Delayed startup of Grid Tie Inverter

Hello all,

I have have grid tied Xantrex STRX2500 and STRX1500 wired in parallel.

Panels:
100W AP-100
Voc 20.1V
Vmp 16.1V
Isc 7.2A
Imp 6.2A

Panels are set up as 4 in series, 5 paralleled into STRX2500 and 3 paralleled into the STRX1500.

Even though the panels are getting sun (angled sunlight), there seems to be quite a delay now to when the inverters start sending power to the grid, compared to the summer. i.e. the inverters start producing sooner in the summer, even with 'similar' angled sunlight (not direct) in the summer.

I assume this has to do with the voltages being higher in the cooler weather (50-55F in the morning) compared to summer. And the panels need to heat up in the sun to lower the panel voltages so the inverter can use the output. Am I thinking correctly?

The inverter display says " No AC dectected " during this "down time" before the inverters start producing. The Vdc at the inverters is showing 75-76V during this "down time"

Some inverter specs (@25-40 C):

Range of operating voltage: STRX2500: 52-75V, STRX1500: 52-85V
Max operating current: 60A DC
Max array Short Circuit Current: 75A DC

I'm guessing this system wasn't designed as well as it should have been (Gried tied since 2001).
«13

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    I think you've got it.
    The Voc of the panels is 20.1 * 4 in series = 80.4 per string. That's already over the max in for the 2500 inverter, and pretty close for the 1500. The smallest amount of cold will put it over on that one as well.

    If you can access the wiring easily and safely you can check this out by shorting the PV output initially, causing it to send Isc through the wiring and warm the panels quickly. Then switch it to the inverter input and see of they don't start right away. If you're not comfortable with handling this kind of DC Voltage don't do it because it can arc and start a fire or shock you.

    Ideally some sort of pre-warming system could be rigged up to automatically overcome this problem. Even a timed load on the PV output would hold the panel Voltage down to where the inverters could start, at which point the load would be switched out so that full power was available to the GTI's.

    EDIT: Checked the Vmp numbers too. Depending on how cold it is getting even that could go over the max in on the 2500; 16.1 * 4 = 64.4 * 1.2 (cold factor) = 77.28
  • balee123balee123 Solar Expert Posts: 86 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    Thanks for the reply. Nice to know my brain still has a little logic still left in it ;)
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    If you can access the wiring easily and safely you can check this out by shorting the PV output initially, causing it to send Isc through the wiring and warm the panels quickly

    I think that putting a load on the panels causes them to run cooler. All the light that hits the panels is either transmitted through, reflected by, or absorbed by the panel.

    If the panel is unloaded, the absorbed light is converted to heat (which warms the panel). When there is a load (even a short circuit), some of the energy is exported out of the panel to create its heat elsewhere.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    balee123 wrote: »
    Thanks for the reply. Nice to know my brain still has a little logic still left in it ;)

    I have also seen the suggestion of just putting a shorting relay across one of the four panels in each string, tied to the voltage output of one panel in the string (the coldest) or tied to a temperature sensor.
    Shorting out one panel will not hurt it in any way, and you would then immediately get the output of 3/4 of your panels as soon as the light hits them, then move up to full output once the panels have warmed up in the sun.
    Just make sure that the relays are DC rated to interrupt the Isc of one panel without any degradation.

    Alternatively, you might even get more daily production in the winter by just putting a shorting wire on one panel per string and leaving it there until summer. Run the calculations and see.

    Yet another alternative, depending on how ambitious you are, would be to rewire your panel array for strings of three instead of four (with two panels left over, unfortunately. Get one more?)
    SMA SB 3000, old BP panels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    Energy comes in many forms. Light is very high frequency, heat much lower. A panel not under load does not convert light energy to electric current so there is no generation of heat from the panel operation, because it is not operating. Decrease the resistance across the output and the Voltage goes down, current goes up. More current = more energy being converted to heat. Since the current is the same everywhere throughout the circuit, the panel heats up from current flow.

    Panels also heat up from sustained time in the sun. Since they do not convert heat energy to electricity (that would be upping the frequency) the circuit will not conduct any of this heat energy away as electrical power.

    In either case the increase in panel temperature will reduce its Voltage to a level that will allow the inverters to come on.

