Voltage Blocking

[oregon_phil]

There was a recent post with 5 panels West and 6 panels East on a 45deg roof using a SolarEdge inverter in one string. The OP opted to discontinue investigation into potential issues, but me being curious about high E/W pitch roofs with one string on a SE inverter, continued looking into the matter.

For those interested in free 3D modeling with shadowing, Google Sketchup 7 was available a couple of years ago. But today, Sketchup was spun off into its own business and now you have to pay for shadowing. Fortunately, somebody on the web archived the old Sketchup 7 for free download.

I went ahead and modeled the house with 45 degree roof and made a stab at the trees. I took AM and PM snapshots of the house at winter and summer solstice. In my mind, I thought the trees would be a bigger issue, but then again my models are only models and don’t show low hanging branches.

What the 3D model showed me is how the steep roof interacts with E/W panel orientation. This particular single phase SE inverters run at a nominal 380 volts. This means you need 7 panels minimum for 60 volt max optimizers and 5 panels minimum for 80 volt max optimizers. The OP’s house in question had 5 panels on the West roof but we don’t know what optimizers were used. If 60 volt optimizers were used, it seems like some voltage blocking will occur but you won’t know how much because you have nothing to compare it to.

Voltage blocking isn’t a theoretical phenomenon because a quick google search yields an extensive discussion regarding the topic.

Search and translate german to english:

Voltage Blocking bei 20kWp SolarEdge Anlage - Strings neu aufbauen?

(Voltage blocking in a 20kWp SolarEdge system - rebuild strings?)

Erfahrungsbericht zum Thema "Voltage Blocking" bei SE Anlage

(Field report on the topic of "Voltage Blocking" at SE plant)

My limited solar knowledge would steer me to use a string inverter with 2 MPPT inputs with rapid shutdown devices or ?? I guess I posted this follow up because I see a potential issue for much lower summer output due to voltage blocking. It’s frustrating because the homeowner won’t know if the system is producing less power due to voltage blocking unless the system doesn’t start up or shuts down during the day.

Just my thoughts. I still wonder what the SE designer app would have said for this system.

[SolTex]

I ask the following question not to push alternative tech nor to pimp for Enphase, but only to expand my general PV knowledge. Would the "voltage blocking" situation described above also occur if the system employed individual micro inverters on each panel? Thanx in advance for any explanation of the Yes or NO.

[SunEagle]

I ask the following question not to push alternative tech nor to pimp for Enphase, but only to expand my general PV knowledge. Would the "voltage blocking" situation described above also occur if the system employed individual micro inverters on each panel? Thanx in advance for any explanation of the Yes or NO.

Micro inverters will reduce the amount of losses due to shading but they are costly and for some more difficult to replace. The overall decision is the homeowners and how their system is wired.

[oregon_phil]

I ask the following question not to push alternative tech nor to pimp for Enphase, but only to expand my general PV knowledge. Would the "voltage blocking" situation described above also occur if the system employed individual micro inverters on each panel? Thanx in advance for any explanation of the Yes or NO.

No voltage blocking for enphase because each microinverter is not dependent on any other microinverter. In the original SE situation above, the SE inverter requires a minimum string length of 8 panels; maximum 25 panels. So you have a string of 6 panels east facing and 5 panels west facing on one string on a 45 deg roof. I suggest reading the article "Voltage Blocking bei 20kWp SolarEdge Anlage - Strings neu aufbauen?" and translate it from german to english in order to really understand the potential problem.

No voltage blocking for a string inverter with one string of 6 panels East facing on one MPPT and one string of 5 panels West facing on another MPPT. East facing and west facing panels act independently of each other when on different MPPT inputs.

Being a string inverter owner, I would have been more inclined to use the second solution of an inverter with 2 MPPT inputs.

[bcroe]

I am not sure I understand the question. But all the panels in a
simple single string need to have the same current generating
capability, to avoid going into bypass mode. That means they are
not only current matched, but have identical orientation, to the sun.

They also must have MPP and OC rating totals falling within the
inverter specs.

With equalizers, additonal restrictions apply, but panels no longer
need to be so closely matched. If for example only half the panels
are producing good power at any time, then half the equalizers must
be able to work within inverter specs while the others essentially
bypass. Bruce Roe

[oregon_phil]

Your last paragraph defines the issue. SE inverters run at 380 VDC and have string boundary conditions of 8 panels minimum, 25 panels maximum.

If 5 panels are in sun and 6 panels are in shade in one string for a single phase SE inverter operating at constant 380 vdc, there's only so much boost an optimizer will perform (either 60 or 80 vdc depending on the optimizer). If it's a 60vdc optimizer, then 300 vdc max for 5 panels. (300vdc max). Or 400vdc max for 80 vdc optimizers. I'm not a EE, but I've read enough of the literature to surmise that this particular string layout might be a problem. We can only determine if this is real problem if panel level power, voltage and current data are analyzed. If only potential solar users would ask questions before the purchase instead of after...

[oregon_phil]

SE must have realized voltage blocking is something they need to address in Europe because I noticed a three phase inverter for short strings and complex roofs. This inverter runs at 375V DC nominal vs 750V DC for a regular European three phase inverter.