By Windy Dankoff
Photovoltaic modules are so reliable that we forget that
things can go wrong! The real world imposes temperature
extremes, lightning and static electricity, moisture and
wind stresses, as well as imperfect manufacturing. Here are
some suggestions for testing and troubleshooting.
Selective shading test - If the array is in a parallel or
series-parallel configuration, this trick will help you
locate a fault without disconnecting any wiring. Find an
object that is large enough to shade at least 4 cells. (A
cowboy hat will do.) Shading just a few cells will drop the
module's output to less than half. With the array connected
and working, monitor the current (or in the case of a
nearby solar pump, just listen to it). Now, shade a portion
of one module. You should see the current should drop
noticeably (or the pump should slow down). If the current
does NOT drop, then the module that you are shading is out
of the circuit. Look for a fault in the wiring of that
module, or of another module that is wired in series with
it.
Fading in the heat
------------------
Occasionally somebody complains of reduced array output
when the sun is hottest. Heat fade shows up most severely
in battery systems. If the difference between the array
voltage and the battery voltage approaches zero, then
current flow can drop nearly to zero. This can also cause a
solar pump to produce less than it should.
The voltage of a PV module normally decreases with
temperature rise. PV manufacturers document this by showing
several lines on the IV curve (the graph of amps vs.
volts), or by stating it in volts per degree of deviation
from 25�C (77�F). Nominal "12 volt" PV modules are designed
to sustain good current flow all the way to 17 or 18V at 25�
C. This allows for voltage drop at higher temperatures. If
heat fade is severe, it MAY be caused by weak PV modules or
by any other weak links in the power chain, including
undersized wiring, poor connections and controller losses.
Here are some tests to isolate these factors.
First, you can confirm heat fading by cooling the array
with water while the system is operating. Monitor the
current. Does it rise to normal? If so, you need to
determine where the voltage drop is severe. Connect a
voltmeter directly to the PV array (or it's combiner box).
Disconnect the array from the controller, in order to read
the open circuit voltage. If it is less than 18V (relative
to a 12V configuration), then part or all of the PV array
may be defective. The selective shading test (above) can
help you locate weaker modules in an array.
Next, reconnect the array to the system. Under good
sunlight, test for voltage drop in the wiring by measuring
the voltage at the array, and then again at the controller
input. Note that voltage drop in wiring will increase in
proportion to the current flow. Next, test for drop in the
controller by measuring the voltage at its PV input, and
then at its battery terminals. Remember, if the battery is
fully charged, the controller SHOULD drop the voltage. If
that is the case, you can bring down the battery voltage by
turning loads on. When the battery is at less than 13.5V
(relative to a 12V system), the controller should allow
full current to flow.
If voltage drop occurs at a single point (at a connector or
within the controller) then concentrated heat will result.
You may feel it, or see signs of heat damage. If voltage
drop is evident at the loads (dimming lights, low voltage
disconnection when batteries are not low) then check for
corroded battery connections.
Burnt terminals
---------------
Years of temperature cycling will occasionally cause a
screw to loosen, or metal to distort. This can be caused by
poor workmanship and/or inferior materials. Add a touch of
oxidation and corrosion, and you get electrical resistance.
Now, keep the current flowing and you get even more heat.
When you repair overheated connections, replace all metal
parts that have been severely oxidized. In worst cases, an
electric arc will jump a gap, melting metal and burning
insulation to a char. Charred terminals on PV modules can
be bypassed by soldering a wire directly to the metal strip
that leads to the PV cells.
Diode failures
--------------
Most PV modules have bypass diodes in the junction boxes,
to protect cells from overheating if there is a sustained
partial shade on them. On rare occasions a diode will fail,
usually as a result of lightning. Most often, it will short
out and reduce the module's voltage drastically. (A shorted
diode will read near-zero ohms in both directions.) If the
module is in a 12V array, there is no need for the bypass
diode so you can remove it. In a 24V array that is unlikely
to experience sustained partial shading, you can remove it.
In any other case, replace it with a silicon diode with an
amps rating at or above the module's maximum current, and
with a voltage rating of 400V or more.