    Inetdog will now elaborate on the physics of this. :D
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter

    Inetdog will now elaborate on the physics of this. :D

    I will be happy to!

    <lecture mode>

    The amount of solar energy hitting the panel is independent of whether the panel is operating or not. So let's look at where it goes:
    1. Some reflects off the surfaces of the glass and the silicon and does not reach the panel at all. Not affected by current.
    2. Some is absorbed by the panel and converted immediately to heat. (Energy from the photons is converted to kinetic energy of electrons and nuclei.) Not affected by current.
    3. Some moves electrons into a particular set of excited states, which physically separate the + and - charged regions of each cell and,
    a. if there is a current path, causes electron movement through the external circuit. Since the resistance of the external components is much higher than the internal resistance of the panel, most of the energy goes into the load circuit, and that energy does NOT heat the panel.
    b. if there is no current path, the electrons with nowhere to go eventually build up a charge which prevents additional electrons from being pumped across the gap. This reduces the probability of light being absorbed by that mechanism, but most likely those same photons will be absorbed in different ways which do not have the potential (sic) to produce power. Or else the electrons move back to their original locations through the internal parallel resistance of the material. They are not likely to be reflected back, so the total heat input to the panel is not decreased.
    c. if there is a current path which is a short circuit, the electrons will move through that path, but their energy will be dissipated inside the panel because that is where the highest resistance is. Same heating effect as b.
    So, the only difference among the three options is that putting a load on the panel (CC, GTI, etc.) will reduce the rate at which the panel warms up by a fraction roughly proportional to the efficiency of the panels (~17%). Either leaving an open circuit or short circuiting the panels will have the same effect of producing maximum heating.
    </lecture mode>

    Comments from the Teaching Assistant (TA):

    The professor has not mentioned where the heat goes to prevent the panels from just getting hotter and hotter without limit. There is heat loss, mostly by convection and conduction to the air, racking system, etc. but also some by infrared radiation back into the sky or down to the roof. As all of these increase with the panel temperature, the result will be that in addition to heating up slower, the loaded panels (case a) will eventually reach equilibrium at a lower temperature than for cases b and c.
    SMA SB 3000, old BP panels.
  • boBboB Solar Expert Posts: 943 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    inetdog wrote: »
    So, the only difference among the three options is that putting a load on the panel (CC, GTI, etc.) will reduce the rate at which the panel warms up by a fraction roughly proportional to the efficiency of the panels (~17%). Either leaving an open circuit or short circuiting the panels will have the same effect of producing maximum heating.


    This is also what I have heard. I wonder how hard it is to measure ?

    boB
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter

    Inetdog, Unless your short circuit is superconducting, it is a load.

    Yes, of course I realize that most resistance in the short circuit is in the panel and therefore most of the current flow will cause heating in the panel, but if there is any resistance at all in the short circuit the shorted panel will run cooler.

    The only way I can see to avoid that conclusion is if the reflectance of the panel changes with the load. I am not aware that it does.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter
    boB wrote: »
    This is also what I have heard. I wonder how hard it is to measure ?

    boB

    Pretty hard, since you'd need "cool light" bright enough to achieve maximum panel current without heating the panel up just from the light falling on it.

    Anyway, in practical terms "pre-loading" the panel will pull the Voltage down below the input max and allow the inverter to start.
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    vtmaps wrote: »
    Inetdog, Unless your short circuit is superconducting, it is a load.

    Yes, of course I realize that most resistance in the short circuit is in the panel and therefore most of the current flow will cause heating in the panel, but if there is any resistance at all in the short circuit the shorted panel will run cooler.

    The only way I can see to avoid that conclusion is if the reflectance of the panel changes with the load. I am not aware that it does.

    --vtMaps

    You are correct that the net heat input to a shorted panel will be lower than that to an open circuited panel. My point is that the amount of difference between the two is insignificant, and is particularly small (factor of 100 or more) compared to the reduction in net heat input you will get with a loaded panel. Loading the panel to the Maximum Power Point will lead to the coolest possible panel.

