Amp Question

joczr2 · February 2017

Hi All. New here. I have 14 solar panels, each rated at 240 watts (30.2 Vmp, 7.95 Imp) being fed into a Frontius Primo 6000 Watt grid tie inverter. With the sun perpendicular to the panels, I've only been able to get 800 watts at the most from the inverter readout. The usual is anywhere from 300-500. I have tested each panels voltage and get ~25 to 27 volts. I should be producing at least 3000 Watts. The cable run from the panels to the inverter is ~60ft. I also used a DC amp clamp on one of the cables as it goes into the house and was getting around 0.9 amps. This is baffling me. Is it possible that a panel can produce its rated voltage but not its rated amps. Our house uses a lot of amps so the need is there I just don't understand. I would sincerely appreciate any help.

BB. · February 2017

Assuming this inverter?

https://www.solar-electric.com/fronius-primo-6000-watt-grid-tie-inverter.html

Recommended PV Power: 4,800 - 9,300 Watts
Nominal Input Voltage: 420 Volts DC
Dual MPPT Inputs
Operating Voltage Range: 80 - 600 Volts DC
DC Conductor Sizes Accepted: #14 AWG to #6 AWG
AC Conductor Sizes Accepted: #14 AWG to #6 AWG
Maximum Output Power: 6,000 Watts
Max. Output Current: 28.8A @ 208V and 25A @ 240V
Grid Connection: 208, 240 Volts single phase
Frequency: 60 Hz
Power Factor Range: 0.85 - 1 inductive/capacitive
Protection Class: NEMA 4X for indoor or outdoor locations
Dimensions (height x width x depth): 24.7 x 16.9 x 8.1 inches
Weight: 47.29 lbs.
Cooling: Variable speed fan
DC Disconnect is included
Built-in Wi-Fi capability

And the solar panels:

14x 240 watts (30.2 Vmp, 7.95 Imp)

Are the panels wired 1x series string of 14 panels?

-Bill

mcgivor · February 2017

To add to Bill's question, is there any shading on any of the panels? This would dramatically reduce output, even if it's partial, on one panel of the series string. Ruling that out of the equation perhaps one, or more panels could bad, dragging the others down, this would require testing each panel individually with a load to verify production of current, not just voltage.

Photowhit · February 2017

joczr2 said:

Hi All. New here. I have 14 solar panels, each rated at 240 watts (30.2 Vmp, 7.95 Imp) being fed into a Frontius Primo 6000 Watt grid tie inverter. With the sun perpendicular to the panels,.........

............I have tested each panels voltage and get ~25 to 27 volts.........

.............. I should be producing at least 3000 Watts.

In use 14 panels at 240 watts should produce roughly 3360 watts (panel rating) x .75 (derating for Normal Operating Cell Temperature) = 2520 watts. I would expect production in this range, not 3000 watts. If the installer did not inform you of this I would be upset.

To me, assuming there is no shading and they are hooked up in a single string that you have a panel with a couple bad internal strings or diodes, I think normal 60 or 72 cell panels are made up of 3 strings of cells with diodes;

bypass diodes protect prevent damage

A single panel with bad strings will limit the whole string of connected panels.

joczr2 · February 2017

Bill. Thanks for the reply. That is the correct inverter. The 14 panels are wired as one string. The inverter salesman advised me to use one string as the inverter required a certain amount of voltage to start. Two string would not be enough.

There is no shading whatsoever on the panels.

Photowhit, I appreciate your comment. How would I troubleshoot the strings or diodes? If each panel puts out the rated voltage, could I still have a bad panel?

Thanks again, I do appreciate the input.

mcgivor · February 2017

The voltage means little, if a panel is damaged, or has shorted diodes, it could still put out voltage, which is why a load or short circuit amp/current test is needed, to verify the current output. Disconnect 1 panel at a time, put your clamp on ammeter over 1 wire, connect +to- and see what current you read, should be close, about 75% of the nameplate short circuit amp ratings.

Estragon · February 2017

Wouldn't the voltage when testing each panel be Voc? If so, isn't an open circuit voltage of 25v pretty low unless the panel is really hot?

Photowhit · February 2017

I don't know of any way to check them, with out load testing each panel.

I'm trying to think of something like a 24 volt DC motor, I think some battery tool motor might work, but they don't draw enough wattage unless under load them selves. Something that you could take with you to put a load on the panel and measure output.

I'm pretty stumped, They make specific DC heating elements for diversion loads, some sort of pure resistance load, like a heating element. An AC element would run at about 1/16th the wattage with 30 volts DC than 120 volt AC so even a 1500 watt heater wouldn't draw enough to test a panel.

I guess just shorting them out, the wire will act as a heating element, and measure the current circulating. I understand the current will flow, and I can't understand why this wouldn't be a valid test, but don't know. There really isn't much resistance.

Any one have any ideas?

mvas · February 2017

mcgivor said:

The voltage means little, if a panel is damaged, or has shorted diodes, it could still put out voltage, which is why a load or short circuit amp/current test is needed, to verify the current output. Disconnect 1 panel at a time, put your clamp on ammeter over 1 wire, connect +to- and see what current you read, should be close, about 75% of the nameplate short circuit amp ratings.

Agreed, you need to simply measure each panel both ways:
1) Voc with DMM in Volts mode
2) Isc with DMM in Amps mode
The bad panel will be obviously different than the good panels.

No other external "load" is required

Estragon · February 2017

As I understand it, you don't want or need a load for a short circuit current test. That's why it's call Isc. The only load is the small resistance in the (hopefully properly sized) wire so power and voltage will be nearly zero, and current will be whatever your clamp meter says.

BB. · February 2017

What is the string voltage (and current) as reported by your inverter in the middle of the day?

Also, any shading, including power lines, can dramatically reduce output.

If your rated Vmp~30 volts, then Voc should be closer to ~35 volts.

What is the brand/model of your panels?

-Bill

joczr2 · February 2017

BB. The DC voltage coming into the inverter is ~600. I was wrong in my earlier comment that the panels put out around 25 V. They put out about 32V. No shading. The panels are:

UpSolar UP-M240P

Vmp 30.2

Imp 7.95

Voc 37.6

Isc 8.40

BB. · February 2017

In the middle of the day, you should be seeing something around:

30.2 volts * 14 panels = 423 volts (cold/bright day)
30.2 volts * 14 panels * 0.81 hot panel derating = 342 volts (hot/noontime sunny day)

Your system should not be reading 600 vdc at all during anything near normal operation.

-Bill

Estragon · February 2017

@BB - I don't know anything about this inverter, but could it be boosting mppt dc voltage to 600v for inverting to AC internally, and that's where the 600v reading comes from?

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