OCPD operation in islanded systems with low available fault current

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pv_n00b

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When designing PV+BESS system that offers an island operation mode, how are people dealing with fault clearing by OCPD when the available fault current from the PV and BESS are much lower than what is available from the utility? For instance, if I have a 100A feeder CB in a panel and there is a fault on the feeder it is protecting the utility might provide 10kA of fault current and trip the CB very fast. But with the utility disconnected and the feeder powered only from the PV and BESS it might see only a few hundred amps of fault current. So it will eventually trip but it will take a long time and maybe the PV or BESS inverters shut down before the CB clears the fault. How can I restore that nice fast trip time when I have such a low available fault current?
 
Not sure how you could address that. Getting OCPDs to clear quickly requires enough current to be in the instantaneous range of the trip curve. A long duraration fault will often result in much more damage to the electrical equipment than a higher current fault of the same type that flows enough current to get into the instantaneous trip range of the breaker.
 
That's the conundrum. I still want fault isolation, but it's more difficult without the high available fault current. With low fault current, our standard system in the USA does not work well. To find a low current ground fault I need something that looks at residual current. But to catch a L-L or 3L fault I'm not sure.
 
pv_n00b said:
When designing PV+BESS system that offers an island operation mode, how are people dealing with fault clearing by OCPD when the available fault current from the PV and BESS are much lower than what is available from the utility? For instance, if I have a 100A feeder CB in a panel and there is a fault on the feeder it is protecting the utility might provide 10kA of fault current and trip the CB very fast. But with the utility disconnected and the feeder powered only from the PV and BESS it might see only a few hundred amps of fault current. So it will eventually trip but it will take a long time and maybe the PV or BESS inverters shut down before the CB clears the fault. How can I restore that nice fast trip time when I have such a low available fault current?
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I would think that inverters would shut down quickly if the impedance of a fault was low enough that it would trip the magnetic component of a breaker that's connected to the utility. I believe inverters can monitor the voltage developed on their output and shut down if that voltage is too low (as a result of a low impedance fault) when the inverter is delivering more than a certain current level. Such operation would be similar to the "foldback" used in DC power supplies to protect equipment and the supplies themselves.
That said, I don't know the details of the internal circuitry within commercially available inverters.
 
I very much doubt that with commercially available inverters you will get a meaningfully longer fault in the first instance. The problem I see is that such inverters tend to be programmed to restart and try again, on the presumption that the shut down was simply from too much load and that the user will remove the load. So if the arc from a fault doesn't blow itself open in the first instance, and also doesn't trip the breaker, it could repeat every few seconds or minutes. Or hours.
 
synchro said:
I would think that inverters would shut down quickly if the impedance of a fault was low enough that it would trip the magnetic component of a breaker that's connected to the utility. I believe inverters can monitor the voltage developed on their output and shut down if that voltage is too low (as a result of a low impedance fault) when the inverter is delivering more than a certain current level. Such operation would be similar to the "foldback" used in DC power supplies to protect equipment and the supplies themselves.
That said, I don't know the details of the internal circuitry within commercially available inverters.
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"It depends".

There's no guarantee that a not-so-bolted short (non-arcing) on a single circuit on a high enough output system will pull the output voltage of the inverters down. The largest battery-based system I ever worked on was 36kVA. A 40 amp 120VAC "load" is noise. It would seem paying closer attention to the trip curves would be better than relying on the inverters to shutdown.
 
synchro said:
I would think that inverters would shut down quickly if the impedance of a fault was low enough that it would trip the magnetic component of a breaker that's connected to the utility. I believe inverters can monitor the voltage developed on their output and shut down if that voltage is too low (as a result of a low impedance fault) when the inverter is delivering more than a certain current level. Such operation would be similar to the "foldback" used in DC power supplies to protect equipment and the supplies themselves.
That said, I don't know the details of the internal circuitry within commercially available inverters.
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I'm hoping to rely on low voltage ride through to keep everything online until the fault clears.
 
Imagine a fault which causes 5kA to flow from the utility and trips the breaker rapidly.

Now imagine the same fault where the PV+BESS system can supply 400A and clearing time might be 10s of seconds.

What has to happen to the system voltage until the fault clears?

-Jon
 
Also to add, these will be PV + BESS microgrid systems in the range of 500kVA-1MVA/2MVAhr+ systems. Commercial building microgrids. From what I can find people are not really digging into the protection issues in these systems when operating in microgrid island mode with low available fault current.
 
winnie said:
Imagine a fault which causes 5kA to flow from the utility and trips the breaker rapidly.

