June 6, 20242 yr Would 9 x 555W Longi's be too much per Mppt? Currently I have 8 x 555W panels per sting and the voltage highest was 388V today. Adding a extra panel will add about 43V and should max at about 431V. What is the thought on this?
June 11, 20242 yr Author On 2024/06/06 at 8:11 PM, Gerrie said: Adding a extra panel Just for info I connected a 9th Longi 555W panel to a the one MPPT (blue) The max voltage sofar was 428.7V as per screenshot, I will monitoring it to see where it might go over the next few weeks. Hopefully it does not get too close to 500VDC otherwise I will disconnect it again.
June 11, 20242 yr 1 hour ago, Gerrie said: Just for info I connected a 9th Longi 555W panel to a the one MPPT (blue) The max voltage sofar was 428.7V as per screenshot, I will monitoring it to see where it might go over the next few weeks. Hopefully it does not get too close to 500VDC otherwise I will disconnect it again. Be careful, cloud edge sun may push you over voltage limits and release the magic smoke. 428v is already close to the max safe recommended 450v.
June 11, 20242 yr @Gerrie I would not go above 450v there i must agree with @WannabeSolarSparky. If you calculate temperature Coefficient of Voc for the 555w Longi 0.265%/°C that equates to 26 x 0.265 = 6.89%@0°C. 9 x 49.95 = 449.95v + 6.89% = 480.95v@0°C that would be to high for my hard limit of 50v below maximum dc input voltage. I would rather stick with 8 panel /string not worth the risk by adding 1 panel. Edited June 11, 20242 yr by TaliaB
June 12, 20242 yr Author 9 hours ago, WannabeSolarSparky said: Be careful, cloud edge sun may push you over voltage limits and release the magic smoke. Thanks WannabeSolarSparky and Talia for the advice I will change back to 8xpanels and try to avoid the smoke.
June 12, 20242 yr 12 hours ago, TaliaB said: @Gerrie I would not go above 450v there i must agree with @WannabeSolarSparky. If you calculate temperature Coefficient of Voc for the 555w Longi 0.265%/°C that equates to 26 x 0.265 = 6.89%@0°C. 9 x 49.95 = 449.95v + 6.89% = 480.95v@0°C that would be to high for my hard limit of 50v below maximum dc input voltage. I would rather stick with 8 panel /string not worth the risk by adding 1 panel. The one thing not easy to quantify would be the 480V is based on open circuit at 0°. This I think can only be reached if the panels were to be disconnected. If they are kept connected to the MPPT the voltage should remain lower. Although cloud edge gives a high power value it is during high current and not close to the Voc voltage. Any inputs on the above reasoning? Edited June 12, 20242 yr by Scorp007
June 12, 20242 yr 44 minutes ago, Scorp007 said: This I think can only be reached if the panels were to be disconnected. If they are kept connected to the MPPT the voltage should remain lower. The reason i always calculate Voc@ 0°C or geographic specific temperatures is for when the output load is at 0W or close to it. Imagine battery fully charged and no load required from inverter. When no power is required from the mppt close to 0A flowing to the controller then Voc voltage could be reached in colder temps. There is 2 conditions where Voc can occur. 1. Open curcuit strings. 2. No load required from Mppt.
June 12, 20242 yr 36 minutes ago, TaliaB said: The reason i always calculate Voc@ 0°C or geographic specific temperatures is for when the output load is at 0W or close to it. Imagine battery fully charged and no load required from inverter. When no power is required from the mppt close to 0A flowing to the controller then Voc voltage could be reached in colder temps. There is 2 conditions where Voc can occur. 1. Open curcuit strings. 2. No load required from Mppt. Thanks. During observation of the Voc I have never had the condition where the battery is full and no load is supplied from the PV.
June 12, 20242 yr 15 minutes ago, Scorp007 said: Thanks. During observation of the Voc I have never had the condition where the battery is full and no load is supplied from the PV. That will definitely be the case when power comes back after load shedding (for any inverter that complies to NRS).
June 12, 20242 yr 25 minutes ago, P1000 said: That will definitely be the case when power comes back after load shedding (for any inverter that complies to NRS). Thanks for this condition. Never thought about it and do not have a Hybrid to see for myself.
