voltage issues

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Eros

Member
Location
United States
I have a combiner box with 5 strings terminating in it. 4 of the strings are reading the proper voltage between positive - negative as well as positive - ground. One string however reads proper voltage between positive - negative but does not have a voltage reading when positive - ground is measured. Any ideas as to why this is happening ?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I have a combiner box with 5 strings terminating in it. 4 of the strings are reading the proper voltage between positive - negative as well as positive - ground. One string however reads proper voltage between positive - negative but does not have a voltage reading when positive - ground is measured. Any ideas as to why this is happening ?
The most likely cause would be a missing negative to ground bond on that string.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The modules are on a common rail system with other modules/strings that have no problem with them, and one of the rails is grounded
The rails ground the frames of the panels, but have nothing to do with connecting any DC wires from the panels to ground.
Normally both the + and the - from the panel are floating.
Depending on your system, the - wire may be grounded at some point along the way or may be left floating. The one in your case appears to have been left floating. Normally the only ground in the system, if any, would be at the inverter.
Did you disconnect each pair of wires from the combiner or at least disconnect the combiner from the inverter before measuring?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I have a combiner box with 5 strings terminating in it. 4 of the strings are reading the proper voltage between positive - negative as well as positive - ground. One string however reads proper voltage between positive - negative but does not have a voltage reading when positive - ground is measured. Any ideas as to why this is happening ?
It sounds to me like that one string doesn't have its negative conductor connected to the inverter. Assuming that it is a grounded system, the path to ground is through the GFP circuit in the inverter.
 

Eros

Member
Location
United States
Currently the + and - leeds are not connected in the combiner at all and I get proper voltage between + and - but not between + and grnd . I performed the same test on all the other strings in the combiner and I got a voltage reading between + and grnd that was correct. So why the voltage to - but no voltage to ground on this string ONLY out of 21 strings in the array ?
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Currently the + and - leeds are not connected in the combiner at all and I get proper voltage between + and - but not between + and grnd . I performed the same test on all the other strings in the combiner and I got a voltage reading between + and grnd that was correct. So why the voltage to - but no voltage to ground on this string ONLY out of 21 strings in the array ?

Since the strings were all presumably identical originally, and since there is initially no connection between - and ground on any of the panels, there must be a deliberate ground connection for each of the strings somewhere out in the array. And the wiring of the one string was done differently or has a broken wire.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Currently the + and - leeds are not connected in the combiner at all and I get proper voltage between + and - but not between + and grnd . I performed the same test on all the other strings in the combiner and I got a voltage reading between + and grnd that was correct. So why the voltage to - but no voltage to ground on this string ONLY out of 21 strings in the array ?
If all your wiring is floating you should not get a voltage reading to ground on any of it. Are there fuses on both the pos and neg conductors or only on the positive ones?

Let me back up a bit... is your inverter transformerless?
 

Eros

Member
Location
United States
The only floating wires are the pos and neg of the problem string. The combiner goes to a charge controller then to a hybrid grid/off grid outback inverter, all of the equipment grounds terminate to a common point. Can't be sure about the transformer. I do recall that the panel had to be installed last in the array which was between panels, could a improperly installed mid clamp(s) cause the issues ? If the wires were broken why would I have voltage from + to -
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The only floating wires are the pos and neg of the problem string. The combiner goes to a charge controller then to a hybrid grid/off grid outback inverter, all of the equipment grounds terminate to a common point. Can't be sure about the transformer. I do recall that the panel had to be installed last in the array which was between panels, could a improperly installed mid clamp(s) cause the issues ? If the wires were broken why would I have voltage from + to -
If the negative wire from a string is not connected to ground somehow - usually through the inverter or charge controller - then you should not see voltage to ground from the positive wire.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
We really should have known from the beginning if this is an (intentionally) grounded system or not. In your last post you finally mentioned Outback, whose systems AFAIK are all intentionally negatively grounded, so I'll proceed on that assumption.

Since the negative-ground connection is at the inverter, any string that does not have it's negative conductor properly connected back to the inverter won't have the negative conductor grounded, and thus no positive to ground-voltage (unless there's also a ground fault on the negative conductor of that string). If you're measuring + to - on the string side of the broken connection, you'll get voltage, which I think is the most likely explanation for your situation. If you measure on the inverter side of the broken connection, you won't get voltage. Since the other strings seem to be grounded via the inverter, the break must be between the combiner busbar and wherever you are measuring. (Golddigger sort of said this.)

Are you measuring at connectors for this string right outside the combiner? If so, did someone install one of them improperly so that they don't make contact when plugged in? When this happens you can often measure the right voltage with your meter when unplugged, but it still doesn't work when plugged.

There are other more far-flung possibilities, involving multiple ground-faults combined with bad connections, but unless you tell us more that doesn't seem worth discussing.

It may also be worth noting that in grounded systems you cannot properly diagnose many string problems unless all negative and positive conductors are disconnected from the combiner busbars. It's usually easy to do the positives with fuses, but the negatives require more planning ahead, such as installing extra connectors outside the combiner.
 
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