Now imagine the same fault where the PV+BESS system can supply 400A and clearing time might be 10s of seconds.

What has to happen to the system voltage until the fault clears?

-Jon
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Depends on the impedance of the fault. While most fault analysis assumes a bolted fault most real faults are line to ground with some impedance in the fault connection.
 
So how do island mode inverters typically behave? Hopefully I am formulating the follow question properly:

Say you have an island-mode inverter with a rating of I0 amps and V0 volts (so a VA rating of I0V0). When it sees an impedance X at its output of not less than X0 = V0/I0, it puts out a voltage V = V0 and a current I = V/X. How do the output I and V change as the load impedance falls below X0, towards 0?

Cheers, Wayne
 
pv_n00b said:
Also to add, these will be PV + BESS microgrid systems in the range of 500kVA-1MVA/2MVAhr+ systems. Commercial building microgrids. From what I can find people are not really digging into the protection issues in these systems when operating in microgrid island mode with low available fault current.
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I think residential systems are at bigger risk because the available currents are lower.

So long as the inverters have a higher output than the not-quite-bolted short, the fault may not clear and the inverter may not trip.
 
wwhitney said:
So how do island mode inverters typically behave? Hopefully I am formulating the follow question properly:

Say you have an island-mode inverter with a rating of I0 amps and V0 volts (so a VA rating of I0V0). When it sees an impedance X at its output of not less than X0 = V0/I0, it puts out a voltage V = V0 and a current I = V/X. How do the output I and V change as the load impedance falls below X0, towards 0?

Cheers, Wayne
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It depends. Remember that inverters are digital equipment. They just keep making their volts and amps as long as the "load" is within their ratings.

For small batter capacity systems they may have a low battery fault cause the inverter to shutdown, but assume the worst -- fully charged battery with sufficient capacity and available current to supply the inverters until the fault clears or the breaker (finally) trips.
 
winnie said:
Imagine a fault which causes 5kA to flow from the utility and trips the breaker rapidly.

Now imagine the same fault where the PV+BESS system can supply 400A and clearing time might be 10s of seconds.

What has to happen to the system voltage until the fault clears?

-Jon
Click to expand...
The inverters probably cut out when the voltage reaches some percentage of nominal which probably happens in a handful of cycles. So no worry about faults being fed for 10s of seconds at a time. But as I said above, the system may try again to supply power and the conventional OCPDs in the circuit may still all be closed. Wash, rinse, repeat until human intervention occurs?
 
tallgirl said:
So long as the inverters have a higher output than the not-quite-bolted short, the fault may not clear and the inverter may not trip.
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I dunno for sure, but I think faults in that category are both the least likely and the least problematic. I mean, if I have a fault that doesn't overload the inverters in a residential setting, then the breakers are not going to quickly trip on utility power either. In some theoretical cases the situation be safer off-grid because inverters will shut down when breakers would never trip. But I also think such 'resistive faults' are few and far between.
 
jaggedben said:
The inverters probably cut out when the voltage reaches some percentage of nominal which probably happens in a handful of cycles. So no worry about faults being fed for 10s of seconds at a time. But as I said above, the system may try again to supply power and the conventional OCPDs in the circuit may still all be closed. Wash, rinse, repeat until human intervention occurs?
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Why would they do that? So long as they are at or below rated output, they just keep putting out the same volts. There will be V = IR losses at the load, but the inverter terminals still have the set output voltage.
 
tallgirl said:
Why would they do that? So long as they are at or below rated output, they just keep putting out the same volts. There will be V = IR losses at the load, but the inverter terminals still have the set output voltage.
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I was responding to a post where Jon stipulated a fault that would draw more than 12 times the inverter rating. No listed inverter is going to maintain the set output voltage at 12 times its current rating.

Perhaps pvnoob's situation with commercial is different, but in the resi work I do I'm near certain that any fault that isn't too much to shut down the inverters is in the thermal (not magnetic) range of any OCPDs. In other words, longer clearing times no matter if the source is utility or microgrid. It's faults that are over the inverters ability to keep outputting voltage where the behavior will be different.
 
Low fault current faults are what drive the adoption of AFCI protection. The arc fault current is not high enough to trip a normal CB before a fire starts and can even be low enough to fall in the normal load current area and not trip at all. So special protection is added to make sure that whenever you vacuum the circuit is protected from the vacuum.
 
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