June 12, 20242 yr Could possibly be simulated by using the PV-on/off switch on the underside of the inverter (example Sunsynk/Deye) under exactly the wrong conditions.
June 20, 20241 yr On 2024/06/11 at 10:33 PM, TaliaB said: @Gerrie I would not go above 450v there i must agree with @WannabeSolarSparky. If you calculate temperature Coefficient of Voc for the 555w Longi 0.265%/°C that equates to 26 x 0.265 = 6.89%@0°C. 9 x 49.95 = 449.95v + 6.89% = 480.95v@0°C that would be to high for my hard limit of 50v below maximum dc input voltage. I would rather stick with 8 panel /string not worth the risk by adding 1 panel. I am also thinking of adding 2 Longi 555w panels per string of which I already have 7 per string. The 480,95 as mentioned above is still below the max input of 500V and I think the chances are incredibly slim for the voltage to surpass the max input of 500V. Why do you recommend not surpassing 450V? What are the risks?
June 20, 20241 yr 1 hour ago, ArnoT said: I am also thinking of adding 2 Longi 555w panels per string of which I already have 7 per string. The 480,95 as mentioned above is still below the max input of 500V and I think the chances are incredibly slim for the voltage to surpass the max input of 500V. Why do you recommend not surpassing 450V? What are the risks? PV modules will “wake-up” every morning with very little sunlight. Because the PV array is sitting outside, the temperatures of the modules will be the exact same value as the night air. Because the sun isn’t actually hitting the array immediately, the modules will not immediately produce any current, which means they will immediately go to open circuit voltage. If the temperature of the module is less than the STC value of 25°C, the module’s Voc value will actually be greater than the value listed on the module’s listing label. Overvoltage on the Mppt could damage component insulation that would generate the magic smoke. My motto rather safe than sorry.
June 20, 20241 yr On 2024/06/20 at 7:48 PM, ArnoT said: I am also thinking of adding 2 Longi 555w panels per string of which I already have 7 per string. The 480,95 as mentioned above is still below the max input of 500V and I think the chances are incredibly slim for the voltage to surpass the max input of 500V. Why do you recommend not surpassing 450V? What are the risks? One could add, there's a +/-3% tolerance on the Isc and Voc values for that panel. Also, the Isc and Voc values are based on STC, but there's also a chance it could be increased at higher levels of irradiance during cloud edge. Basically you'd be skating on thin ice, and you might be okay for a long time, until one day maybe the heavens align against you, and you exceed safe values. If you're interested in 10 panels per MPPT, maybe rather consider 2 strings of 5 in parallel, or the simplest would be to just stick to 8 in series per MPPT. On a lighter note, tomorrow is the solstice, just hang in there, there's brighter days to come. Edited June 23, 20241 yr by GreenFields
June 21, 20241 yr 10 hours ago, TaliaB said: If the temperature of the module is less than the STC value of 25°C, the module’s Voc value will actually be greater than the value listed on the module’s listing label. This is not 100% correct. The Voc at 25°C is based on a irradiation of 1000. Early morning the irradiation could be 200 or any other value below 1000. This in turn would cause the actual Voc to be say 25V instead of 50V per panel for the panels under discussion. This for even 10 panels would be 250V. This far exceeds the start up voltage of the MPPT and as even the lowest current is produced would drop the Voc even further. I am not saying one should not use the Voc at STC but the risk is much lower due to the low Voc under low irradiation conditions. The over voltage can still come in at other possible alignment conditions as per @GreenFields. This point that the Voc is much lower when at below 1000 irradiation has been explained in earlier topics. Currently in Gauteng my Voc=48V panels at 06h30 would only have a 7V/panel. At about 06h45 this will go up to a level that switches on the MPPT on my Axpert and the voltage would drop to 35V/panel under a very low Watt load. Thus the panel never got close to the STC Voc level due to low irradiation. I am sure those members with irradiation meters will be able to confirm this behavior by measuring the Voc. I stand to be corrected as the above is based on actual values. Actual values can even differ with a high % and not get us close to the risky level for this stated condition.
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