690.8(A)(1)(a)(2)

[ggunn]

Is there an on line calculator that will take module, location (NREL), and orientation data to produce results compliant with 690.8(A)(1)(a)(2)?

 

[wwhitney]

The informational note references this software, which is a free download: https://sam.nrel.gov/

Cheers, Wayne

 

[ggunn]

The informational note references this software, which is a free download: https://sam.nrel.gov/

Cheers, Wayne

Thanks, but that software is so powerful and does so many things that to do any one simple thing is not at all simple. All I need is a plot of Isc for a specific module in a specific orientation at a specific location on the summer solstice that I can use to justify exceeding the published Isc X 1.25 limit with two strings on an inverter MPPT. It only exceeds the limit by a small amount, and I'm pretty sure the orientation alone (10 degree tilt with 210 degree azimuth) would be enough to get under the limit, but justifying it is an issue.

 

[wwhitney]

So the idea is that Isc is proportional to irradiance?

PVWatts's Hourly Data export includes a "plane of the array" irradiance column for each hour, so you could use it to calculate a rolling 3 hour average column and then find the maximum value of the rolling 3 hours average.

Cheers, Wayne

 

[ggunn]

So the idea is that Isc is proportional to irradiance?

Of course. I is always proportional to irradiance.

 

[solarken]

This is an interesting topic/problem. I configured a simple system in NREL SAM, with a single module, REC370AA, and a single microinverter, Enphase IQ8H-240-72-2-US, and ran a simulation with 10Deg tilt and 210deg azimuth for my location here in Ohio. The tool does provide a graph of Isc, and a whole other bunch of values. But, not sure it will help. In my opinion, the problem is the way NEC calculates max PV ckt current based on Isc. PV ckts in the real world will not really see Isc for any significant length of time. It is a lab measured value in ideal conditions. I was surprised that SAM does not directly give DC operating current. But you could export the hrly data including the DC operating voltage and the inverter input kW into a spreadsheet and calculate the current for each hr of the year, and see what the max current is, or average over three hrs as the NEC states. The top graph is Inverter input and inverter output power, the bottom graph is module operating voltage and Isc. I just picked one day to graph that looked to have a high power production.

 

[wwhitney]

In my opinion, the problem is the way NEC calculates max PV ckt current based on Isc.

690.8(A)(1)(a)(1) does reference short circuit current, but 690.8(A)(1)(a)(2) just says "highest 3 hour current average". So that sounds like an operating current, not a short circuit current.

Cheers, Wayne

 

[ggunn]

690.8(A)(1)(a)(1) does reference short circuit current, but 690.8(A)(1)(a)(2) just says "highest 3 hour current average". So that sounds like an operating current, not a short circuit current.

If operating current is higher or lower because of irradiance, Isc will vary as well.

 

[electrofelon]

Thanks, but that software is so powerful and does so many things that to do any one simple thing is not at all simple. All I need is a plot of Isc for a specific module in a specific orientation at a specific location on the summer solstice that I can use to justify exceeding the published Isc X 1.25 limit with two strings on an inverter MPPT. It only exceeds the limit by a small amount, and I'm pretty sure the orientation alone (10 degree tilt with 210 degree azimuth) would be enough to get under the limit , but justifying it is an issue.

What about simply calculating the projected area of the module that faces the sun during the most ideal time and comparing to if it was normal to the sun?

 

[wwhitney]

What about simply calculating the projected area of the module that faces the sun during the most ideal time and comparing to if it was normal to the sun?

I.e. if the sun's location is denoted by a vector S on the unit hemisphere, and the module's normal vector is N, then computing S dot N, which will be the cosine of the angle formed by S and N.

In the northern hemisphere, if the panel's azimuth is 180 degrees and tilt angle is T, then the best case for irradiance is when the sun's azimuth is also 180. At latitude L, the sun's tilt at solar noon will between L-23.5 degrees (the earth's tilt) to L + 23.5 degrees over the course of the year. So if T is in that range, the highest irradiance is achieved, the sun does pass directly normal to the panel. Otherwise, the best case will give a factor cos (L-T-23.5) on the summer solstice.

When the panel azimuth is not 180 degrees, the above needs to be corrected, as the sun will be lower in the sky when its azimuth matches the panel azimuth than it is at solar noon. One could look more closely at the sun equation of time to figure out the correction.

However, PVWatts, and presumably SAM, compute the panel irradiance as not just the direct (beam?) irradiance, but also add a component for diffuse irradiance. I'm not sure how that's computed. So extracting the irradiance data from one of those programs is probably simpler and more accurate.

BTW, I did a quick PVWatts run for my location and 10 degree tilt, 210 degree azimuth and then briefly looked at the panel irradiance data. The highest value was over 1000 W/m^2, maybe it was 1050 W/m^2? One more reason for the 125% factor in 690.8(A)(1)(a)(2), the short circuit current may exceed STC conditions (saying whether it actually did would also require a model for cell temperature and effects on Isc).

