Central Inverter location

Status
Not open for further replies.
Grouch

Grouch

Senior Member
Location
New York, NY
Can the central inverter be on the rooftop? or does it have to be located in the main service room? anything in the NEC that dictates its location?
 
Grouch1980 said:
Can the central inverter be on the rooftop? or does it have to be located in the main service room? anything in the NEC that dictates its location?
Click to expand...

There is nothing in the NEC that dictates it. General requirements like equipment clearances and following manufacturers instructions apply. You may need additional disconnecting means if the inverter is not at a location where having disconnects grouped with it meets other requirements.
 
Grouch1980 said:
Can the central inverter be on the rooftop? or does it have to be located in the main service room? anything in the NEC that dictates its location?
Click to expand...

What do you mean by "central inverter"?

To me, this usually means 100kW+ inverter that is at least the size of a refrigerator, and would need to be crane-lifted in to position. You will likely need a structural assessment to determine whether the roof is strong enough to support its concentrated weight and seismic load.

I have seen people use the term "central inverter" for anything that isn't a microinverter, which would include string inverters. It's much more common to see string inverters on a rooftop, either along a roof-level wall, or on a rack to allow them to lay nearly flat where the manufacturer allows it (commonly 15 degrees).
 
No, much less than 100kw. Since I'm new to the field, I may be calling things incorrectly. I'm thinking of a small inverter (less than 100kw), with say 2 or 3 MPPT's. Would that be a central inverter, or a string inverter? I'm definitely not referring to the microinverters, which is an inverter at each solar module.
 
String inverter is usually used in contrast to microinverter. But it should not be thought to imply one for every string either.
With a combiner or multiple input terminals, a string inverter can handle multiple strings in one or more arrays.
Array inverter is a seldom used term.
 
Grouch1980 said:
Can a string inverter be considered a central inverter? When is the term 'central inverter' correct to use?
Click to expand...

The term "string inverter" indicates that it is built for individual strings (aka source circuits) to be routed directly to the inverter. Typically a number of strings that you can count on your fingers, or possibly fingers and toes together (e.g. 12 strings). Some string inverters offer the ability to use a combiner in the array field, and combine the strings in advance of connecting to the inverter. Other string inverters may strictly require string-level input connections, because the ampacity of each input terminal is only enough for the amps of an individual string, and they don't have a terminal to bypass the string-level fuses or bulkhead connectors. It is common that you see a line-up of string inverters grouped along a wall, that are paralleled with a grouped panelboard to combine their outputs. This may appear to be a reason why they are called string inverters, but the real reason for the term has to do with them being built for individual strings to connect directly to each one.

The term "central inverter" indicates that it is not built for individual strings to terminate at the inverter, but rather that the inverter design expects you to use combiners in the field, and DC feeders to route each combined output to the inverter. You'll usually see several combiners connected to the same central inverter. Some central inverters have a built-in re-combiner to fuse or breaker the outputs as they are combined again. Other central inverters may require a third-party re-combiner instead.

The exact kilowatt threshold between string inverter and central inverter has been increasing over the years, and there is some overlap. The higher you can get the DC and AC voltages, the more kilowatts you can fit in the same physical size and weight. It used to be (early 2010's) that isolation transformers were built in to the inverter, and that meant that the inverter had considerably more weight and physical size, compared to an equivalent kW rating today. Two factors that have improved the amount of power you could fit in a given weight and physical size, are the elimination of transformers in inverters by allowing for both polarities of DC to be ungrounded at equal/opposite voltages, and the increasing amount of utilization voltage governing the size of the DC string.

This also means that AC voltage plays an important role in how much power you can fit in a given physical size, since a ~15 kW inverter at 208V is about the same physical size as a ~33 kW inverter at 480V. It used to be that physical size of inverter correlated to kW rating of inverter, when there were transformers as part of the design. But now, it correlates more with the output amps of the inverter, rather than the kW rating.
 
Last edited:
Thanks for the info! So I have 3 questions:

1. Where you said "Some string inverters offer the ability to use a combiner in the array field, and combine the strings in advance of connecting to the inverter."... wouldn't that be considered a central inverter? or would the low KW rating define it as a string inverter?

2. Where you said "Some central inverters have a built-in re-combiner to fuse or breaker the outputs as they are combined again"... The re-combiner would only be looking for the DC outputs of the combiners correct? in other words, you can't connect strings directly to the re-combiner; by definition a re-combiner only accepts the outputs of other combiners?

3. A central inverter without a built-in recombiner... it can only receive one input from a combiner? So one central inverter per combiner?
 
Grouch1980 said:
1. Where you said "Some string inverters offer the ability to use a combiner in the array field, and combine the strings in advance of connecting to the inverter."... wouldn't that be considered a central inverter? or would the low KW rating define it as a string inverter?
Click to expand...

No. What defines it as a string inverter, is the fact that is it manufactured with the possibility of directly connecting individual strings, whether you choose to build it that way or not. It also has to do with the relatively few strings that it is meant to use per inverter, as opposed to hundreds of strings as is the case with a central inverter.

2. Where you said "Some central inverters have a built-in re-combiner to fuse or breaker the outputs as they are combined again"... The re-combiner would only be looking for the DC outputs of the combiners correct? in other words, you can't connect strings directly to the re-combiner; by definition a re-combiner only accepts the outputs of other combiners?
Click to expand...

Correct. With a central inverter's re-combiner, you cannot connect individual strings to it. Its fuse or breaker positions are built for hundreds of amps each, and won't have the ability to connect the #10 or #12 wire of an individual string within the lug ranges available. A re-combiner or subcombiner, is built for combining the outputs of multiple combiners, as opposed to combining individual strings.

