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

[Alaska Man]

What you mean to say is that if each string is X volts at Y amps the combined output will be X volts at 3Y amps. The power from one string would be XY watts and from the combination you get 3XY watts.

Are you asking me or telling me?

[Volusiano]

I see, so that's how that works! I was thinking...how can you combined two or three strings of 5kW and it not come out to 10kW or 15kW total?

If you understand the fundamental of electricity, it'll be easy to see how the whole thing works.

Each panel when energized can produce a certain amount of voltage and current at its output (and these 2 parameters can vary in an IV curve). The voltage is like water pressure, current is like amount of water. Power is voltage * current, or like water flow. Energy is power * time, or like volume of water produced in a given time.

Voltages add up in series while current is forced to remain the same on a string of panels connected in series.

Currents add up in parallel while voltage is forced to remain the same across the branch of multiple strings connected in parallel.

Power can add up either in a series string panels or parallel strings of panels connected together, because Power = Voltage * Current. So as long as voltages add up (while current is fixed) or currents add up (while voltage is fixed), power is added up by either of these parameters.

If you already know all this, then ignore the stuff above. I just thought it may be helpful in case you're not familiar with the basics, which is what I gathered from your line of questioning.

[inetdog]

Are you asking me or telling me?

Telling you.
What I should have said is that what you wrote is flat out wrong, but if you change to naming the voltage of each string instead of the wattage of each string it becomes a true statement.

[inetdog]

Inetdog, can you tell by looking at the specs of of the 12400 inverter how this would be wired up if there are 48 250W panels but there can't be more than 21 panels per string (5,250W)? Would you just do 3 strings of 16 panels, going to a combiner box, and then from the combiner box to one (of the two) inputs on the inverter?

I figure that the answer to that is probably in the footnotes for the "**" and "***" in the table. That is where it would tell you whether you can apply full maximum power or current to a single input or have to split it up between the two.
The part of the table that you pasted in does not have the answer you are asking for.

[HX_Guy]

Thanks Volusiano, that's a great example...and no, I don't know jack about how electricity works.

So if voltages get added up when panels are wired in series, and you have say 16 panels in a string with each panel being 30Vmp, does that mean you have 480 volts at 8.3 amps? And 3 strings wired in parallel would still equal 480V correct, but at 25 amps? Is that compatible with the specs of the inverter in question?

I see it lists "Max Input Voltage" as 500 volts but what's confusing is that the spec sheet for the optimizers stats you can do up to 5,250W per string, which would mean 21 250W panels, which in turn means 630 volts.

[inetdog]

Thanks Volusiano, that's a great example...and no, I don't know jack about how electricity works.

So if voltages get added up when panels are wired in series, and you have say 16 panels in a string with each panel being 30Vmp, does that mean you have 480 volts at 8.3 amps? And 3 strings wired in parallel would still equal 480V correct, but at 25 amps? Is that compatible with the specs of the inverter in question?

I see it lists "Max Input Voltage" as 500 volts but what's confusing is that the spec sheet for the optimizers stats you can do up to 5,250W per string, which would mean 21 250W panels, which in turn means 630 volts.

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.

[HX_Guy]

I figure that the answer to that is probably in the footnotes for the "**" and "***" in the table. That is where it would tell you whether you can apply full maximum power or current to a single input or have to split it up between the two.
The part of the table that you pasted in does not have the answer you are asking for.

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

[Alaska Man]

Telling you.
What I should have said is that what you wrote is flat out wrong, but if you change to naming the voltage of each string instead of the wattage of each string it becomes a true statement.

Yeh I saw that after I re-read it. I was trying to explain the concept. My bad.


I'm out.

[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.

[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. 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]

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.

[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

[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]

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?