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Will PV Current Imp Cause Inverter Tripping/Shutdown?

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    Will PV Current Imp Cause Inverter Tripping/Shutdown?

    Hey guys,

    As far the current involved in a PV module, the maximum current that it can produce is the "Isc", and "Imp" is produced on the peak operating conditions. This is what I understand (Correct me if i'm wrong).

    With this in mind, while selecting the inverter, I have:

    module with Isc = 8.97A and Imp = 8.41A
    Inverter with input Isc = 675A and Imp = 356A

    I have 45 strings going into one inverter with 23 strings in MPPT1 and 22 strings in MPPT 2.

    Here,
    - The Isc from the strings to the inverter is (8.97A x 1.25 x 23) + (8.97A x 1.25 x 23) = 504.5625A which is less than 675A
    - The Imp from the strings to the inverter is (8.41A x 23) + (8.41A x 22) = 378.45A Greater than inverter's 356A

    Will this cause the inverter to shutdown or trip?

    Ps: 100kW inverter
    305W Modules
    1.5 DC/AC Ratio (Central inverter which supports upto 1.75 DC/AC ratio)

    #2
    I think the Imp can exceed the Inverter's max without damage. I admit I have never really thought about it because I have never been remotely close to an inverters max current, but I dont see it any different then exceeding the inverter power with a high DC/AC ratio. I would think the inverter is smart enough to drive the MPP away form ideal to clip current just as it does to clip power. I Cant imagine the inveter will just use current until it destroys itself. I think just like the DC/AC ratio, its something you want to model to see that any loss of production from clipping is acceptable.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

    Comment


      #3
      I very much doubt that the inverter will shut down or trip. There is almost no way I can imagine that an inverter could allow a 1.75 DC/AC ratio and not end up in this situation. It will almost certainly just limit the current to something less than the IMP of the array and therefore clip energy production. And since you have a DC/AC ratio of 1.5 this should be something you are expecting. It's pretty common these days as module prices have dropped and interconnection costs haven't.

      Consult the inverter manual and manufacturer to double check that you are not voiding the warranty.

      Comment


        #4
        Originally posted by electrofelon View Post
        I think the Imp can exceed the Inverter's max without damage. I admit I have never really thought about it because I have never been remotely close to an inverters max current, but I dont see it any different then exceeding the inverter power with a high DC/AC ratio. I would think the inverter is smart enough to drive the MPP away form ideal to clip current just as it does to clip power. I Cant imagine the inveter will just use current until it destroys itself. I think just like the DC/AC ratio, its something you want to model to see that any loss of production from clipping is acceptable.
        This is exactly my thought too, and in this case the energy loss is acceptable. But we are experiencing random shutdown and restart of the inverter during peak hours. The inverter manual says even in worst conditions 1.75 DC/AC ratio is acceptable as long as I meet the inverters Isc and Voc, which are fine in my case. I will have to check the thermal limitation to see if that causes this problem.

        Comment


          #5
          Originally posted by jaggedben View Post
          I very much doubt that the inverter will shut down or trip. There is almost no way I can imagine that an inverter could allow a 1.75 DC/AC ratio and not end up in this situation. It will almost certainly just limit the current to something less than the IMP of the array and therefore clip energy production. And since you have a DC/AC ratio of 1.5 this should be something you are expecting. It's pretty common these days as module prices have dropped and interconnection costs haven't.

          Consult the inverter manual and manufacturer to double check that you are not voiding the warranty.
          Yeah, I was skeptical about this DC AC ratio too, but the manual says it is acceptable to do it according to their "tests" under rough conditions. But yeah we are having this shutdown problem and trying to figure out why.

          Comment


            #6
            There is a potential issue with DC/ac ratios being too high, where the point on the I/V curve needed to clip that power is outside of the inverter's capabilities. It seems that with some basic measurements and discussion with the manufacturer would identify of that is what is going on. I also seems to me that specifying a DC/ac ratio that is acceptable is too crude, and one would need to look at where the array parameters sit within the inverter's capabilities.
            Ethan Brush - East West Electric. NY, WA. MA

            "You can't generalize"

            Comment


              #7
              Oh, so the inverter is shutting down. Yeah, what electrofelon said then.