There is a rule that requires fuses to be serviceable, so it is common to need disconnects in advance of fuses of a subcombiner, in order to de-energize their inputs, without having to walk all over the field to find the combiner. A central inverter usually has a master DC disconnect, and if built with a re-combiner, it would have fuses or breakers for each combiner input. In order to de-energize the fuses within sight, you need a DC disconnect in advance of the inverter. This is the advantage of specifying DC breaker re-combiners, if one is available for your equipment, because the disconnecting means and OCPD are integrated in the same component.

3. A central inverter without a built-in recombiner... it can only receive one input from a combiner? So one central inverter per combiner?
Click to expand...

Not necessarily. It could be that it is built for just one combiner, or it could be that it is built with the expectation that you'll use a separately-installed re-combiner adjacent to it on the pad.

I guess a 4th question... are string and central inverters all available with MPPT's? Or it depends on the manufacturer?
Click to expand...

If the inverter is built to work without DC-to-DC converters (aka optimizers), the MPPT feature will be integrated in to the inverter. By contrast, if the inverter is built to work with module-level optimizers, the MPPT feature is built in to the optimizers, and there is no MPPT in the inverter. How many MPPT zones an inverter will have, will be inverter-specific. Another term for "MPPT zone" is "input channel".

Some inverters only have one MPPT zone, while others have multiple. When connecting to a common MPPT zone, your logical layout of strings has to be a "rectangle". I.e. a uniform number of modules in a string. Otherwise, it will have to compromise on the outputs of dissimilar strings, and reduce the performance of both. When an inverter has multiple MPPT zones, each MPPT zone can have a string configuration that is independent of all the other zones. But still, within an MPPT zone, you need a uniform string size and module type. You may be able to realistically get away with mixing slightly different modules (same mfr, same product family, but slightly different power ratings) on the same MPPT zone, whose current and voltage are within close margins of one another, but for best results, use identical modules and identical strings, within any given power-processing zone. You also need to pay attention to the power and current constraints of each MPPT zone. It is best to unify the power distribution among the MPPT zones as much as practical.

What an MPPT zone is, is a section of the inverter that parallels the group of strings, and then sends it through a DC-to-DC converter, to convert the voltage to the inverter's "preferred" DC voltage, where it is internally paralleled with the outputs of all the other MPPT zones. The input of the DC-to-DC converter voltage will trace the I-V curve of the string, and dynamically search for the "sweet-spot voltage" where voltage and current "play nicely together" to optimize the output power of the collection of modules.
 
Last edited:
Great stuff. Thanks for the replies.

My only follow-up question would be for #3: "Not necessarily. It could be that it is built for just one combiner, or it could be that it is built with the expectation that you'll use a separately-installed re-combiner adjacent to it on the pad."

So in either case, the central inverter only takes one input only? Whether through a combiner or re-combiner?
 
Grouch1980 said:
Great stuff. Thanks for the replies.

My only follow-up question would be for #3: "Not necessarily. It could be that it is built for just one combiner, or it could be that it is built with the expectation that you'll use a separately-installed re-combiner adjacent to it on the pad."

So in either case, the central inverter only takes one input only? Whether through a combiner or re-combiner?
Click to expand...

A central inverter could have multiple MPPT zones. I've seen that it is more commonly the case that the entire inverter has 1 MPPT zone, for central inverters. In any case, a central inverter is built with the expectation that you combine your strings in the field, away from the inverter, and connect the combiner output DC feeders to the inverter or adjacent recombiner. It could be built for multiple combiners to be combined in the DC section of the inverter, multiple combiners to be combined in an external re-combiner, or for just one combiner input.
 
ah I see. I forgot about the inverters that have multiple MPPT's. so you can have many combiners, and their outputs connect to the MPPT's on one central inverter. Do i have that right?

back to the re-combiner... they can be adjacent or built-in? and say it's a built-in recombiner... that one output would typically go into one MPPT? So the recombiner connection to the MPPT would all be factory wired.
 
Grouch1980 said:
ah I see. I forgot about the inverters that have multiple MPPT's. so you can have many combiners, and their outputs connect to the MPPT's on one central inverter. Do i have that right?
Click to expand...

The exact breakdown of how combiners associate with MPPT zones will vary, based on the details of the equipment involved. It is likely for combiners to be deployed at a 1-to-1 assignment with the MPPT zones, and it is may be the case that multiple combiners to connect to the same power-processing zone.

Generally, you would avoid spanning multiple MPPT zones or multiple inverters with the same combiner, unless you have information that allows you to do so. A combiner could be built for two combination zones (essentially it would be two combiners in the same enclosure). Another way to do this, is if the inverter allows you to parallel two MPPT zones together. In the latter case, you would then lose the advantage of having multiple zones, and they would both collectively behave as if they were one MPPT zone of twice the power rating.

back to the re-combiner... they can be adjacent or built-in? and say it's a built-in recombiner... that one output would typically go into one MPPT? So the recombiner connection to the MPPT would all be factory wired.
Click to expand...

If there's a built-in recombiner, it would most likely be factory-wired to the inverter. This is the case, regardless of whether the combiner outputs connect to one MPPT or multiple.
 
Awesome. Thanks for all the help. yeah I figured my last set of questions is really manufacturer dependent. But this was good stuff. I'm new to the PV field, so trying to learn as much as i can.

Thanks everyone.
 
Status
Not open for further replies.
Top