Article 710: Stand-Alone Systems

[Grouch]

I read through article 710 in the 2017 NEC (not a great feat, it's not even a page).

I have the following questions:
1. For 710.15(A), it states "Power supply to premises wiring systems shall be permitted to have less capacity than the calculated load. The capacity of the stand-alone supply shall be equal to or greater than the load posed by the largest single utilization equipment connected to the system." Where it says power supply or stand-alone supply, is this the ESS and the solar PV array, assuming you have both? or is this only referring to the ESS, regardless whether or not you have a PV array?

2. For 710.15(B), where it mentions 'building disconnecting means', are they referring to the back-feed breaker on the backup loads panel, assuming you are using that to interconnect the stand alone system?

and finally 3. For 710.15(D), it states "Energy storage or backup power supplies are not required." If you have no ESS, how would you have a back-up loads panel then? The solar PV array can't power that on its own without an ESS / multi-mode inverter.

Thanks again for the help!

 

[ron]

Article 710 is intimately connected to Articles 702 and 705.

Q1 - I would think that both systems, the ESS and PV together make up the "power sources", and as long as one or both can support that largest load, you are good.

Q2 - The circuit conductors terminate in the disconnecting means for that (those) power sources, so I would agree it is the backfed breaker in your application.

Q3 - 710 can apply to standby generators, fuel cells, and several types of PV inverters that can operate in island mode.

 

[tallgirl]

Article 710 is intimately connected to Articles 702 and 705.

Q1 - I would think that both systems, the ESS and PV together make up the "power sources", and as long as one or both can support that largest load, you are good.

Q2 - The circuit conductors terminate in the disconnecting means for that (those) power sources, so I would agree it is the backfed breaker in your application.

Q3 - 710 can apply to standby generators, fuel cells, and several types of PV inverters that can operate in island mode.

My only comment is that "solar PV array" is irrelevant, and what matters is the PV and ESS inverters.

 

[Grouch]

Article 710 is intimately connected to Articles 702 and 705.

Q1 - I would think that both systems, the ESS and PV together make up the "power sources", and as long as one or both can support that largest load, you are good.

Thanks. Not that I would do this, but this implies that if the ESS provides 50% of the power for the largest load, and the PV provides the other 50% of the power for that largest load, we're good?

Q3 - 710 can apply to standby generators, fuel cells, and several types of PV inverters that can operate in island mode.

Understood.

 

[Grouch]

My only comment is that "solar PV array" is irrelevant, and what matters is the PV and ESS inverters.

Are you referring to how a solar PV array can be oversized, with its rated power greater than the inverter? (where you design for a DC:AC ratio greater than 1). So the actual power to take into account is that of the inverters.

 

[pv_n00b]

Thanks. Not that I would do this, but this implies that if the ESS provides 50% of the power for the largest load, and the PV provides the other 50% of the power for that largest load, we're good?

710 is agnostic toward the power production source. So a question about how a particular power production source works is not relevant to this section. It could be the new PV-only backup system Enphase just launched with no battery. Doesn't matter.

 

[pv_n00b]

2. For 710.15(B), where it mentions 'building disconnecting means', are they referring to the back-feed breaker on the backup loads panel, assuming you are using that to interconnect the stand alone system?

The building disconnecting means will be defined in the code section specific to the type of power production system being used. The conductor sizing requirement here is intended to prevent someone from sizing the conductor based on the load if the load draws less current than the power production source can supply. The conductor has to be sized to the full output of the power production source even if the load is small.

and finally 3. For 710.15(D), it states "Energy storage or backup power supplies are not required." If you have no ESS, how would you have a back-up loads panel then? The solar PV array can't power that on its own without an ESS / multi-mode inverter.

Thanks again for the help!

Basically, it is saying you don't need to have a backup power supply or energy storage to take over if the power production source shuts down.

 

[tallgirl]

Are you referring to how a solar PV array can be oversized, with its rated power greater than the inverter? (where you design for a DC:AC ratio greater than 1). So the actual power to take into account is that of the inverters.

For a stand-alone system the PV array and inverter output may be completely unrelated depending on the type of storage, whatever is charging the storage, and the inverters.

For a grid-tied system, you will have a greater DC:AC ratio to increase the amount of time the inverters are producing their rated output.

For a standalone system the PV array is sized for average daily consumption, days of autonomy (how long before you run out of storage and would need an external energy source), depth of discharge, and availability of a backup power system. It's an entirely different collection of math and will depend on the equipment you're using.

