300A breaker tripping when backfed

[wwstrick]

How did you verify the amps? Note that a clamp on ammeter can verify the amplitude but not the direction of current. Can you confirm the monitoring doesn't show anything weird for load currents? Both CTs are in the right direction?

All that asked, I'm currently leaning against the CT installation being the issue.

Do you know the response time of the Sol-Ark PCS? How sure are you that it is limiting export to 160A within a second or two after load changes? Does the tripping correspond to instances of high load or fluctuating load?

Your 10-45kW load represents a variance of something like 145A. Supposing 35kW of load dropped all at once, and this happens at the same time the inverters were exporting 160A, you are now export 305A. Or maybe more on one line? If the Sol-Ark response to this isn't faster than the breaker trip time then that could be causing the trips, given you apparently have 375A of total inverter output available.

I also wonder if the breaker trip setting is correct, as someone asked above. A lower actual breaker trip point would exacerbate what I just described.

Is it possible to limit the total output of all inverters to less than 300A (or whatever the breaker trip setting really is) regardless of load and export? That would ensure that the inverters never output enough current to trip the breaker at 300A, even if all 45kW of max load drops at once. If the issue persists after that, I think the breaker is somehow to blame, although I can't explain how if it doesn't have GFP and is otherwise suitable for backfeed.

The tripping has happened with the export limited to 20A. It is not exceeding the 300A. It just happens when exporting. It has happened at 150A and at 15A.

 

[jaggedben]

The tripping has happened with the export limited to 20A. It is not exceeding the 300A. It just happens when exporting. It has happened at 150A and at 15A.

How granular is your data or your measurements that you know it isn't exceeding some number if amps for, say, 5 seconds?

 

[wwstrick]

How granular is your data or your measurements that you know it isn't exceeding some number if amps for, say, 5 seconds?

Not very, but if it is limited at 20A and is continually reporting 20A I doubt it has hit 300A. Tomorrow I will be there with a monitor that will catch any amount of overcurrent.

 

[PWDickerson]

I downloaded the catalog for that breaker series, and it looks like that breaker has some pretty fancy capabilities. This is from the Catalog:

RP Reverse active power: With a constant trip time (t = k), this trips when the total active power – in the opposite direction of the current exceeds the set threshold.

D Directional overcurrent: This form of protection is able to recognize the direction of the current during the fault period and thus detect if the fault is upstream or downstream of the circuit-breaker. The protection, with a fixed time trip curve (t=k), intervenes with two different time delays (t7bw and t7fw), according to the current di- rection. In ring distribution networks, it enables the identification and disconnection of the area in which a fault has occurred, while maintaining operation in the rest of the installation.

I copied this from section 3/42 of the SACE TMAX XT catalog, document number 1SDC210100D0206 - 2021.04. Maybe the breaker is set to trip at relatively low values of reverse current. I don't have experience with this kind of equipment.

 

[wwstrick]

I was onsite yesterday and we found that the site owner ran the neutral cable from the meter through the breaker. The high current was on the neutral and did indeed pass 300A when the breaker tripped. We did not have a CT on the neutral and could not see that. We shut down the solar system, activated the solar bypass switch and found that neutral varied from 20A to 45A when the two legs were running 60A to 100A. When we activated backfeeding to the grid, even at a limit of 20A, the neutral cable kept increasing to where the breaker tripped. We are planning to shut off buildings on the site to isolate the source of the problem.