Cheers, Wayne

 

[electrofelon]

I.e. if the sun's location is denoted by a vector S on the unit hemisphere, and the module's normal vector is N, then computing S dot N, which will be the cosine of the angle formed by S and N.

In the northern hemisphere, if the panel's azimuth is 180 degrees and tilt angle is T, then the best case for irradiance is when the sun's azimuth is also 180. At latitude L, the sun's tilt at solar noon will between L-23.5 degrees (the earth's tilt) to L + 23.5 degrees over the course of the year. So if T is in that range, the highest irradiance is achieved, the sun does pass directly normal to the panel. Otherwise, the best case will give a factor cos (L-T-23.5) on the summer solstice.

When the panel azimuth is not 180 degrees, the above needs to be corrected, as the sun will be lower in the sky when its azimuth matches the panel azimuth than it is at solar noon. One could look more closely at the sun equation of time to figure out the correction.

However, PVWatts, and presumably SAM, compute the panel irradiance as not just the direct (beam?) irradiance, but also add a component for diffuse irradiance. I'm not sure how that's computed. So extracting the irradiance data from one of those programs is probably simpler and more accurate.

BTW, I did a quick PVWatts run for my location and 10 degree tilt, 210 degree azimuth and then briefly looked at the panel irradiance data. The highest value was over 1000 W/m^2, maybe it was 1050 W/m^2? One more reason for the 125% factor in 690.8(A)(1)(a)(2), the short circuit current may exceed STC conditions (saying whether it actually did would also require a model for cell temperature and effects on Isc).

Cheers, Wayne

I wondered if the simple projected area method was too simple. So do you think it is overly conservative or under conservative?

 

[wwhitney]

I wondered if the simple projected area method was too simple. So do you think it is overly conservative or under conservative?

I think projected area is a quite exact way to compare the direct (beam) irradiance from the sun during cloud-free conditions. If you also have a way to determine the diffuse irradiance, then you can compare the total irradiance.

Cheers, Wayne

 

[solarken]

I think projected area is a quite exact way to compare the direct (beam) irradiance from the sun during cloud-free conditions. If you also have a way to determine the diffuse irradiance, then you can compare the total irradiance.

Cheers, Wayne

SAM calculates all that, direct, diffuse, and a bunch of other effects, so it seems that is the best way to get the max current, or the highest 3hr avg current. But I still think the NEC settling on Isc as the primary basis for PV current calculations is too conservative. The string current will never be as high as Isc, except for perhaps a very brief transient period, because the inverter is harvesting energy from the panel, and will not pull current as high as Isc because the voltage would be zero at Isc. I think Isc x 1.25 is too conservative for sizing conductors. Looking at the OP though, is the issue that the Isc of two parallel strings would exceed the input rating of the MPPT input on the inverter? If that is the case, maybe you don't want to approach the Isc rating of the MPPT input, since maybe the inverter could be damaged by short durations approaching Isc.

 

[wwhitney]

The string current will never be as high as Isc

How about when there's a direct + to - DC short while the array is irradiated at 1050 W/m^2?

Cheers, Wayne

 

[solarken]

How about when there's a direct + to - DC short while the array is irradiated at 1050 W/m^2?

Cheers, Wayne

Well then the current may be about 1.05 x Isc
You have a point, the NEC omits requiring OCPD for PV conductors generally because the ckt is current limited, so it may seem odd to argue that it is too conservative. But especially in many parts of the country and with nonideal array orientations, it ends up being overly conservative, since the real world Isc would likely be lower than the STC value.

 

[ggunn]

As it turns out, the inverter company clarified that we should be using Isc rather than Isc X 1.25 to comply with their maximum short circuit current limit per MPPT input, so the problem went away.

 

[electrofelon]

As it turns out, the inverter company clarified that we should be using Isc rather than Isc X 1.25 to comply with their maximum short circuit current limit per MPPT input, so the problem went away.

Hmmm but doesn't that contradict what code says?

 

[ggunn]

Hmmm but doesn't that contradict what code says?

No, it doesn't. The upper short circuit current limit on an inverter MPPT input isn't a code issue. Wire sizing and OCPD are calculated by NEC rules.

 

[electrofelon]

No, it doesn't. The upper short circuit current limit on an inverter MPPT input isn't a code issue. Wire sizing and OCPD are calculated by NEC rules.

Ok I agree. I was thinking of the statement in the voltage section that states: "The maximum voltage shall be used to determine the volt‐ age rating of conductors, cables, disconnects, overcurrent devices, and other equipment" and there is no analogous statement in 690.8 requiring the calculated current be applied to equipment.