 

[Grouch]

For a stand-alone system the PV array and inverter output may be completely unrelated depending on the type of storage, whatever is charging the storage, and the inverters.

For a grid-tied system, you will have a greater DC:AC ratio to increase the amount of time the inverters are producing their rated output.

For a standalone system the PV array is sized for average daily consumption, days of autonomy (how long before you run out of storage and would need an external energy source), depth of discharge, and availability of a backup power system. It's an entirely different collection of math and will depend on the equipment you're using.

So what you're saying is, the PV array or battery size is not what matters... it's the inverter output ratings?

 

[Carultch]

So what you're saying is, the PV array or battery size is not what matters... it's the inverter output ratings?

Yes. For design of the AC infrastructure needed to connect the inverter to the grid (the breaker, the panelboards, the disconnects, the feeders, the transformers, etc), it is irrelevant what is connected on the DC side. You could put 5kWdc on a 100kW inverter, and you would still design the AC side for the AC current associated with the inverter's full rating as specified on the datasheet, regardless of whether it will realistically occur or not. You size the AC infrastructure in terms of the maximum possible output current, whether it will ever happen or not.

You of course, cannot create energy from nothing, so given 5kWdc on a 100kW inverter, it will only produce a little less than 5kW after accounting for inefficiencies. The full 100kW inverter's current rating would be entirely hypothetical, if you were to do this in practice.

In the event that you do have a high DC:AC ratio, and you do have more available power on the DC side than the inverter can handle, what the inverter will do, is shift the voltage above the maximum power point, and "leave sunlight on the roof" in the form of thermal energy in the modules, while they operate closer to their open circuit voltage, away from their ideal point.

For a grid-tied system, you will have a greater DC:AC ratio to increase the amount of time the inverters are producing their rated output.

I would suggest "could", rather than "will". It's a design decision, subject to the hard limit of the inverter manufacturer on this ratio, for how much you saturate the inverters, and not something you necessarily would desire to do. Unless your situation is AC-constrained, and you don't have the option to add more inverter power.

 

[tallgirl]

I would suggest "could", rather than "will". It's a design decision, subject to the hard limit of the inverter manufacturer on this ratio, for how much you saturate the inverters, and not something you necessarily would desire to do. Unless your situation is AC-constrained, and you don't have the option to add more inverter power.

Obviously it depends on what the power conversion side is able to do.

Having available energy for conversion doesn't mean the inverters must use it. The financial decisions can be somewhat complex, but electrically it's a question of what the conversion hardware will do.

 

[ggunn]

I would suggest "could", rather than "will". It's a design decision, subject to the hard limit of the inverter manufacturer on this ratio, for how much you saturate the inverters, and not something you necessarily would desire to do. Unless your situation is AC-constrained, and you don't have the option to add more inverter power.

DC:AC ratios of 1.2 or so are typical in grid tied PV systems, and higher ratios are not uncommon. For several reasons PV modules in the field rarely if ever experience Standard Test Conditions, and some clipping of DC power at peak production times is not necessarily a bad thing, since the system will produce more at off peak times than it would if it were sized to never clip.

 

[Grouch]

In the event that you do have a high DC:AC ratio, and you do have more available power on the DC side than the inverter can handle, what the inverter will do, is shift the voltage above the maximum power point, and "leave sunlight on the roof" in the form of thermal energy in the modules, while they operate closer to their open circuit voltage, away from their ideal point.

Very interesting.

On a related note, what happens in this scenario? you have a PV array and inverter to provide power to the main panel and also the back-up loads panel. If the grid goes down, the PV array would be trying to deliver the same amount of power to only the back-up loads panel (let's assume we have a lot sunny days). Once the battery is fully charged, where does the excess power from the PV array go, since the main panel is disconnected, and there's also no grid to send power back to? Does the PV inverter do what you described above, and shift the voltage higher so you get less current being delivered to the back-up loads panel?

 

[wwhitney]

Once the battery is fully charged, where does the excess power from the PV array go

Any grid forming (microgrid-capable, standalone, off-grid, etc) inverter needs to balance the microgrid, and if there are other grid following (grid interactive, grid tied, etc) inverters in the microgrid, have a way to shut them off. For the case of a grid forming ESS inverter, and a grid following PV inverter, often the technique used is to raise the AC frequency; the grid following inverter is designed to taper power output with frequency, or at least just shut down once the frequency gets too high.

