Trying to understand how multiple strings are connected to an inverter.

[bcroe]

Hi Bruce, I'm looking to power a VFD as well as a single phase inverter from a PV array. I would like to maximize the power from the array so don't want to separate the strings between the inverters. The combiner you mentioned....would it be work to connect 4 strings to it and have two outputs of which one is connected to the inverter and the other to the VFD? Basically both VFD and inverter has access to all the power from the array. would it not cause problems with mppt or any other problems?

Alex

You can use a single output combiner box to bring your strings together. You don't want
2 different loads connected to the array at the same, otherwise the MPPT or other schemes
will fight each other instead of tracking a stable operating point. If you can't change between
one or the other ONLY, you need some kind of power reserve to stabilize things, like a grid
tie or a battery that powers the loads. Bruce Roe

[Alex115]

Question

My combiner box has 2 separate output circuits. Bruce Roe[ATTACH=CONFIG]4293[/ATTACH]

Hi Bruce

Im looking to power a VFD as well as a single phase inverter from a PV array. I would like to maximize the power from the array so don't want to separate the strings between the inverters. The combiner you mentioned....would It be work to connect 4 strings to it and have two outputs of which one is connected to the inverter and the other to the VFD? Basically both VFD and inverter has access to all the power from the array. would it not cause problems with mppt or any other problems?

Alex

[inetdog]

Has anyone looked into how the optimizers work? From my understanding, the optimizers on each string need to communicate with the inverter. Does combining everything on the DC level into one input to the inverter still allow for this communication and the optimzers to work as advertised?

Also, does the input limitation of the inverter still have to cover the NEC 125% of the max output current after being combined at a fused connection in the DC combiner panel?

The optimizers and inverter either have a separate control wire for communication or more likely impose an RF signal on top of the DC to carry the information among them. The string inverter is acting as the master and is sending instructions to the individual optimizers to arrive at the optimal 360 volt input to the inverter.
And, yes, the central inverter needs to know the geometry of the optimizer array, in terms of one string or more strings and which optimizers are in which string, in order to send the right instructions.

Basically each optimizer is a DC to DC convertor which is being told by the inverter what the string current should be, and it will adjust its voltage to produce that current from the available panel power. Then the inverter more slowly adjusts the string current it is asking for to match what the panel string can produce at a set voltage of 360 volts.

[MikeG]

What about how these optimizers work?

Has anyone looked into how the optimizers work? From my understanding, the optimizers on each string need to communicate with the inverter. Does combining everything on the DC level into one input to the inverter still allow for this communication and the optimzers to work as advertised?

Also, does the input limitation of the inverter still have to cover the NEC 125% of the max output current after being combined at a fused connection in the DC combiner panel?

[bcroe]

Something else to share, SolarEdge (as do others I'm sure) has public monitoring accounts you can peek in on. This is an account from someone here in Phoenix, I LOVE how you can see what each panel is doing.

http://monitoring.solaredge.com/solaredge-web/public?name=John%20Emerson#layout_

Very interesting to see how the system works. The lighter the shade of blue, the more those panels are producing. This person seems to have most panels facing south, a few facing east and some facing north(?).

You can click and see which panels are connected to which inverter/string.

Peculiar that the east facing panels are producing very very little right now, maybe they have a shade issue or what else could it be?

For rooftop that could help. I prefer to occasionally just run around my ground mounted
system with a clamp on DC ammeter. Shadows vs output tell the story for 10 strings of
various orientations. Any failure would be obvious, with everything in pairs. Bruce Roe

[HX_Guy]

Sorry about the link. It was actually from a manufacturer rep (giving a presentation to a sales company I think). Haven't found anything nearly as detailed from SolarEdge themselves explaining how their system actually works.

Something else to share, SolarEdge (as do others I'm sure) has public monitoring accounts you can peek in on. This is an account from someone here in Phoenix, I LOVE how you can see what each panel is doing.

http://monitoring.solaredge.com/solaredge-web/public?name=John%20Emerson#layout_

Very interesting to see how the system works. The lighter the shade of blue, the more those panels are producing.
This person seems to have most panels facing south, a few facing east and some facing north(?).



You can click and see which panels are connected to which inverter/string.



Peculiar that the east facing panels are producing very very little right now, maybe they have a shade issue or what else could it be?

[HX_Guy]

Great presentation on how the power optimizers work, the technical stuff starts at 21:25 in the video:

If you find a link from the manufacturer please post it - not from sales companies.

[inetdog]

Hmm, interesting point. When talking to tech support yesterday, they said that the inverter holds the voltage at an optimum 360V at all times by adjusting the voltage of the optimizers (or the optimizers adjust their voltage based on what the inverter needs, one of the two). Looking at the optimizer spec sheet, it says it's operating output voltage is 5 - 60...which mean you could have up to as many as 72 panels (per string??!!! In theory anyway? ) or as little as 6 panels.