              Comment


                #8
                Originally posted by electrofelon View Post
                There is a potential issue with DC/ac ratios being too high, where the point on the I/V curve needed to clip that power is outside of the inverter's capabilities. It seems that with some basic measurements and discussion with the manufacturer would identify of that is what is going on. I also seems to me that specifying a DC/ac ratio that is acceptable is too crude, and one would need to look at where the array parameters sit within the inverter's capabilities.
                This surely makes sense, does that mean arrays’ power-voltage curve doesn’t intersect the inverter’s operating range?

                Comment


                  #9
                  What's the shut down error message? Most inverters these days are pretty good about telling you why they are shutting down. Random shutdowns during peak production are also a sign of possible AC overvoltage due to a high AC voltage drop and high AC voltage from the utility.

                  Comment


                    #10
                    Originally posted by pv_n00b View Post
                    What's the shut down error message? Most inverters these days are pretty good about telling you why they are shutting down. Random shutdowns during peak production are also a sign of possible AC overvoltage due to a high AC voltage drop and high AC voltage from the utility.
                    Unfortunately there was no error message. This was found out only via monitoring system. We now have spread out the strings from this inverter to other inverter evenly. We don't see any issue until now after restringing. Lets see what happens!

                    Comment


                      #11
                      Information to clarify the query, and replies

                      Originally posted by TheElectrician View Post
                      Unfortunately there was no error message. This was found out only via monitoring system. We now have spread out the strings from this inverter to other inverter evenly. We don't see any issue until now after restringing. Lets see what happens!
                      Based on the original query by TheElectrician, and replies to that query from others, I'd like to clarify several points that have been posted, most of which are excerpted for brevity:

                      electrofelon: I think the Imp can exceed the Inverter's max without damage.

                      DL-PE: This is correct. Inverters won't be damaged if the maximum power point current from the PV array exceeds the inverter's maximum rated DC input current.

                      The query by TheElectrician implies much more PV DC wattage is connected to the inverter input than is required to generate maximum AC output. By UL 1741 requirement, grid-interactive inverter AC output current may not exceed the maximum current rating in the inverter's specification, regardless of the total DC power available. This in turn limits the DC input current. If an oversized array of (say) 175% is connected to an inverter, the output power waveform through the course of the day has a steeper slope up early in the day and will last later into the afternoon than an inverter with an array that's 100% of the inverter's DC current rating. It may mean the system isn't going to operate

                      The only parameter of importance in this aspect of a system is to ensure the PV array output voltage does not exceed the inverter's maximum DC input voltage rating under all conditions of operation. (PV voltage goes up with colder temperature.)



                      TheElectrician
                      ... and in this case the energy loss is acceptable.

                      DL-PE:
                      No energy is lost. It's simply not converted from solar DC to grid-connected AC. However, in this context, it's easily understood you're referring to this unconverted energy. It simply means the system efficiency is less than ideal. Typical DC-to-AC sizing is 1.25 to 1. (125 kW of PV for 100 kW of inverter-rated AC output.) This may vary depending on circumstances. 1.75 to 1 is much higher than good design practice would suggest.


                      TheElectrician
                      But we are experiencing random shutdown and restart of the inverter during peak hours. The inverter manual says even in worst conditions 1.75 DC/AC ratio is acceptable as long as I meet the inverters Isc and Voc, which are fine in my case. I will have to check the thermal limitation to see if that causes this problem.

                      DL-PE:
                      There's an important clue here. "Random shutdown and restart of the inverter during peak hours."

                      Shutdown could be due to any of several possibilities:

                      1. Inverter over-temperature due to continuous operation at 100% of its ratings for extended periods of time. Ambient temperature would also be a factor.

