testing short circuit current

[J Green Dragon B]

PVC Testing.

A solar panel can be modelled as a current source in parallel with a diode and with some internal resistances. The current sourced will be related to the level of incident light and other factors (partial shading, poor incident angle, etc.).

Each panel (PV Cell) has an internal parallel (shunt) and series resistance which means even when the output is short-circuited, there is still a load across these two resistances (effectively in parallel in short circuit conditions) for the current source in the PV Cell to create a voltage across.

Given three unknowns, i.e. Is=current source current (amps), Rsh and Rse (ohms) we need three equations (and sets of measurements) to find their values. Short circuit (s/c) and open circuit (o/c) conditions give rise to two of these, the third coming from testing with a load somewhere between 0 (s/c) and infinity (o/c) ohms. A low test resistance will draw a lot of current and should be suitably rated, as should all cables and connections (especially for short-circuit testing). Given typical values of Rsh and Rse, I'd go for a test resistance of about 100ohms. Under short circuit conditions, Isc ~ Is.

I am currently preparing an analysis of the equivalent circuit for the particular PV Cell I have, showing how to derive equations to solve for Is, Rsh and Rse. Modelling the diode and diode current Id is more complex.

[Sunking]

Statistically, an imperfect ("real world") MPPT tracker will be above Vmpp as much as below Vmpp during the bulk charging state.

Yep like hunting dove or quail with a shotgun, or hand grenades, all you got to do is get close.

You have been around long enough to know designing electrical or electronics is: Measure 3 times with a micrometer, mark it with chalk, and cut it with an axe.

[greenHouse]

Yep Vmp is a very hard target to hit as it is always in constant motion and changing. But the only time you will be operating in the Voc to Vmp range is with battery systems near the end of charge cycle as current tapers off. But let's belabor that point. All I am saying is can model and design with it being a current source, the trick it to find the right resistance to sink it into.

Statistically, an imperfect ("real world") MPPT tracker will be above Vmpp as much as below Vmpp during the bulk charging state.

Get a high-resolution logging multi-meter out and watch an array during active tracking. I've got a Fluke 289 and might upload some tracks if the sun would just come out ...

[Sunking]

Yep Vmp is a very hard target to hit as it is always in constant motion and changing. But the only time you will be operating in the Voc to Vmp range is with battery systems near the end of charge cycle as current tapers off. But let's belabor that point. All I am saying is can model and design with it being a current source, the trick it to find the right resistance to sink it into.

[greenHouse]

My point here is you are correct when you said a panel is a voltage source from Voc to Vmp ( a poor one at that). However that is a moot point because we would never want to operate a solar panel above Vmp. We are only interested in operating a solar panel Imp > Isc range and therefor makes it a current source.

I'd look at dI / dV -- this is an argument I've gotten into with a number of "continuous MPPT" fan-boys.

The change in current for voltage greater than Vmpp isn't a step function. I think the use of I-V curves is misleading and would rather see power versus voltage or current curves. The I-V curves look like a cliff, and that's why people get so jiggy about Vmpp being some kind of magic number.

[Sunking]

The best model, and Sunking will hit me when I say this, is that a PV cell is a =voltage= source from Voc to Vmpp and a =current= source from Impp to Isc.

No I am not going to hit you

As you know there is no ideal (independent) voltage or current sources right? This is simply because there is no unlimited power source. Both voltage and current sources are Dependent or Limited by the power source.

My point here is you are correct when you said a panel is a voltage source from Voc to Vmp ( a poor one at that). However that is a moot point because we would never want to operate a solar panel above Vmp. We are only interested in operating a solar panel Imp > Isc range and therefor makes it a current source.

[greenHouse]

we live in a real world, i will believe what my meters tell me before i trust a simulator, we CAN measure the Isc (or as close as the existing resistance will allow), we CAN measure the Vsc (because there is existing resistance), we can calculate the resistance based on the Vsc and Isc, we CAN calculate the power as well.

disagreeing with this is not practical in any terms for someone trying to measure short circuit current, or please show me some results to prove the point if you must

Not sure who you're responding to.

The "smiley face" diagrams are wrong, on so many levels, for physical PVs as to be useless.

The best model, and Sunking will hit me when I say this, is that a PV cell is a =voltage= source from Voc to Vmpp and a =current= source from Impp to Isc. This dual nature of physical PV cells is why the series-parallel rules for constructing arrays are the way they are.

They are also why Vmpp isn't infinity ...

[greenHouse]

[QUOTE=avandalen;16307]Don

[Sunking]

You can build a circuit to represent a solar cell for a simulator, it is easy.

[avandalen]

Right, but you can't say that the open circuit voltage of a current source is infinity. Your diagrams are great for theory, but as RifRaf said, you've got to look at the real world, not some simulator.

And there is absolutely nothing at all unsafe about leaving a solar cell open circuit.

Don’t misunderstand me. With simulation you can do a lot of tests on PV panels that are not executable in the real world. I can simulate exactly what happens with shadow, broken cells etc. But my argument was indeed theoretically.
The open circuit voltage of a current source is infinity, but solar cells are diodes with a current source, and thus the voltage is about 0.55V max. Of cause you can left open a PV panel.

[greenHouse]

Yes of course we can only measure the short circuit current, not the power or MPP.

Right, but you can't say that the open circuit voltage of a current source is infinity. Your diagrams are great for theory, but as RifRaf said, you've got to look at the real world, not some simulator.

And there is absolutely nothing at all unsafe about leaving a solar cell open circuit.

[RifRaf]

we live in a real world, i will believe what my meters tell me before i trust a simulator, we CAN measure the Isc (or as close as the existing resistance will allow), we CAN measure the Vsc (because there is existing resistance), we can calculate the resistance based on the Vsc and Isc, we CAN calculate the power as well.

disagreeing with this is not practical in any terms for someone trying to measure short circuit current, or please show me some results to prove the point if you must

[avandalen]

Hmm.. Multisim is supposed to use Spice , did you use a stock PV module, or did you use one you built yourself ?

I simulated my solar bike PV panel on sort circuit.

[avandalen]

What voltage did you used when you shorted out the array?

P = V x I

For I equal to Isc, V is equal to 0, and P also equals zero. So ...

Yes of course we can only measure the short circuit current, not the power or MPP.

[greenHouse]

I wonder what the problem is, I have made a simulation in Multisim of a whole PV panel with short circuit. Maybe I have overlooked something.

What voltage did you used when you shorted out the array?

P = V x I

For I equal to Isc, V is equal to 0, and P also equals zero. So ...