Inverter Maximum Operating Input Current?

[310 BLAZE IT]

Hello,
I am sizing a PV source circuit and I am trying to remember how code handles the DC input circuits (and why):

I have an inverter with a maximum input current of 180A. If the current exceeds this amount, my understanding was that the MPPT will adjust to a higher voltage by increasing its impedance and derate the input power while keeping the input current below this value. Using 690.8(A)(1)(a)(2) does not explicitly state that the inverter limiting the input current is the reason for allowing an exception to this rule, however I believe it should be.


If I go into SAM which is reccomended in the note below, I get a maximum hourly current of ~11.5Ax 15 strings = ~172A

Now if I crank up the DC:AC ratio, the same current appears despite the clipping losses being quite high, meaning SAM is incorrect:

the PE seal should not be needed for overloading the conductors but rather overloading the inverter in accordance with mfr requirements, so I feel this code section should be revised to reflect this and not reference SAM...

Thoughts?

 

[pv_n00b]

To calculate the PV array current you either use 690.8(A)(1) or (A)(2). The inverter input limit is used in (A)(2).

 

[310 BLAZE IT]

OK thats what I thought was the intent of the code but it isn't clear in the language:



A datasheet listed maximum input current is distinct from an 'inustry standard method maximum current calculation'. I think a third item should be added here for the next code cycle that differentiates systems which have a significantly higher DC:AC ratio and therefore reach max power before max current is able to increase from those which simply have an inverter which limits the input current.

 

[electrofelon]

I am a bit confused. 690.8(A)(1)(a)(2 is just telling you HOW to calculate the current, nothing beyond that.

 

[310 BLAZE IT]

I can use engineering judgement to see that the inverter input current is limited by the inverter, but this should be spelled out more explicitly in the code, so that when it is given on an inverter datasheet, it is able to be unquestionably used.

Based on my plots above, the suggested methods for how to calculate the maximum current for 690.8(A)(1)(a)(2) given within the informational note do not consider current clipping. Current and power clipping are different functions within the inverter and should both be considered if they exist.

 

[ggunn]

Most commercial inverter data sheets show a maximum DC current calculated as Isc X 1.25 for the inverter as a whole and per MPPT channel. Apparently there is a maximum amount of clipping they can do. I follow those guidelines irrespective of array orientation.

 

[pv_n00b]

(A)(2) covers the situation where the load device limits the amount of current it can draw from the array. (A)(1) covers the case where there is no limit on the load and the current is calculated based on the maximum amount the array can output. There is no reason to change (A)(1) to incorporate (A)(2) into it.
It's almost impossible to use (A)(2) anyway because of the OCPD requirement. I would need a fuse at the module for string inputs and at the combiner output for string circuits combined before the inverter. That's not something our currently available equipment supports.

 

[310 BLAZE IT]

Agreed on (A)(2) with OCPD not really being an option going into inverter. I feel that (A)(1)(a)(2) works for this situation, but it should be spelled out, and perhaps a 1.25% factor added. It gets complicated when analyzing this on top of even more complicated set of voltage drop calculations.

 

[tallgirl]

Most commercial inverter data sheets show a maximum DC current calculated as Isc X 1.25 for the inverter as a whole and per MPPT channel. Apparently there is a maximum amount of clipping they can do. I follow those guidelines irrespective of array orientation.

Correct.

The electronics may not be able to respond fast enough to raise the voltage into the lower current side of the MPPT window. Cloud edge and cold panel effects can add up significantly, and I've seen equipment destroyed by 150+% over rated maximum on systems that were "designed" to never exceed 100% of rated maximum.

 

[pv_n00b]

Correct.

The electronics may not be able to respond fast enough to raise the voltage into the lower current side of the MPPT window. Cloud edge and cold panel effects can add up significantly, and I've seen equipment destroyed by 150+% over rated maximum on systems that were "designed" to never exceed 100% of rated maximum.

Yup, totally agree. There are, or at least were, inverters that had hard limits in the manual for maximum output array current and that was it. Others had the maximum input current the inverter could process listed but the array maximum output could be higher. Out of the box, these inverters could usually do at least a DC/AC ratio of 120%. With the permission of the inverter manufacturer I have gone up to 150% and have heard of people going higher. Some DC coupled PV+BESS systems have very high DC/AC ratios when only looking at the PV array and the system inverters and not the battery. Special design considerations are needed for these systems to protect the inverters.

 

[310 BLAZE IT]

Edge of cloud is gone due to UL1741 ramp rate function programmed by default in all grid codes if the inverter has been built in the last 5 years

You are confusing damage from an improperly rated busbar short circuit rating probably with bifacial modules on any 1000v inverter that doesn't limit current. The inverters adjust their maximum power point to keep current below the limit, instead of riding it through on systems that are fully thermally optimized. Ever look at waveform of a faulted inverter? It's the same magnitude and duration on the DC side if there is impedance to stop the fault current. Any array fault would apply short circuit current to the sub combiner, so its extremely vulnerable if designed improperly.

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[310 BLAZE IT]

Interesting Followup

The contractor installed the wrong size conduit with no approved wire submittal so we were forced into our initial design before increasing wire size... Luckily I was able to verify both the sma core 1 and our combiner box had 90c terminal and per 110.14 had 90C on wire, lugs, and 'device' which is normally the kicker, so didn't need to use this section.

I also talked to the SAM developer and he had no clue this code article referenced his software so there is a forum post with a solution on how to display DC current with the software so you can then do manual coding or excel to calculate this minimum.

Do paid software platforms model this? How about additive with voltage drop(s)

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[pv_n00b]

I also talked to the SAM developer and he had no clue this code article referenced his software so there is a forum post with a solution on how to display DC current with the software so you can then do manual coding or excel to calculate this minimum.

Do paid software platforms model this? How about additive with voltage drop(s)

I have not looked at all the paid software but I can say that PVSyst does not provide this information. We use PVSyst for financial models and SAM to get the expected Isc values. It would be nice if PVSyst provided this information.

 

[pv_n00b]

Edge of cloud is gone due to UL1741 ramp rate function programmed by default in all grid codes if the inverter has been built in the last 5 years

I don't think 1741 has stopped natural processes. Edge of cloud effects can still increase PV module output over the STC ratings. The 125% adder to Isc is there to account for random natural processes that can increase the module output over STC. Personally, I think it's always been a bogus adder since it requires a statistically unlikely set of circumstances to come together to make it happen. Might as wall say the 125% boost is in case a small car full of clowns pulls up and they all hold up mirrors to shine more sunlight on the array. It's a leftover from the 80s when the CMP did not understand how PV modules worked and were scared of them.