DC trunkage

[jaggedben]

Differing string voltages is by no means a non-issue. Older inverters would do okay finding the maximum powerpoint voltage with multiple strings paralleled together. But overall system production would suffer. In small residential systems it was also constraining if you couldn't make all the strings the same length and face them in the same direction. In utility scale systems, my understanding is that the biasing at higher voltages is pretty detrimental to the panels longevity, although to my knowledge it's still standard practice on the largest systems where O&M labor costs are actually cheaper than spending more $$$ on electronics for every panel and losing efficiency therein.

There are other, arguably bigger factors that also drove the adoption of micro-inverters and DC optimizers in residential and commercial, mostly having to do with safety (not working with uncontrolled high DC voltage) and the rapid shutdown requirement. But string inverters had already evolved in the direction of multiple MPPTs before rapid shutdown, because it increased system production and design flexibility. Not exactly one inverter/string, but no longer paralleling as many strings on a common bus before going to the inverter input.

 

[wwhitney]

Differing string voltages is by no means a non-issue.

Sure, but what about differences in string voltage among identical length strings of identical panels in identical orientations? Due to random variation in manufacturing. That is the case that I was referring to as something I expected to be a non-issue. [In that in my example I specified two strings of the same length.]

Cheers, Wayne

 

[electrofelon]

My understanding is that in practice this is a non-issue, but perhaps those with some actual experience could confirm. : - )

Cheers, Wayne

But there's no guarantee that 2 sets of panels will have the same output voltage. I suspect that's part of the move towards one inverter/string.

In a perfect world it would be one string per MPPT input. SMA sunny boy inverters, for example, have 3. Three phase string inverters have 6 or more. I worked on many 300kw-3MW systems over several years and they all utilized string inverters with one string per MPPT. I don't know to what extent the designers wanted to avoid combining same length strings.

I never worked on a center inverter system, but a friend of mine who is just told me they are using a DC combiner to combine sets of 27 1500VDC strings. I can contest to the extra labor of installing a zillion string inverters, so I suspect they get better value from the labor savings even if they are losing some production by MPPTing every string.

 

[jaggedben]

Sure, but what about differences in string voltage among identical length strings of identical panels in identical orientations? Due to random variation in manufacturing. That is the case that I was referring to as something I expected to be a non-issue. [In that in my example I specified two strings of the same length.]

Cheers, Wayne

You still have to consider soiling, shade, and internal module failures, on top of manufacturing tolerances. Again, these things are better dealt with on very large ground mounts than on residential buildings or many commercial ones. I don't do utility scale solar, but from what I've read and seen at trade shows, string level monitoring is being sold as a solution for these kinds of issues. If you have dozens of strings tied to the same bus you don't want a failure in one of them to drag the whole system down or cause further damage to that string over time. At utility scale it may be sensible to address such issues with service calls. On residential you want the systems to never need this kind of attention, because a single service call consumes so much more of the return on investment.

 

[electrofelon]

By the way I meant "attest" not "contest" in my previous post

 

[wwhitney]

You still have to consider soiling, shade, and internal module failures, on top of manufacturing tolerances. Again, these things are better dealt with on very large ground mounts than on residential buildings or many commercial ones. I don't do utility scale solar, but from what I've read and seen at trade shows, string level monitoring is being sold as a solution for these kinds of issues. If you have dozens of strings tied to the same bus you don't want a failure in one of them to drag the whole system down or cause further damage to that string over time.

Thanks for the explanation.

Cheers, Wayne

 

[electrofelon]

You still have to consider soiling, shade, and internal module failures, on top of manufacturing tolerances. Again, these things are better dealt with on very large ground mounts than on residential buildings or many commercial ones. I don't do utility scale solar, but from what I've read and seen at trade shows, string level monitoring is being sold as a solution for these kinds of issues. If you have dozens of strings tied to the same bus you don't want a failure in one of them to drag the whole system down or cause further damage to that string over time. At utility scale it may be sensible to address such issues with service calls. On residential you want the systems to never need this kind of attention, because a single service call consumes so much more of the return on investment.

It's an interesting discussion. From the utility scale side, my perspective was from that of an installer and I saw saw string inverters as horrible from a labor perspective. This one project, about 2.7 MW, had something like 70 inverters. The amount of time to mount and connect all those was was very high. Plus pulling all the string home runs....... Central inverters fed with some number of combined strings seemed so much more efficient labor wise. But.....as you say there is the perspective of string level monitoring and easier locating and isolation of problems which the string inverters will do.