Hello. I’m Neil Forcier, an application engineer with Agilent Technologies, and I’m going
to be talking to you about testing photovoltaics with a DC electronic load. So one great advantage
to using a DC electronic load to test photovoltaic devices is its power handling capability.
So right here, I have the Agilent N3301A. And in this half-rack 3U size, I can sink
up to 600 watts of power and a maximum of 120 amps in that small space. So the idea
here is electronic loads have high power handling capability at a low cost. So I can get those
600 watts between 4 and 5K$US.
So unlike a four-quadrant DC power supply, an electronic load provides a much more inexpensive
solution at high power handling capability. So it’s great for testing high-power/high-current
PV devices like large area cells, PV modules, PV panels, and also concentrated photovoltaics or CPV.
So, basically, I’m going to show you an example, where I’m going to run a simple test and
capture the IV curve, or the IV curve characteristics, of a PV panel. So in my test setup, I have
my DC electronic load, which I’m going to refer to as an E-load throughout the video.
I have my boost power supply, which I’ll talk about later, and explain in more detail,
what a boost power supply is. And I have my PV panel. And then, finally, I have my light source.
Now this is, in no way, the typical light source someone would use to test photovoltaics.
But for the budget of this video, it’s just a low-cost light to basically prove the functionality
of what I’m going to show you.
So you heard me mention that one of the great advantages to using a DC electronic load for
PV testing is its high power handling capabilities at a fairly low cost. And that’s compared
to four-quadrant power supply type devices. Now the other great thing about a DC electronic
load is it has the built-in voltage and current measurements, so you don’t need an outside
instrument to monitor the voltage and current.
So, for instance, Agilent’s N3301A has current and voltage measurement capabilities built-in,
in parallel, so it can measure voltage and current. Also, it’s in the form of digitizers
- voltage and current digitizers. So you can control how fast you want to go through the curve.
So, for instance, if you’re doing flash testing, you can easily cover a panel that’s 250 watts,
50 volts open-circuit voltage in ten milliseconds. Or if you want higher accuracy, you can take
longer digitized measurements, and it’ll give you more average measurements for higher
accuracy. So it’s very flexible, as far as time for measurement. So high power handling
and flexibility in measurement capability are two advantages of using the electronic load.
Okay. So before I get started on the test, let me explain the boost supply. So one characteristic
of a DC electronic load is the fact that as you get closer to 0 volts, as the voltage
drop across the electronic load leads get closer to 0 volts, its current handling capability
begins to fall. And as you get around 0 volts, its current handling capability basically drops to zero.
Now for PV devices, this is a real problem. Because when we have 0 volts across our PV device
under test, that’s when it’s in its short-circuit condition, and that’s where we want to measure
the short-circuit current flowing out of it.
Well, if the electronic load can’t sink any current at 0 volts, that’s a real problem.
So what we’re what going to use the boost supply for is basically to combat that characteristic
of electronic loads. And that’s not just for Agilent electronic loads; that’s for
all electronic loads.
So the N3301A, in particular its current handling capability, begins to drop off around 2 volts. So I’m
going to move quickly to a diagram, and show you how the boost power supply is hooked up in the test system.
So now you’re looking at a schematic representation of the test setup we have, and you can see,
on the left, the PV device is our solar panel. Connected to the PV device, all the way on
the right, is the electronic load.
And there in the middle, connected between the common of the PV device and the common
of the E-load, is the boost supply, which I pointed out. And the boost supply’s job
is to do exactly what its name says; it’s to boost the voltage potential of the PV device up.
So as I mentioned earlier, the E-load, its current handling capabilities, begin to drop
as the voltage begins to lower. So as I said for the Agilent E-load, that happens around 2 volts.
So we’re going to set our boost supply for 3 volts, just to be safe, and so
that’s going to keep our PV device at a potential of 3 volts at all times.
So if you look on the bottom here, we have some writing. And, basically, when the boost
supply is at 3 volts, and the E-load has a 3-volt drop across it, there is a 0 volt potential
across the PV device, meaning you’re in a short-circuit condition. So at that point,
when the boost supply is at 3 volts, and we have a 3-volt drop across our electronic load,
the PV device is in a short-circuit condition, and all of its current is flowing out through
the E-load and through the boost power supply.
Also, let’s say our PV device has an open-circuit voltage of 20 volts. So if our electronic
load is at a high resistance or an open condition, it’s going to have 23 volts dropped across it,
20 from the open-circuit condition of the PV device and 3 from the boost supply.
So when we do our measurements, we just have to compensate for those 3 volts.
Keep in mind though, the boost supply, as we step through the different voltages,
is not adding any current to it; it’s just boosting the voltage potential of the PV device up.
So one important consideration in this setup is when you’re using a boost supply for
PV testing with an E-load, you want to make sure that the maximum amount of current that’s
going to flow out of the PV device, that the boost supply can handle that amount of current.
For instance, if our PV device has a short-circuit current of 7 amps, our boost power supply
has to be able to handle 7 amps, because the current is flowing through the boost supply.
Other than that, our boost supply can be a pretty basic power supply. It doesn’t need anything fancy.
So now we have our panel under an illuminated light source - our flood lights. We have
our boost supply on. And so now I’m going to turn on our electronic load. So the electronic load
has very high power handling capabilities, so we’re probably going to hear the fan come on a little bit.
Okay. So I have this setup we just saw in the figure. I have our light powered on,
so our panel is illuminated. Now this panel is about a 40-watt panel. Since our flood lights
are less than an ideal source of light, we’re not going to get 40 watts out of there.
We’ll get, maybe, 12 watts.
So what I’m going to do is run this - just step through the IV curve and capture the
IV curve characteristics of the panel with the E-load. And I’m going to use Agilent’s
VEE software to do that. So I wrote a very simple program that’s going to capture the
IV curve, as well as the PV curve, and show some measurement parameters.
Okay. So here are the results of our quick test. You can see, on the left, we have some
of our parameters: our max power point, our open-circuit voltage, our fill factor,
and our short-circuit current. And then on the right, you can see the graph of our IV and PV curves.
Now, of course, these were both built using the built-in voltage and current measurement
capabilities of the electronic load. I measured 100 points for each. And, of course, we used
those voltage and current measurements to build both the IV and the PV curve.
So fairly simple - one instrument to do the loading of the PV panel as well as make the measurements.
And then we had one other additional instrument - a basic power supply to serve as our boost power supply.
So as you can see, it’s pretty simple doing the test with the electronic load. For more
information on Agilent’s DC electronic loads, see the link at the top. For more information
on Agilent’s solar test solution and other solar resources, see the middle link. There
you can find an application note entitled: “IV Curve Characterization in High-Power
Solar Cells and Modules”. It’s an application note that gives you more detail on using electronic
loads for PV testing.
And then, finally, in Electronic Design, you can find an article entitled: “A Photovoltaic
Max Power Point Tracking Algorithm for DC Electronic Loads”. And that article talks
about using an ideal max power point tracking algorithm for using electronic loads. This
is helpful for when you’re testing PV panels or concentrated photovoltaics outdoors.
The article can be found on the third link, at the bottom, or you can simply type in
MPPT algorithm electronic load into the Google search engine, and it’ll be at the top of the list
Thank you very much.