300A breaker tripping when backfed

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wwstrick

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Antigua, Guatemala
I will be headed to a site in Guatemala next week totry to find the cause of a 300A main breaker tripping when backfed even with less than 5A. The breaker is new ABB breaker that is rated for backfeeding. The site is 120/240 volt. The inverters have been configured to limit the backfed current to 160A and they are doing that. If the system is set to not "sell" energy to the grid everything runs fine. When set to sell to the grid the breaker will trip within a few hours of backfeeding to the grid. They had an old breaker there and replaced it thinking that the breaker was the cause. The new breaker is doing the same thing. I am trying to think of any potential cause to know what I am looking for in case there is nothing obvious when I get there. The site is remote and I do not want to make two trips. Does anyone have any ideas
 
What is the breaker tripping on?

Overload
GF
IT

Might want to bring monitoring equipment with you to help find out.

It's a good bet there is nothing wrong with the breaker so likely it is doing exactly what it is supposed to be doing.

Maybe an intermittent short in the invertor.
 
I should clarify that the inverters are 6 SolArk 15kW and they are combined through a Terrasmart ACC-6-100-N4 AC combiner with individual fuses for each inverter. The combiner then goes through its own breaker to the supply side tap. It would seem to me that the problem would need to be between the combiner output OCPD and the main breaker at the meter. I plan to go to the site and put monitors at the supply side tap and at the main feeder breaker. Then I will allow the system to backfeed to the grid. And then, who knows?
 
How do you know the inverters are limiting export current? What are the measurements of current at the main breaker during operation thru a sunny day til it trips? Does the actual current measured at the main breaker match what the CT's feeding the Sol-Ark say? The combined output at full 15kW production could be 375A at 240V.
Also, how/where is the PV system interconnected? If the main is 300A, how are you landing 375A of inverter capacity, on which piece of equipment? Do you have a 1-line?
 
wwstrick said:
I should clarify that the inverters are 6 SolArk 15kW and they are combined through a Terrasmart ACC-6-100-N4 AC combiner with individual fuses for each inverter. The combiner then goes through its own breaker to the supply side tap. It would seem to me that the problem would need to be between the combiner output OCPD and the main breaker at the meter. I plan to go to the site and put monitors at the supply side tap and at the main feeder breaker. Then I will allow the system to backfeed to the grid. And then, who knows?
Click to expand...

So to be clear...
The inverters are connected by supply side tap, and it's the load breaker that's tripping? Even though it's not backfed?
 
wwstrick said:
... The inverters have been configured to limit the backfed current to 160A and they are doing that. ...
Click to expand...

They've been configured to limit their total output to 160A, or to limit exports to the grid to 160A? Different things.

solarken said:
... Does the actual current measured at the main breaker match what the CT's feeding the Sol-Ark say? ...
Click to expand...

This is a good point. I wonder if it could be a CT ratio setting issue. Independently verifying the currents being reported by the Sol Ark would be one thing I'd be prepared to do.
 
As others have said, I would start out by looking for errors in CT configuration. If you replaced the breaker that is tripping, and it is still tripping, it is very likely working correctly. Use an amp clamp to confirm actual current through the breaker and see if it matches that indicated by the CT's.
 
solarken said:
How do you know the inverters are limiting export current? What are the measurements of current at the main breaker during operation thru a sunny day til it trips? Does the actual current measured at the main breaker match what the CT's feeding the Sol-Ark say? The combined output at full 15kW production could be 375A at 240V.
Also, how/where is the PV system interconnected? If the main is 300A, how are you landing 375A of inverter capacity, on which piece of equipment? Do you have a 1-line?
Click to expand...
I have measured the current at the CTs that are connected to the inverters and they match. The inverters are limiting the current according to the CTs. I will be rechecking the CTs on arrival. The system is interconnected at a point about 20 meters from the main feed breaker. The grid is fed into the inverters through a 500A breaker that is connected to a fused combiner for the six inverters.
 
PWDickerson said:
As others have said, I would start out by looking for errors in CT configuration. If you replaced the breaker that is tripping, and it is still tripping, it is very likely working correctly. Use an amp clamp to confirm actual current through the breaker and see if it matches that indicated by the CT's.
Click to expand...

jaggedben said:
They've been configured to limit their total output to 160A, or to limit exports to the grid to 160A? Different things.



This is a good point. I wonder if it could be a CT ratio setting issue. Independently verifying the currents being reported by the Sol Ark would be one thing I'd be prepared to do.
Click to expand...
The export to the grid has been limited to 160A. The CTs were reading correctly a couple of weeks ago, but I will recheck them.
 
wwstrick said:
The main breaker at the service feed is tripping and only when backfed. It will eventually trip even with the backfed current limited to 20A
Click to expand...
So it's not a supply side tap? Is the breaker that's tripping in series between the inverters and the grid, or not? If not, what is the CT arrangement to capture the backfeed?
 
jaggedben said:
So it's not a supply side tap? Is the breaker that's tripping in series between the inverters and the grid, or not? If not, what is the CT arrangement to capture the backfeed?
Click to expand...
The breaker that is tripping is the 300A main feed breaker at the service entrance. That breaker is directly connected through another breaker to the fused combiner that connects to the 6 inverters. It is in series between the inverters and the grid. The CTs are on those cables.
 
wwstrick said:
The breaker that is tripping is the 300A main feed breaker at the service entrance. That breaker is directly connected through another breaker to the fused combiner that connects to the 6 inverters. It is in series between the inverters and the grid. The CTs are on those cables.
Click to expand...
A one line drawing would facilitate the discussion.
 
The main breaker is an ABB XT4N 300A. I have not yet seen it, but it does not have GFP or RCD. The attached diagram is for an installation with 3 inverters. This one is just like it, but with 6 inverters.
 

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wwstrick said:
The main breaker is an ABB XT4N 300A. I have not yet seen it, but it does not have GFP or RCD. The attached diagram is for an installation with 3 inverters. This one is just like it, but with 6 inverters.
Click to expand...

In the diagram you attached, the CT's will only measure the current going from the inverters to the grid. But it would not include the current supplied to the loads by the inverter's load output terminals, or supplied by the grid directly to the loads in bypass mode. Wouldn't you want the CT's to measure the total net current flowing to or from the grid?
 
synchro said:
In the diagram you attached, the CT's will only measure the current going from the inverters to the grid. But it would not include the current supplied to the loads by the inverter's load output terminals, or supplied by the grid directly to the loads in bypass mode. Wouldn't you want the CT's to measure the total net current flowing to or from the grid?
Click to expand...

I don't see the need. The inverters know how much current they are outputting, and they can add or subtract what they measure at the CTs to determine what is being consumed by the loads. The diagram is how most manufacturers do it.

I do agree that if the CTs are somehow not measuring, say, half of the six inverters, you will have one sort of problem or another. But I have a feeling something else is going on.
 
wwstrick said:
The main breaker is an ABB XT4N 300A. I have not yet seen it, but it does not have GFP or RCD. The attached diagram is for an installation with 3 inverters. This one is just like it, but with 6 inverters.
Click to expand...

So the breaker that is tripping is at the spot below the meter labeled '200A Knife Disconnect, Fused'?
 
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