Cheers, Wayne

 

[Carultch]

Ultimately, the answer to where the excess energy goes, is module heating. Not to any more heating than you will have at open circuit, which the modules are already capable of withstanding. And good thing the modules can do this, because it is better for the heat to be spread out over the large area of the panels, instead of concentrated in the inverter.

The grid-forming BESS inverter will have a way to communicate its limit to the grid-following inverter, either through its communication system, or through the frequency it produces. The only thing the grid-following PV inverter can do to limit its power output, is to "leave sun on the roof". This is what it will do, regardless of what source of information is specifying its power limit. In concept, you could get the same result if you shift the IV curve toward Isc, but it is much safer to shift to Voc instead. Shifting to Voc will stagnate the charges in the modules, and keep the heat out of the wiring.

 

[ggunn]

Very interesting.

On a related note, what happens in this scenario? you have a PV array and inverter to provide power to the main panel and also the back-up loads panel. If the grid goes down, the PV array would be trying to deliver the same amount of power to only the back-up loads panel (let's assume we have a lot sunny days). Once the battery is fully charged, where does the excess power from the PV array go, since the main panel is disconnected, and there's also no grid to send power back to? Does the PV inverter do what you described above, and shift the voltage higher so you get less current being delivered to the back-up loads panel?

What he is describing is clipping, which all grid tied inverters do when the array is capable of producing more power than the inverter can convert too AC. When a system with both a battery inverter and PV inversion is running off grid the battery inverter can shift the AC frequency when the batteries get full so that PV inverter(s) will either drop off line or throttle back their production to protect the batteries. When this happens I believe that what goes on on the DC side is the same as what Carultch described (moving the MPPT point on the DC IV curve to reduce power for inverters that respond to frequency shift) but the trigger for the process is different.

 

[Grouch]

Nice. Thanks for the explanations... there's a lot going on I see behind the scenes between the multimode and PV inverters.

 

[tallgirl]

Ultimately, the answer to where the excess energy goes, is module heating. Not to any more heating than you will have at open circuit, which the modules are already capable of withstanding. And good thing the modules can do this, because it is better for the heat to be spread out over the large area of the panels, instead of concentrated in the inverter.

The grid-forming BESS inverter will have a way to communicate its limit to the grid-following inverter, either through its communication system, or through the frequency it produces. The only thing the grid-following PV inverter can do to limit its power output, is to "leave sun on the roof". This is what it will do, regardless of what source of information is specifying its power limit. In concept, you could get the same result if you shift the IV curve toward Isc, but it is much safer to shift to Voc instead. Shifting to Voc will stagnate the charges in the modules, and keep the heat out of the wiring.

I went looking for an explanation of what you've described and the answer is the temperature rise is negligible. Were you thinking that the difference between Pmpp and what's currently being produced is lost as heat?

https://www.researchgate.net/post/Temperature-rise-in-unloaded-solar-cell

See the (currently) last response.

 

[jaggedben]

I went looking for an explanation of what you've described and the answer is the temperature rise is negligible. Were you thinking that the difference between Pmpp and what's currently being produced is lost as heat?

https://www.researchgate.net/post/Temperature-rise-in-unloaded-solar-cell

See the (currently) last response.

If the energy that normally is turned into electrical energy delivered over the wiring isn't delivered, then where does it go? Doesn't it have to go somewhere? The only other possibility I can think of is that the cells would change color and reflect more of the sunlight away from the mods, but I don't think that happens.

More energy lost as heat doesn't necessarily translate to a meaningful module temperature rise, since the module also radiates heat all the time.

Those guys posting at the link you found are clearly a lot smarter than me, but I didn't see them say anything that fundamentally contradicts what Carultch said.

 

[tallgirl]

If the energy that normally is turned into electrical energy delivered over the wiring isn't delivered, then where does it go? Doesn't it have to go somewhere? The only other possibility I can think of is that the cells would change color and reflect more of the sunlight away from the mods, but I don't think that happens.

More energy lost as heat doesn't necessarily translate to a meaningful module temperature rise, since the module also radiates heat all the time.

Those guys posting at the link you found are clearly a lot smarter than me, but I didn't see them say anything that fundamentally contradicts what Carultch said.

The electrons only move when the circuit is completely.

A solar cell is essentially a diode and photons create movement in the electrons which can only go one way, in the usual fashion with diodes. They don't "have" to go anywhere.