    Just to put some numbers to it: If Isc of the panel is 6 amps, Imp is 5 amps, and Vmp is 20 volts, the loaded panel will be losing 100 watts that would otherwise go into heating it.
    If the short circuit resistance is .1 ohm ( a ridiculously high resistance for a short circuit -- look at the wire tables) then it will develop a voltage of .6 volts and a power loss to the panels of 3.6 watts. With a more realistic value of .01 ohms, the voltage will be .06 volts and the heat loss from shorting will be only .36 watts. Any resistance in the leads inside the panel will just contribute to warming up the panel, so all that counts is the wiring completely outside the panel.
    SMA SB 3000, old BP panels.
  • boBboB Solar Expert Posts: 943 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    Pretty hard, since you'd need "cool light" bright enough to achieve maximum panel current without heating the panel up just from the light falling on it.

    Anyway, in practical terms "pre-loading" the panel will pull the Voltage down below the input max and allow the inverter to start.


    Temperature rise due to sun shining is expected of course. I would measure temperature rise over ambient with the short, at Voc and at Vmpp.
    Data logging the cell temperature should show this but what I was wondering is how much it changes and if it is noticeable outside of the
    noise. I may just have to try this and see. I ~think~ it may have been BP that said this happens but I never saw or read about
    this phenomena first hand.

    What is the part about lowering the V towards Vmpp so that the inverter turns on ? Usually it's the inverter itself
    that drags the voltage down.

    Sorry... I'm afraid I have lost track of the original quest here. Guess I better read it again. This energy change thing is interesting.

    boB
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    boB wrote: »

    What is the part about lowering the V towards Vmpp so that the inverter turns on ? Usually it's the inverter itself
    that drags the voltage down.

    Sorry... I'm afraid I have lost track of the original quest here. Guess I better read it again. This energy change thing is interesting.

    boB

    I think that the concern is that when Voc is high, the inverter may shut down protectively and not draw enough current to lower the voltage to the point where the protective circuitry will allow the inverter to start and keep the voltage low. I imagine this depends to some extent on just how the high voltage protection on the inverter is built.

    Just how does the inverter handle the slow rise of available current when the string voltage is high from the time dim light first hits the panels? If Voc is below the protective threshold, I could see the inverter periodically trying to draw current to see if enough is available for operation, or just presenting a fixed resistive load and not starting up until the voltage rises to a threshold.

    Also, would the MPPT search run the risk of letting the string voltage go above the protective limit, shutting the inverter down until Voc drops sufficiently?

    When you know the design of the particular inverter (and I am sure you do know a few!) these questions can be answered.

    So one idea being floated was whether adding an external resistive load (maybe switched off once the inverter starts) would keep the inverter from ever seeing cold Voc. Another was reducing the string voltage by taking out (shorting) one panel (until the inverter starts or until the temperature rises, or other logic.) And yet another was shorting out the whole string to try to get it to warm up faster.
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    inetdog wrote: »
    My point is that the amount of difference between the two is insignificant

    I certainly agree with that. Deja Vu, let's see... where have I had this conversation with you.... Oh, here it is, from last August:
    vtmaps wrote: »
    Unless your short circuit is superconducting there is some heat produced. I do agree with you that the effect is minimal and of little or no practical consequence. Sort of like turning off the radio in the car to get better gas mileage.
    inetdog wrote:
    Unless your radio is attached to a 1000 watt audio amplifier.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter
    boB wrote: »
    Temperature rise due to sun shining is expected of course. I would measure temperature rise over ambient with the short, at Voc and at Vmpp.
    Data logging the cell temperature should show this but what I was wondering is how much it changes and if it is noticeable outside of the
    noise. I may just have to try this and see. I ~think~ it may have been BP that said this happens but I never saw or read about
    this phenomena first hand.

    What is the part about lowering the V towards Vmpp so that the inverter turns on ? Usually it's the inverter itself
    that drags the voltage down.

    Sorry... I'm afraid I have lost track of the original quest here. Guess I better read it again. This energy change thing is interesting.

    boB

    The OP (original post/poster or original problem in this case) was experiencing a situation where the GT inverters weren't coming on as early as they should in cold temps. These particular inverters have rather low input Voltage maximums, and it appears to be a situation where the panel Voc is high and the inverters are staying off due to perceived high Voltage. I even calculated that under the right conditions the Vmp of the one array cold would be above the input limit of the inverter.

