Why monitor voltage?


#1

just wondering why you monitor voltage especially AC?

i was thinking if i could monitor DC voltage for each string i can see if its getting shaded or of there seems to be an issue with a panel (not sure what else you can get from the dc voltage)

what can you learn/diagnose with ac voltage?
anything else you can learn/diagnose with DC voltage?


#2

AC Voltages are typically measured at the main service panel. Since the capacity of your Utility Service is so significantly larger than a typical inverter, the voltage measured is almost completely driven by the the voltage delivered by the Utility. Therefor AC Voltage measurements provide info on the your Utility Service rather than your PV System. This could be helpful information if your inverter is going off line due to voltages outside it’s operating range.

While monitoring the DC Voltage of a string can be helpful, typical current monitoring is more useful. Utility scale systems will monitor both current and voltage and compare the values to IV curves provided by the manufacturer to determine if there are issues with the panels but this is not typical for residential systems.

I do monitor AC System voltage on my system but that’s basically a side benefit of the equipment I use to measure consumption.


#3

Monitoring the DC will depend a bit on where the reading is from, at the panels the readings are more stable (using a multi meter), but from the inverter it show DC dancing around a lot as its converting that to AC, the more amps it can convert the more the DC readings change (this is the DC reading from the inverter not with a multimeter at the terminals)


#4

B-man you have a fronius inverter??? if so just read the DC voltage straight of the inverter


#5

yeah i guess i can thought it might be interesting to see the data corresponding to when it gets shading but its not something im going to look at once the fun wears off. or does it ever :joy:

yeah i do. as above post it doesn’t sound like Im going to get anything decent from the readings anyway


#6

well with the DC voltage of a string and the amps at the same time,and with the output amps and AC voltage all from the same screen on Fronius solar web, history tab you can work out the overall Efficiency of your system


#7

The overall efficiency could also be computed from Power Input (aka PV power) and Power Output. Most inverters are capable of show those two measurements already computed (with voltage and amps you compute power than compute efficiency).

I think logging DC can tell you when something happen to block light into your panels. While DC readings do swing, the swing under load is somewhat in a ball park. For instance, one of my arrays swings in between 340 and 370 volts under load. This swing depends on the quality/quantity of light hitting panels, load (from inverter), temperature (de-rating panels), etc. But, in a sunny day with few clouds, if a cloud pass between direct sunlight and panels I can see a huge drop outside the ballpark (like 250V for some minutes). In a very very hot day, the swing range shift downs a dozen volts or shift up in a cold winter day (very rare here I must say). What I want to say is that swing in DC volts will not invalidate its readings. On the other hand, monitoring AC voltage is very very stable, here, a 220V AC will vary from 219 to 223 with very few occasions of 217V, all variations inside range proposed by regulatory agencies. No surprise at all.

In my case I am logging DC voltage on standard voltage register and AC voltage on extended register. I did the opposite at first but for me it is meaningful to see string voltage alongside generation/power quantities and AC voltage on the other page at pvoutput. - my 2 cents

BRs


#8

i have found the fronius solar web can graph that so i haven’t bothered doing anything about it i might just keep it located there for interest sake