One string not reaching the inverter limits it still overcomes the inverter that shuts down and restarts

[Spanish Flyer]

I'm having a bit of a problem from one of my 7 strings; my two Kostal Piko MP Plus 5.02 inverters can take as much as 600 V from either of their two MPPT's and 5.500 W, but for some reason one of the strings as soon as I reach 530V and producing 4.200 W, the inverter shows an error stating the DC current as being too high, switches off and restarts after a few minutes... It will continue to do so as long as the sun is providing the same power. I have tried swapping the strigs between the two inverters, but then the other inverter will fail...

What is more, I installed a brand new Piko MP Plus 5.02 and it is the same problem.
How come not reaching the limit in voltage or watts it still makes all three inverters fail to work? the only thing I have not checked is the amps, but on DC there is not much to calculate having the voltage and watts produced...(around 8-10 Amps).

One more thing; both my Kostal inverters have been working with their strings flawlessly for more than two years (take into consideration the third brand new inverter, having the same problem).

What can go wrong with one string, other than decaying some production percentage after two years?? I'm totally baffled..

 

[timselectric]

Are your panel frames grounded well?
If a panel is leaking voltage to the frames, it could possibly cause issues.

 

[Mattb4]

What are your panels specifications and how are they connected? Since the problem seems to occur with the one string have you done voltage readings from it in full sunlight with a DMM (one that can handle +600vDC) without it connected to your MPPT? When the issue first came up was there anything being done different about the time that it started versus the previous 2 years of operation?

ETA: One more thing, have you attempted to operate with just the one problem string and not two at the same time even if wired to different inputs?

 

[740GLE]

Whats the max PV current on the SCC?

Whats the ISC of the Array?

Can you swap that array to another SCC? Maybe you just have a bad unit, warranty time.

 

[Spanish Flyer]

Are your panel frames grounded well?
If a panel is leaking voltage to the frames, it could possibly cause issues.

Hi timselectric, thanks for your comments;
Regarding the earthing of the panels, in the area where I live professional installers all agree that being a mountainous area with many Spring/Summer storms, earthing the panels on the roof make it an attractive target for them, so no one as far as I know installs the panels with an earth if the installation is on the roof.
Mind you, there is a clear possibility that at a certain stage, and after some time, any panel could derive through the frame and chassis to the roof (wood and tiles), but if anything, the production would drop drastically both in voltage and amperage.

One thing I've noticed however is that the voltage reaches its maximum quite early in the morning; right now for example (10 AM), the problematic string that brings the Kostal inverter to shut down and produces the "Too high DC current" is already reading 530 V, and basically it does not go any higher when the inverter fails... The Kostal Piko MP Plus 5.02 has a PV DC max. voltage per string at 600-750 V, so that is another baffling fact for me.

 

[timselectric]

Regarding the earthing of the panels

I was referring to grounding, not earthing.
Bonding the panel frames to your electrical grounding system.
I understand that some regions refer to this as earthing. But I want to be clear that I am not talking about a connection to a ground rod or earth spike.

What could be happening is that a panel on one string is leaking positive voltage to the frame.
And a panel from another string is leaking negative voltage to the frame. And intermittent contact between the frames is causing voltage spikes.

Since most systems have a common negative.
The string that you believe is the problem is probably the one that is leaking positive voltage.

Voltage measurements will have to be taken at the array. Between positives, negatives, and frames.

 

[Spanish Flyer]

What are your panels specifications and how are they connected? Since the problem seems to occur with the one string have you done voltage readings from it in full sunlight with a DMM (one that can handle +600vDC) without it connected to your MPPT? When the issue first came up was there anything being done different about the time that it started versus the previous 2 years of operation?

ETA: One more thing, have you attempted to operate with just the one problem string and not two at the same time even if wired to different inputs?

Hi Mattb4 and thanks for your comments,
In deed I have tried disconnecting the offender string and since the only two strings that go over the 500 Volt were on the same Kostal inverter, I immediatly swapped one of the strings to the other Kostal inverter (the rest of the inverters (DEYE and Growatt) cannot handle 500 V DC)
This is what I have found:
Now both Kostal inverters have one string that produces around 370 V and the other that goes over the 500 Volts.
And listen to this... Now BOTH inverters fail for the same reason at some stage; that means that I have two problematic ant totally independent strings, and I cannot blame the inverters since I tried with a spanking new inverter out of the box and it also fails.

I'm also sure that the panels during Spring in clear, cold days or even with some small white clouds can deliver much more than design, but still the readings I get from my meters seem they are behaving.... Or not?

Now I'm looking for an adjustable voltage/watt switch limiter that is also auto resettable and can work with DC voltages over or reaching the 600 Volts... I know such a thing does not exist, but dreaming is free...