                      2. Abnormal grid conditions. In this case, most likely the grid voltage at the inverter exceeds the allowed maximum. This can be caused by poor connections in the inverter AC output circuit, insufficient ampacity in the inverter conductors .. or simply a high grid voltage. When forcing current into conductors, and back into the grid, there's a certain amount of voltage rise in the circuit. Removing load from the grid also can cause a voltage rise. 10% is the maximum allowed variance. Thus presuming a 277/480 volt three-phase topology, if any phase should exceed the 10% voltage allowance, it's required that output from all three phases cease. The inverter must sense "good" grid within acceptable parameters for a period of five minutes before it can again begin exporting power.

                      3. Out-of-tolerance line frequency is also a possibility, albeit somewhat distant. Grid-interactive inverters are required to cease exporting power if the frequency falls below 59.7 Hz, or goes above 60.5 Hz. Depending on the location of the installation, the inverter may be required to operate outside of these limits.



                      * jaggedben
                      I very much doubt that the inverter will shut down or trip. There is almost no way I can imagine that an inverter could allow a 1.75 DC/AC ratio and not end up in this situation. It will almost certainly just limit the current to something less than the IMP of the array and therefore clip energy production. And since you have a DC/AC ratio of 1.5 this should be something you are expecting. It's pretty common these days as module prices have dropped and interconnection costs haven't.

                      Consult the inverter manual and manufacturer to double check that you are not voiding the warranty.


                      DL-PE:
                      I included the entire context of Ben's reply, because it's all relevant.

                      The inverter will not cease to generate power simply because of a high DC-to-AC ratio. There may be more DC available than the inverter can convert to AC, but all that happens is the inverter output attains its specific maximum output power, and does not go above it. Thus, when the daily energy is plotted, the chart has a flattened area during the middle of the day. Due to the resemblance of this chart to an audio waveform that has been "clipped", the same term is often mistakenly applied to a flattened energy plot in AC systems. "Clipping" implies distortion. There is no distortion taking place here. Instead, the correct term would be "limited". However, most people take them to mean the same thing.


                      * electrofelon
                      There is a potential issue with DC/ac ratios being too high, where the point on the I/V curve needed to clip that power is outside of the inverter's capabilities. It seems that with some basic measurements and discussion with the manufacturer would identify of that is what is going on. I also seems to me that specifying a DC/ac ratio that is acceptable is too crude, and one would need to look at where the array parameters sit within the inverter's capabilities.

                      DL-PE:
                      No. This is incorrect. There is no point at which the DC-to-AC ratio can be "too high". Inverters will use as much DC power as is needed to attain their maximum rated AC output power. The rest goes unused. Yes, this may result in the inverter operating at a point on the array's voltage-current curve that isn't exactly at the "maximum power point", but this is harmless. The only "damage" is to the bank account of the system owner, when they pay much more for the system than was necessary.


                      * pv_n00b
                      * What's the shut down error message? Most inverters these days are pretty good about telling you why they are shutting down. Random shutdowns during peak production are also a sign of possible AC overvoltage due to a high AC voltage drop and high AC voltage from the utility.

                      DL-PE:
                      The reference to AC over-voltage shutdown is absolutely correct. If this is what's occurring, you can watch for the inverter to resume generating power after a required five-minute wait time, during which the grid voltage and frequency must remain within acceptable limits. If during that five-minute period another anomaly should occur, the five-minute time starts back at zero.


                      * TheElectrician
                      * Unfortunately there was no error message. This was found out only via monitoring system. We now have spread out the strings from this inverter to other inverter evenly. We don't see any issue until now after restringing. Lets see what happens!

                      DL-PE:
                      If in the rewiring the total power to any one inverter is such that it never attains 100% of its rated output power, this may be sufficient reduction to keep the grid voltage within acceptable limits.

                      For those who may not be familiar with solar panels, their "rated" output and what's actually achieved tends to be around 80% of the ratings. Thus, a solar panel rated 300 watts DC under "Standard Test Conditions" will deliver around 240 watts in real life. Sunlight intensity and ambient temperature directly affect their total output power. Sun angle and other variables also factor into this equation.


