out of the inverter," I have the MATE hooked up to a PC via a serial cable. Both the MATE and WattPlott software indeed show "power to loads" AND "excess to grid" amounts. It shows about 500w going to "loads" first, then about 1000w "selling" to grid; while the batteries are being charged from the array at 28v (24v system). I have the sell point at 29v.

    Isn't that what it is supposed to be doing? If not, what is supposed to happen to the excess power if the array produces more power than is needed to fully charge the batteries and keep them floating? What have I missed?

    What would reducing the size of the inverter do in this case?

    Thanks
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #6
    Re: Sell voltage set point- Outback FM80
    jkeener wrote: »
    OK...now I am lost-

    "When you us a GT system there is no "power to loads/excess to grid" there is only power out of the inverter."

    What is being referred to is that with a hybrid system, you have three destinations for the PV power:
    1. To the battery, via a method that either takes the DC directly to the batteries or runs a charger from the AC. (The latter is not a good idea!)
    2. To the inverter and from there to AC connected loads. (Variable depending on what you have switched on.)
    3. From the inverter Back to the grid (sell).

    The hybrid system will have separate connections for the grid AC and the AC to the loads so that it can measure them independently.

    For a straight GT system, there is only one connection between the inverter and the grid. Both the utility and the local loads are on the other side of that connection. As a result the two destinations are:
    1. To the loads via the GTI output
    2. To the utility via the GTI output.

    The inverter will always produce as much AC out as the incoming PV power will allow. Whether that ends up going to local loads with makeup power purchased from POCO or ends up going to local loads with the excess sold to POCO will depend only on the difference between the PV power and the load power. The GTI has no control over this.

    A lot of people would like to have a control for their GTI which would "turn off the sell function", but since the GTI does not know where its output is going, it cannot do that.
    It would be possible to implement such a function by providing separate load-side and POCO-side power metering equipment and feeding that information to the GTI controller. But no readily available non-hybrid system has that capability designed into it.

    The closest thing currently available, and promoted mainly to the UK market, is a system which meter the loads and the GTI output and turns on additional loads to keep from trying to feed power back to POCO.
    SMA SB 3000, old BP panels.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Sell voltage set point- Outback FM80

    Or to put it another way you're seeing the loads on the respective terminals. The output for each is controlled by those loads, not by the inverter. It's a nuance (or is that nuisance?), but it's there.

    The size of the inverter is directly related to the capacity of the battery bank in terms of "evening out" the Voltage fluctuations caused by the charge controller switching on/off rapidly to maintain a set Voltage. If there isn't enough capacity, the Voltage at the inverter can fluctuate too much causing 'sell' to kick on and off too.

    Your plan of setting the sell Voltage to near the theoretical Float value is a good one (making sure the charge controller has the same settings). Do you have the remote temp sensor and the HUB connecting the FM80 with the GVFX? That would help make sure both units agree on what the battery Voltage actually is.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Sell voltage set point- Outback FM80

    Forgot something.
    You've got 2300 Watts of panel. Normally that could produce ( 2300 * 0.77 / 28 ) 60 or so Amps of current. 60 Amps of current on 232 Amp hours of battery is 25% charge rate potential at the full Voltage. That is an awful lot of current to expect those batteries to filter. The fluctuations in supply/demand on the DC side would be rough on them over time.

    If allowed to go down to 24 Volts that array could theoretically try to put over 70 Amps to that battery bank for charging: a 30% charge rate.

    Probably your best bet is to increase the battery bank size.

    Those are the same batteries I've got, btw: East Penn/Deka/US Battery PS2200 right? :D I like them so far, but oddly enough am considering doubling the bank (and array) for my off-grid system. I have the equivalent inverter as well: VFX3524.
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  • jkeener
    jkeener Registered Users Posts: 19 ✭
    #9
    Re: Sell voltage set point- Outback FM80

    There appears to be some confusion on the Outback equipment. The GVFX/FM80 is specifically designed to be able to turn "sell" on or off. The MATE controls that function. It has to be turned on or off by the user. Any selling only occurs after handshaking with the grid.

    http://www.youtube.com/watch?v=9smMlG-3mbk&feature=share&list=UU21t-7wcbR5TTFSK5CUmq0Q


    Additionally, the inverter is powered by both AC and DC. The cc can charge the batteries via solar or the inverter can change via AC from grid. The inverter also has a pass through mode allowing the attached loads to be powered by grid AC only.

    Since placing the fx in sell mode, it only stops selling at night. There is no on/off fluctuation as mentioned. I do however agree I need more batteries.

    Not sure if it makes a difference but my sub panel is powered full time by the inverter. It is not a off grid sub.
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  • jkeener
    jkeener Registered Users Posts: 19 ✭
    #10
    Re: Sell voltage set point- Outback FM80

    The array max amps is 39. FYI

    Here are two screen shots of the WattPlott. One from AM and one from this evening.

    My battery float is 26.4


    Attachment not found.Attachment not found.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Sell voltage set point- Outback FM80

    My mistake: I thought you were dissatisfied with how your system was functioning and wanted to know how to make it better. Apparently it works to your satisfaction.

    Sorry I spoke.
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  • jkeener
    jkeener Registered Users Posts: 19 ✭
    #12
    Re: Sell voltage set point- Outback FM80

    LOL no need to apologize. Healthy debate is, well, healthy.
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