    We see this often up here with MPPT controllers. One reason why the MidNite Classic is so desirable in northern climes. :D

    So again the important thing here is 'clamping' the Voc down so the panel can warm up (by whatever means) with the resulting V-drop that will allow the inverters to start earlier.

    As panel efficiency improves, this situation will change. At <20% efficient not much power falling as light on the panel actually has anything to do with electrical production!

    Might try a panel in a dark room and feed it the rated Isc from a power source and see how much the panel temp changes. It won't be much.

    BTW, for those who don't know and may be reading this, panels are a current source not a Voltage source. The change in temp has a significant effect on the Voltage, but not the current. These things (panels) don't always behave the way you think they should.
  • boBboB Solar Expert Posts: 943 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    The OP (original post/poster or original problem in this case) was experiencing a situation where the GT inverters weren't coming on as early as they should in cold temps. These particular inverters have rather low input Voltage maximums, and it appears to be a situation where the panel Voc is high and the inverters are staying off due to perceived high Voltage. I even calculated that under the right conditions the Vmp of the one array cold would be above the input limit of the inverter.

    We see this often up here with MPPT controllers. One reason why the MidNite Classic is so desirable in northern climes. :D

    So again the important thing here is 'clamping' the Voc down so the panel can warm up (by whatever means) with the resulting V-drop that will allow the inverters to start earlier.


    OK, so as for that original concern then, the system needs to have a PV module (or more) taken out of the string(s) to keep the cold weather Voc down.
    PV Inverters don't have over-voltage protection that I am aware of. Since just dropping the input voltage down ~some~ will mean they have
    to dissipate power as heat, they just don't do that (AFAIK that is)... They ~could~ do that but the inverter manufacturers normally assume that the
    system is designed around max voltages and coldest temperatures. In fact, the NEC says the system must be designed properly to avoid
    that over-voltage problem.

    If the inverter could disconnect its input capacitors, at least temporarily, then it could SHORT the PV array to reduce the voltage and that
    would heat things very little (crowbar) but I don't know of any GT inverters that do even that. Let me know if you find one.

    But either of these would add cost to the inverter as well as make it somewhat larger.

    Apollo Solar had that PV module shorting device. I don't know if you could even still get them but
    the system being designed around the temperature range should work way better without having to
    reduce your power output by getting rid of that module. OF course, PV is real cheap these days
    so maybe it's OK to just throw away PV for reasons like this.
    boB
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    boB, as you know this cold weather Voc problem is common to off-grid systems. :roll:

    Here we have a case of two older GT inverters with a low input Voltage around 75 maximum. It's pretty easy to exceed. Clamping the Voc down so that the inverter will start and then removing the clamp so full panel power can go to the inverter is the thing. Only some of the math indicates that even the Vmp will exceed input maximum on the one inverter if the weather is cold enough.

    Nice thing about newer GTI's is that the operating Voltage is in the hundreds and usually has a 200 Volt usable range; not difficult to get an array to perform with those specs even in cold weather.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    vtmaps wrote: »
    Inetdog, Unless your short circuit is superconducting, it is a load.
    If a short circuit is superconducting, it is an infinite load. A no load condition is an open circuit.

    http://nextlol.com/img/28381/
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    ggunn wrote: »
    If a short circuit is superconducting, it is an infinite load. A no load condition is an open circuit.

    inetdog's point was that a short circuit has no resistance and therefore no power is dissipated in the "load". In that way a short circuit and an open circuit result in minimum heating of the "load" and maximum heating of the panel.

    There is some difference (between open and short circuit) in where in the panel the heat is dissipated.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    In the real world all circuits have resistance. There is no such thing as "a short circuit has no resistance". Nor will the resistance in a shorted panel output be equal at all points; the wiring will be the lowest resistance and have the greatest heating effect from the current flow. If you add resistance the current flow drops and the heating likewise. Curiously the panel's current output will also drop.

    None of which has any bearing on the difference in energy type from photons bouncing electrons around without consequence at the speed of light to converting said activity into electrical current through a circuit which creates current and thus converts some of that energy to heat.

    It'd be a darn fun experiment to set up. Anyone want to do it?

