Basic operation of a GTI

Sun Dog
Sun Dog Solar Expert Posts: 115 ✭✭
in Grid Tie and Grid Interactive Systems #1
If a grid tie inverter is set to output 240VAC but can start with as little as 150VDC does that mean that it is boosting the incoming DC, or have I got my terminology/understanding all wrong?

I see that the MPPT voltage range into a GTI is very broad but is there normally a tighter "optimal" range for any given inverter? I found a graph on the Sunny Boy website that showed a couple percent difference between an input of 175V and 400V but nearly no difference between 400V and 480V.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Basic operation of a GTI
    Sun Dog wrote: »
    If a grid tie inverter is set to output 240VAC but can start with as little as 150VDC does that mean that it is boosting the incoming DC, or have I got my terminology/understanding all wrong?

    You are correct. Look at micro-inverters: one panel, usually with a Vmp of 30, on the input but 240 Volts out.
    I see that the MPPT voltage range into a GTI is very broad but is there normally a tighter "optimal" range for any given inverter? I found a graph on the Sunny Boy website that showed a couple percent difference between an input of 175V and 400V but nearly no difference between 400V and 480V.

    Also correct. If you look at the specs of several you will see things like "minimum Voltage for operation" or "start-up Voltage" and sometimes "maximum MPPT Voltage" (meaning input V above that doesn't get turned into more power). Look at the specs on this Fronius for example: http://lib.store.yahoo.net/lib/wind-sun/IG-PLUS-V-datasheet.pdf

    Note the "MPPT Voltage Range" of 230-500, yet the "DC Start-up Voltage" is 245. This is because it wants to make sure there's enough power available before it starts to work, but will allow the Voltage to sag below this once in operation so that minor variations such as passing clouds don't shut the whole thing down.
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  • Sun Dog
    Sun Dog Solar Expert Posts: 115 ✭✭
    #3
    Re: Basic operation of a GTI

    So as far as efficiency, is there typically little difference within the MPPT voltage range? I would have guessed that an input of slightly more than 240V would have been optimal but perhaps it doesn't really matter. I have seen some efficiency graphs for solar MPPT charge controllers and normally the closer the output and input voltages are, the higher the efficiency. I assumed the same would hold true for a GTI.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #4
    Re: Basic operation of a GTI

    As I understand, roughly, there are different sources of losses in an AC inverter (GT or Off Grid)...

    One is "switching losses". When transistors turn on and off they charge and discharge the gate capacitance. E=1/2*CV2. Here, higher voltage on the DC side means more switching losses.

    Another is current/resistance losses. E=I2R. Wires, circuit board traces, transformer windings. To a degree, higher voltage means lower current on the DC side of the inverter.

    Probably have other losses (transformer losses, control logic, etc.)--Some which may vary based on power/voltage/current through the device--And others that are fixed (i.e., 30 watts just to run the electronics).

    Where do the two curves cross? Probably is different for each brand/model out there.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Basic operation of a GTI
    Sun Dog wrote: »
    So as far as efficiency, is there typically little difference within the MPPT voltage range? I would have guessed that an input of slightly more than 240V would have been optimal but perhaps it doesn't really matter. I have seen some efficiency graphs for solar MPPT charge controllers and normally the closer the output and input voltages are, the higher the efficiency. I assumed the same would hold true for a GTI.

    That is going to depend on the particular GTI, as you can have a lot of variations on a them. In general they are meant to operate at an "ideal" V*I point just like a controller, but that point can have quite a wide range in reference to the output Voltage. Again, the micro-inverters run at about 1/10 the input Voltage but provide the same output Voltage and also have efficiency in the high 90's percent range.

    Since the output of panels is not a constant through the day there really is no reason to try and pin down a precise V*I point of maximum efficiency for any given GTI. Maybe in lab, but not in the real world. For one thing they have to be able to accommodate different arrays using different panels, which won't all have the same Vmp anyway. Most of the manufacturers have string design tools which will allow you to work out a best array for a given panel.

    As far as efficiency variance goes, you are looking at a difference of less than 10% and probably less than 5% across the allowable Voltage range. You can't really expect better.
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  • Sun Dog
    Sun Dog Solar Expert Posts: 115 ✭✭
    #6
    Re: Basic operation of a GTI

    Thanks guys, I appreciate the info and explanations.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #7
    Re: Basic operation of a GTI
    Cariboocoot wrote: »
    Note the "MPPT Voltage Range" of 230-500, yet the "DC Start-up Voltage" is 245. This is because it wants to make sure there's enough power available before it starts to work, but will allow the Voltage to sag below this once in operation so that minor variations such as passing clouds don't shut the whole thing down.
    Come on, man; you know that a passing cloud won't affect the voltage much if at all. :D

    Now, here in Texas on a hot day the voltage may sag below the startup voltage, and that's where that range will keep the inverter going.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Basic operation of a GTI
    ggunn wrote: »
    Come on, man; you know that a passing cloud won't affect the voltage much if at all. :D

    Now, here in Texas on a hot day the voltage may sag below the startup voltage, and that's where that range will keep the inverter going.

    You haven't seen the clouds we get in the Cariboo! :D
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