One last question: If the voltage is not over the 600 V limit and the wattage when the inverters fail is about 4.300 W, being the limit over 5000 W, the only thing I have not been able to measure is the Amps when it fails... Could the amps in a DC device go over the roof, when V and W are within operating figures???
Thank you all once more

 

[Spanish Flyer]

I was referring to grounding, not earthing.
Bonding the panel frames to your electrical grounding system.
I understand that some regions refer to this as earthing. But I want to be clear that I am not talking about a connection to a ground rod or earth spike.

What could be happening is that a panel on one string is leaking positive voltage to the frame.
And a panel from another string is leaking negative voltage to the frame. And intermittent contact between the frames is causing voltage spikes.

Since most systems have a common negative.
The string that you believe is the problem is probably the one that is leaking positive voltage.

Voltage measurements will have to be taken at the array. Between positives, negatives, and frames.

Yes timselectric, you are right; over here in Spain to us earthing and grounding are the same and when done to the panels, my eathing means sending one ground/earth line all the way to the main junction box and connecting it to the general ground array. Actually it is "not allowed" to install a new spike to earth within the house surroundings, since between both there could be a current differencial that could produce unwanted current flow between them.
I believe that checking each string for "leaks" is a must, and I must have an electricial have a look at that ASAP
One phase and neutral 230 V AC

 

[LydMekk]

I am also in Spain. You are misunderstanding the ground spear from the rails function...
I also put a ground spear (copper) in the ground near my rails under my panels.
It is ONLY my rails/panel frames which are attached to this - nothing else.
This will NOT interfere with your house earthing where your fuse panel and inverter should be connected, ok?

I agree that other 230V stuff should not be grounded in separate copper spears as the house really should have only one point of grounding. But that is not what we are talking about there.
We are only pointing out that the rails and the frames of the PV should be grounded. Nothing else on that earth spear.

Works fine here even in heavy lightning - I am living on a high hilltop.

 

[timselectric]

I am also in Spain. You are misunderstanding the ground spear from the rails function...
I also put a ground spear (copper) in the ground near my rails under my panels.
It is ONLY my rails/panel frames which are attached to this - nothing else.
This will NOT interfere with your house earthing where your fuse panel and inverter should be connected, ok?

I agree that other 230V stuff should not be grounded in separate copper spears as the house really should have only one point of grounding. But that is not what we are talking about there.
We are only pointing out that the rails and the frames of the PV should be grounded. Nothing else on that earth spear.

Works fine here even in heavy lightning - I am living on a high hilltop.

That's actually the opposite of what I am saying.
The panel frames should be connected to the existing grounding system. With no extra auxiliary ground rods.
One grounding system for everything.

 

[timselectric]

Works fine here even in heavy lightning - I am living on a high hilltop.

It only appears to work because it hasn't been called upon to do its job, yet.
If your panels develop any leakage to the frames. This will create a voltage difference between the earth around your ground rod and the house grounding system. If anyone comes in contact with both, this is a Hazzard.

 

[Mattb4]

Hi Mattb4 and thanks for your comments,
In deed I have tried disconnecting the offender string and since the only two strings that go over the 500 Volt were on the same Kostal inverter, I immediatly swapped one of the strings to the other Kostal inverter (the rest of the inverters (DEYE and Growatt) cannot handle 500 V DC)
This is what I have found:
Now both Kostal inverters have one string that produces around 370 V and the other that goes over the 500 Volts.
And listen to this... Now BOTH inverters fail for the same reason at some stage; that means that I have two problematic ant totally independent strings, and I cannot blame the inverters since I tried with a spanking new inverter out of the box and it also fails.

I'm also sure that the panels during Spring in clear, cold days or even with some small white clouds can deliver much more than design, but still the readings I get from my meters seem they are behaving.... Or not?

Now I'm looking for an adjustable voltage/watt switch limiter that is also auto resettable and can work with DC voltages over or reaching the 600 Volts... I know such a thing does not exist, but dreaming is free...

One last question: If the voltage is not over the 600 V limit and the wattage when the inverters fail is about 4.300 W, being the limit over 5000 W, the only thing I have not been able to measure is the Amps when it fails... Could the amps in a DC device go over the roof, when V and W are within operating figures???
Thank you all once more

You did not list your panel specs or how they are connected. Since you are getting a fault on the strings that are higher voltage either they are exceeding the SCC limits or your SCC is incapable of operating up to its limits.

Regarding your last question W=VA. If you know the watts and the voltage you know the amps.

 

[Spanish Flyer]

As I mentioned earlier, my photovoltaic system goes back to 2017, when I installed my first 8 panels and a Kostal inverter I no longer have. since then the system grew year after year and now I have a total of 69 panels that range from the first 8 with 230W and 24V to the latest ones JA Solar with 455W and 48V. In this photo you can see where they go and how they are distributed; every single string has all panels connected in series.
Hope you can get a better idea with the image...