Hi, I needed to replace one of my inverters that’s been running fine for the last 5 years, so I bought the exact same one. It’s been offline for about a month. The breakers from the panels have been off as well. When last used the strings averaged 130 to 140 volts each and were well within the voltage specs for the inverter. I installed the new inverter using the same connections, when I turned it on it faulted with an overvoltage warning for the solar connection. I checked, one string was showing 235 volts the other 456 volts. What could cause the strings to gain so much voltage? I did the turn on early in the morning when there was almost no solar power available. Non of the other strings for the charge controllers connected to the same system were out of normal range. Any suggestions will be greatly appreciated. thedog
That is very strange. I have never heard anything like that happening before. The only way solar can increase voltage is at lower temperatures.
The only logical answer is that you are comparing the system operating voltage (Vmp) with the array open-circuit voltage (Voc). What inverter are you using?
How many panels are in each string? What is the Voc of the panels are you using?
In early morning, I can imagine the MPPT controller on one of your two strings ramping up its internal impedance to its maximum value – and that might put the system into an overvoltage condition. If it was just a warning, i.e. if the 456V didn’t exceed maximum voltage spec on the inverter, but it was outside the operating range of its MPPT, then … I think it’s mostly a question of whether the warning-condition will “clear itself” once there’s enough irradiance to let the MPPT on that string “find” an operating voltage at which the string can produce some power, or whether there’s an audible alarm that, even if you manage to learn how to ignore it, may cause you to disregard any faults, or any other warnings.
You say it’s the “exact same” inverter but there could well have been an update to its firmware – and this could cause it to behave quite differently, especially in cases outside of its normal operating conditions (e.g. when there’s an overvoltage condition on your mains supply, assuming you’re grid-tied).
Five years of longevity isn’t a brilliant track record for an inverter… so I think it’s possible that it’s been running very “hot” and you might get better longevity and comparable efficiency out of a beefier inverter. That’s irrelevant of course, unless your replacement inverter fails within five years and you’re shopping around for a different model.
I think you’re probably right. I’ve been away so I haven’t been able to work on it, but now the new inverter has shut down and the batteries show a SCF fault when I turn the breakers back on. Same as the old inverter.
Can you provide the panel Voc and number of panels per string? Then we can easily calculate the max string voltage at any temperature.
You need to check the next condition:
VOC (corrected by temperature) x modules in string < Max. Input voltage
You can calculate VOC (corrected by temperature) in the next way:
- pick the VOC (STC) and the temperature coefficient (Tvoc) from the solar panel datasheet
- get the minimum temperature in your location
- VOC(corrected) = VOC * (1 + (minTemp - 25) * Tvoc)
Example:
VOC = 30V
Tvoc = -0.3 %/C
minTemp = -10C
VOC(corrected) = 30 * (1 + ((-10 - 25) * -0.3/100)) = 33.15V