    You need a panel, some intense light (such as halogen shop lights - preferably behind a glass barrier to reduce the infrared effect) that can illuminate the panel close to its Isc rating, Ammeter to check same, and thermometer fastened to panel.

    Step one would be to check the current output at short circuit to be sure you're getting enough light on the panel.
    Step two would be measure the temp of the panel with no circuit on it until panel temperature ceases to rise.
    Step three would be to connect the short circuit again and see if panel temperature: A). drops; B). remains the same; C). increases.

    You can't do this outdoors because of the variations in ambient temp and light.
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    In the real world all circuits have resistance. There is no such thing as "a short circuit has no resistance". Nor will the resistance in a shorted panel output be equal at all points; the wiring will be the lowest resistance and have the greatest heating effect from the current flow. If you add resistance the current flow drops and the heating likewise. Curiously the panel's current output will also drop.

    None of which has any bearing on the difference in energy type from photons bouncing electrons around without consequence at the speed of light to converting said activity into electrical current through a circuit which creates current and thus converts some of that energy to heat.

    It'd be a darn fun experiment to set up. Anyone want to do it?

    You need a panel, some intense light (such as halogen shop lights - preferably behind a glass barrier to reduce the infrared effect) that can illuminate the panel close to its Isc rating, Ammeter to check same, and thermometer fastened to panel.

    Step one would be to check the current output at short circuit to be sure you're getting enough light on the panel.
    Step two would be measure the temp of the panel with no circuit on it until panel temperature ceases to rise.
    Step three would be to connect the short circuit again and see if panel temperature: A). drops; B). remains the same; C). increases

    You can't do this outdoors because of the variations in ambient temp and light.

    If anyone actually does this, just to complete the experiment (and also get an idea of whether or not your experiment is sensitive enough to measure any effect!) try the third option of putting either an MPPT controller driving discharged batteries in Bulk at max panel output or loading the panel down with a calculated resistance which will put the panel close to the MPP. (I predict that the end temperature will be lower and that there will be no difference between the first two cases.)
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter

    i've kind of lost track of what you guys are trying to determine and why. are you trying to heat up the pvs to lower the voc? there is internal resistances in a pv that will dissipate some power. good luck on achieving much with that, but in any case a bit of power is dissipated there and any time power is dissipated then some heat is generated. also, of note, that loading down the pvs will also lower the voc, which is what i believe boB was referring to with apollo solar's attempts at the problem. it does not have to be a short circuit and we aren't trying to define the term here either as it will have resistance, albeit low, but a short is not necessarilly needed to load down the pvs enough.
  • inetdoginetdog Solar Expert Posts: 3,121 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    niel wrote: »
    i've kind of lost track of what you guys are trying to determine and why. are you trying to heat up the pvs to lower the voc? there is internal resistances in a pv that will dissipate some power. good luck on achieving much with that, but in any case a bit of power is dissipated there and any time power is dissipated then some heat is generated. also, of note, that loading down the pvs will also lower the voc, which is what i believe boB was referring to with apollo solar's attempts at the problem. it does not have to be a short circuit and we aren't trying to define the term here either as it will have resistance, albeit low, but a short is not necessarilly needed to load down the pvs enough.

    Here is a summary of what I think the thread has been wandering around:
    1. The OP reports that his GTI will not start on cold mornings until the sun has been shining for awhile. The speculation is that the GTI will not start until the string voltage drops below its Max Input Voltage limit.
    2. Proposed solutions were to a. apply a load to drop the string output down to a value (greater than Vmp but less than Voc) which would allow the inverter to start and then hold the voltage down on its own (unless the MPP scan takes the voltage above the limit again!) b. bypass one panel to drop the string voltage. Note that either of these first two might lose more power than would be gained by getting the GTI to start earlier.) or c. somehow heat the panel.
    3. The recent subject of contention is which of these options would result in the panels heating up faster:
    a. Leaving them open circuited.
    b. short circuiting them, or
    c. drawing something close to Imp from them.
    4. My contention is that both a and b will deliver the same rate of heating, and that will be the best you can do, while c will actually heat the panels more slowly since energy is being extracted from them into an external load.
    Feel free to jump in from that springboard. :-)
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter

    save me the trouble of looking back so somebody tell me just how far above the max is the voltage going? also state what that max voltage on the inverter is if you can.

    in any case i think loading it down some will be a benefit as some production would be better than none, which is what you get being too far over the max voltage point of the inverter. it may pay to experimentally determine how much loading would be required in order to allow the inverter to operate. this could then be employed as either heat tapes or power resistors and could even be put under the pvs to warm them directly if precautions are taken. i would not like the idea of it being automatically switched in or out of line as i believe it should be supervised.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    Niel;

    This is an unusual case in that the GTI's operate at very low Voltage: <80. As such it only takes a Volt or two to go over the limit on Voc and keep them from starting.

