Why there is a negative current flowing into solar panels

Hi,

I have a big solar farm with multiple combiner boxes are connected to a big inverter.
The inverter has a number of combiner boxes that are connected to the same DC (+) bus in the inverter. Each combiner box has 20 strings connected to it.

Let's say I have 10 combiner boxes that are connected to an inverter, and while the inverter is running I see 3 combiner boxes recording negative current while the 7 of them are recording positive current, and there is no ground fault on the inverter.
Also the negative current does fluctuate between positive and negative side and the reason is not CT error since we also verify the negative current on the other end of the cable with a clamp meter.

Just wondering if there is someone who ever seen such a phenomenon in the solar field?

Regards,

 

[IRstuff]

That would appear to be the downside of paralleling things like batteries, etc. The weaker outputs will actually pull current from the stronger outputs.

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[xnuke]

Just wondering if there is someone who ever seen such a phenomenon in the solar field?

Yes, that can happen. Sounds like you may need blocking diodes to prevent reverse current flow.

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[jfpe]

Is this a new installation? We have seen this on start ups when combiner boxes are connected with reverse polarity.

-JFPE

 

[Islander1]

Yes, it is a new installation. Was it constantly negative current or fluctuating between negative and positive?

 

[cranky108]

Did you know that panels that are in the shade, or at night, will consume energy? That is why you might need blocking diodes. But blocking diodes can consume energy while you are producing power.
So which is the greater loss, panels consuming power, or the blocking diodes?

 

[Islander1]

I understand that shaded panels will have lower output voltages and that will create some current to flow towards a panel with lower voltage from the other panels with higher voltage. But in my case, the current fluctuation happens too fast and I cannot really justify it cloud coverage.
Also it seems to happen only on the current combiner boxes rather than happening on all them.