|Troubleshooting a PV Array
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 andworking, 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 (see "Batteries: How to Keep Them
Alive" in SunPaper 1, or at our website).
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.
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.