Testing High Power Solar Cells and Modules Using Agilent’s Electronic Load

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.

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