                      Summary:
                      Based on information provided by "TheElectrician", if the random issues continue .. my first recommendation is to monitor the voltage of all three phases. If the voltage on any phase exceeds 110% of nominal, the entire inverter will cease to export power (within two seconds), and will stay in that mode until the grid has been within acceptable parameters for five minutes. If you're present when the inverter goes offline, and it resumes exporting five minutes later, this is clear evidence of an abnormal grid condition. If the voltage is within a volt or two of 110%, this is an indication of pending problems, and it takes only a small increase in the voltage to cause the inverter to cease. It was indicated some of the PV were moved to another inverter. Does it also suffer the same intermittent behavior? If so, it strengthens the "high line voltage" assertion by PV Noob. If the issue affects just one inverter, it indicates bad connections in the output between the inverter and the point of connection to the utility.

                      An over-temperature shutdown is not clear a possibility, as it would result in the generation of an error code, and would leave the inverter offline for a period of time to cool. Many inverters also include a pre-shutdown warning, and may even reduce the AC output to prevent additional heating.

                      As a brief aside .. please don't be distracted by the "Junior Member" status I seem to have acquired here. This is my 48th year involved in the solar industry.

                      My apologies for the lengthy reply.

                      .

                      Comment


                        #12
                        Originally posted by DL-PE View Post
                        No energy is lost. It's simply not converted from solar DC to grid-connected AC.
                        When a solar array is clipped on the DC side by an inverter, it just means that the MPPT circuitry is driving up the resistance the array sees until the current it is generating falls below what the inverter can process. You're correct in saying that electrical energy is not lost, but that's because the excess electrical energy is not produced at all. The radiant energy imparted onto the array by sunlight that is not converted to electricity is lost, though.

                        Comment


                          #13
                          Originally posted by DL-PE View Post


                          * electrofelon
                          There is a potential issue with DC/ac ratios being too high, where the point on the I/V curve needed to clip that power is outside of the inverter's capabilities. It seems that with some basic measurements and discussion with the manufacturer would identify of that is what is going on. I also seems to me that specifying a DC/ac ratio that is acceptable is too crude, and one would need to look at where the array parameters sit within the inverter's capabilities.

                          DL-PE:
                          No. This is incorrect. There is no point at which the DC-to-AC ratio can be "too high". Inverters will use as much DC power as is needed to attain their maximum rated AC output power. The rest goes unused. Yes, this may result in the inverter operating at a point on the array's voltage-current curve that isn't exactly at the "maximum power point", but this is harmless. The only "damage" is to the bank account of the system owner, when they pay much more for the system than was necessary.
                          I believe I am correct on this one. See this article https://www.solarpowerworldonline.co...yper-clipping/
                          Ethan Brush - East West Electric. NY, WA. MA

                          "You can't generalize"

                          Comment


                            #14
                            Originally posted by electrofelon View Post
                            I believe I am correct on this one. See this article https://www.solarpowerworldonline.co...yper-clipping/
                            I agree that it may be possible for there to be a problem, but it's only when the DC to AC ratio is too high and the orientation and location of the array is such that these higher DC currents from the array will be generated. Here in Texas for an array with a 180 degree orientation and 30 degree tilt I wouldn't want to "overload" its inverter more than 15% or so. For a system in Canada with a less than optimum orientation, I would bump up the DC:AC ratio significantly because the array would never get anything approaching STC conditions.

                            Comment


                              #15
                              Originally posted by ggunn View Post
                              I agree that it may be possible for there to be a problem, but it's only when the DC to AC ratio is too high and the orientation and location of the array is such that these higher DC currents from the array will be generated. Here in Texas for an array with a 180 degree orientation and 30 degree tilt I wouldn't want to "overload" its inverter more than 15% or so. For a system in Canada with a less than optimum orientation, I would bump up the DC:AC ratio significantly because the array would never get anything approaching STC conditions.
                              Right. I am not saying this is a common problem, and there are lots of variables: DC/AC ratio, orientation, array characteristics and how they lie in the inverter's window, design of the inverter, etc. But becomes more of a potential concern when you start getting DC/AC ratios up in the high 1's.
                              Ethan Brush - East West Electric. NY, WA. MA

                              "You can't generalize"

                              Comment

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