    Modern GTI's with their hundreds of Volts operating range are a lot easier to deal with in that respect.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    In the real world all circuits have resistance. There is no such thing as "a short circuit has no resistance". Nor will the resistance in a shorted panel output be equal at all points; the wiring will be the lowest resistance and have the greatest heating effect from the current flow. If you add resistance the current flow drops and the heating likewise. Curiously the panel's current output will also drop.
    Of course the current drops. What you are doing when you add resistance is moving along the IV curve for the module; the current drops very slowly and the voltage increases quickly for a bit as you move off Isc, but then as you increase R more, the change in current speeds up and the change in voltage slows down until you reach Voc. With different resistances you can plot points to generate the IV curve itself.

    Use wirewound resistors and use pliers to handle the low values; they get HOT.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter

    yes, ggunn knows what i'm driving at here in that it wouldn't take too much loading to drop the voc to an acceptable area to allow gt inverter operation.

    do be sure of the planned resistor's dissipation as they can not only get hot, they can possibly self destruct if needing to dissipate more power than they are rated for. as an example, if one were to have a resistor that passes .089a (89ma) with 60v on that resistor it is dissipating .089a x 60v = 5.35w and i like to have roughly a 2x leeway as a minimum for a resistor wattage rating. in the case of solar you would want to use the highest voc voltage it can attain as the reference voltage in determining wattage dissipation.

    this is where the formuli i = e/r and w = v x i come in handy.
    i = current in amperes
    e = volts
    r = resistance in ohms
    w = watts
  • balee123balee123 Solar Expert Posts: 86 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    Hi. I am the OP and had an interesting thing happen this morning. This was one of the coldest morning here in San Diego; it was in low 40's as the sun started to come up....burrrrrrrrrrrrrrr ;). There were no clouds or marine layer this morning and almost as soon as the sun rose, the inverters started producing. For what it's worth, the sun now rises directly perpendicular to the panels, i.e. ~90 degrees to the direction the panels are facing (~200 degrees). Can anyone explain this in light of this thread's recent discussion?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Delayed startup of Grid Tie Inverter

    One of those confluences of circumstances.

    Every now and then systems behave in an unexpected way because all the variables (or possibly planets) line up in just the right way. Most of the time it is an aberration that can be ignored. If it tends to happen more often then not, then it is an average condition that has to be dealt with.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Delayed startup of Grid Tie Inverter

    even though the sun can't hit the pvs directly there is plenty of back scatter radiation from the atmosphere that would illuminate the pvs somewhat. apparently you have enough of that scattered light to make the system operational. don't be too confused about it either as you can see before the sun is actually up and is an example of that back scatter radiation.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,959 ✭✭✭
    Re: Delayed startup of Grid Tie Inverter
    balee123 wrote: »
    Hello all,

    I have have grid tied Xantrex STRX2500 and STRX1500 wired in parallel.

    Panels:
    100W AP-100
    Voc 20.1V
    Vmp 16.1V
    Isc 7.2A
    Imp 6.2A

    Panels are set up as 4 in series, 5 paralleled into STRX2500 and 3 paralleled into the STRX1500.

    That perfectly fine, the Sunties are rated to 125V, where is all this silly talk of 80V come from? ... geez, go away for a year an who is running the asylum?

    The front-end of the Sunties is 150V components, no different than say boBs MX60's ... The Voltages listed by the OP are the typical mppt range, not open circuit values, but there is nothing in the code to turn off the units ... and I know as I have worked on the firmware :)

    as to the OP's issues, if these are not UPG models ( and they should have been upgraded in 2003 for free ) no ac detected is just that, AC, not DC
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