Does that make sense?

It sure seems to, yes.

[HX_Guy]

Hmm, interesting point. When talking to tech support yesterday, they said that the inverter holds the voltage at an optimum 360V at all times by adjusting the voltage of the optimizers (or the optimizers adjust their voltage based on what the inverter needs, one of the two). Looking at the optimizer spec sheet, it says it's operating output voltage is 5 - 60...which mean you could have up to as many as 72 panels (per string??!!! In theory anyway? ) or as little as 6 panels.

Does that make sense?

One other question (of course ), the specs of the panel say NOCT of 48º, which is Nomial Operating Cell Temperature...so when you look at calculating all those Temp Coefficient values, do you go from the 48º as a starting point or what temperature starting point do you use? Edit: Nevermind, the spec sheet says the STC is 25º C, so I guess you use that as a starting point. What is the NOCT 48º then?

[inetdog]

By the way, am I supposed to be looking at Vmp or Voc when calculating the string voltage and inverter input voltage?

The specs of my panel are:

Voltage at Max Power: 30.93 Vmp
Current at Max Power: 8.08 Imp
Open Circuit Voltage: 37.68 Voc
Short Circuit Current: 8.63 Isc
Series Fuse: 20A
Tempe Coefficient (Pmax): %/degree Cº: -0.46
Temp Coefficient (Voc): %/degree Cº: -0.34
Temp Coefficient (Isc): %/degree Cº: +0.07
NOCT: degrees C: 48

The string Vmp must be higher than the lowest supported DC input for the inverter. (which depends on the AC line voltage you connect to)
The string Voc must not be greater than the maximum safe DC input voltage for the inverter.
But all that goes out the window when you introduce optimizers into the equation. The optimizer can hold its output down below the Voc or even Vmp of the panel that is connected to it.

[HX_Guy]

By the way, am I supposed to be looking at Vmp or Voc when calculating the string voltage and inverter input voltage?

The specs of my panel are:

Voltage at Max Power: 30.93 Vmp
Current at Max Power: 8.08 Imp
Open Circuit Voltage: 37.68 Voc
Short Circuit Current: 8.63 Isc
Series Fuse: 20A
Tempe Coefficient (Pmax): %/degree Cº: -0.46
Temp Coefficient (Voc): %/degree Cº: -0.34
Temp Coefficient (Isc): %/degree Cº: +0.07
NOCT: degrees C: 48

[HX_Guy]

That's correct, the SolarEdge inverter will only work with their optimizers, you can't use regular panels with them since there is no MPPT circuitry built into the inverter.

About the shading, from what I've understood so far, you have to keep at least 6 panels unshaded and the string will still work due to something about the inverter bumping up the voltage to 60V per panel and it needs 360V to operate. Does that make sense?

And true again about the convenience thing...but sucks you can only do 5250W per string, so on their 12,400W inverters, you need 3+ strings and still need a combiner box.

[inetdog]

OK. Looks like there is no input power/current limitation that depends on whether you use one MPPT input or both. I would look at the actual installation manual to be sure though.

I guess it helps to read the whole thing. The Solar Edge inverters are designed to be used only with Solar Edge optimizers.
The two DC inputs are just two terminal block pairs, not two separate MPPT inputs. In fact the input is really probably not even a fully capable MPPT input in the same sense that it would be for a general purpose string inverter.
Instead it works together with the optimizers to get the two strings to produce the same output voltage at whatever current each string is capable of.

No need to worry about shading effects at all as long as the disparity between the two strings does not exceed the operating range of the optimizers. I have not investigated that.

The only reason that there are two sets of DC inputs is as a convenience for you to be able to connect two strings without using an external combiner box.

[HX_Guy]

The literature for the optimizers does not know what kind of inverter you are using and so cannot tell you whether or not the string will end up exceeding the the inverter input limit.
You (or your installer) have to design the system to meet all of the different limitations at the same time, because only you know what all of the components are.

They actually assume you are going to use one of their inverters (because I think you have to), which is why it's confusing.

Though it doesn't know what kind of panels you'll be using, I guess there are panels out there that could have a combined voltage of 500 or less and produce 5250 watts?

[inetdog]

Ok, here is the whole thing


The ** and *** state:


** Limited to 125% for locations where the yearly average high temperature is above 77˚F/25˚C and to 135% for locations where it is below 77˚F/25˚C
**
*** A higher current source may be used; the inverter will limit its input current to the values stated

OK. Looks like there is no input power/current limitation that depends on whether you use one MPPT input or both. I would look at the actual installation manual to be sure though.