short circuit current

 can somebody please explain to me how i can use this short circuit current
in my calculations and what it is good for.  i know electronics math, just
maybe have forgotten if ever know what this is on pv cells.  do i use it for
max calculations as to keep below that current at all times?  or what?

pardon if i sound lame at the moment.

wolf

 

I *think* PV cells can't really put out more current than short-circuit, so
you don't have to try and 'keep below that current at all times'.

Circuit protection devices (fuses and breakers) have a couple of different
ratings.  Working voltage and trip settings are probably familiar to you.
But another is current-interrupting rating.  It is important that your
protection devices have a interrupting rating *higher* than the available
short-circuit current.

For example, say you have a modest 20 amp circuit being supplied by a normal
residential service.  Something goes wrong and the wiring ends up with a
dead short.  *MUCH* more than 20 amps flows through the wire and circuit
breaker and the breaker tries to open.  If the power source's short-circuit
current is really high, when the breaker opens it may not be able to
'interrupt' the current and an arc continues passing current through the
breaker to the short.

Most breakers for AC panels used in residential service have interrupting
ratings of 10k or more (newer breakers, the '10k' is clearly visible on the
exposed body of the breaker).  That means if the shorted circuit draws as
much as 10 000 amps from the 120V line, the breaker can *still* interrupt
the current flow (at least once).

With PV, the cells short-circuit current isn't hundreds of times higher than
normal operating current, so it isn't *too* big a deal.  But batteries are
another matter.  If your pv setup has some lead-acid batteries, their
short-circuit current could be quite high (over a thousand amps).

And circuit protection devices for AC will not work nearly as well on DC.
Be sure to use circuit protection devices for DC, and that can interrupt the
maximum short circuit current that *could* occur.

This help you??

daestrom

 daestrom wrote:


Thats what I think too,but we could be wrong?

(Please correct me if I'm wrong here too..)
I think to figure the max power (W) from a solar cell/panel you measure
the short-circuit current,and then measure the open circuit voltage, a
little ohms law,multiply the current by the voltage(P=ExI),and you get
the max output in Watts.

For instance,My small panel outputs about 250ma (short circuit)and about
16V (open circuit/"no load") in good bright light.Right about 4 Watts.
(I did say small..It's an old Solec panel.)

Answer any questions?

 

Several wrong concepts were expressed here.

The product of short circuit current and open circuit voltage is not
maximum power.  The power out of a solar cell at any specific
irradiance and temperature varies with the operating voltage.
At zero volts (short circuit current) the power out is zero because
the voltage component is zero.  Likewise at open circuit voltage
the current is zero, resulting in zero power output. As the operating
voltage voltage is varied from zero to open circuit voltage, the
current varies from the short circuit current to zero.  At any specific
voltage there is a defined current.  The product of this voltage and
current is power.  This solar cell characteristic can be plotted as
a curve (called an I-V curve).  At some point along this curve the
product of voltage and current will reach a maximum.  This point
is called the maximum power point and is defined as the point
with the maximum power voltage and the maximum power current.

For a PV array, the open circuit voltage is used to determine
the voltage ratings of the system elements such as wires, fuses,
switching devices.  See the US national Electrical Code.

The short circuit is likewise used used to determine the current
 ratings (ampacity) of the system elements such as wires, fuses,
switching devices.  This is why these ratings appear on PV
module labels.



Bill Kaszeta
Photovoltaic Resources Int'l
Tempe  Arizona  USA
bill@kaszeta.org

 


Thanks for your explaination. What is somewhat confusing to me is how this
is measured, since
these would seem to me unmeasurable theoritical points unless a bit of
extrapolation is occuring,

Am I correct to assume that the measurement of open circuit voltage is
measured across very
high resistance load, and short circuit current is measured across a very
low resistance load?
Can these be measured with a simple meter, using the meters built in loads,
and the extrapolation of the meter be relied upon to closely interpolate the
results? Or is the recognized proceedure for measuring short circuit current
and open circuit voltage and plotting I-V curves something more complicated?

In measring my own panels, that were without a rating label, I simply took
voltage and current readings off or the output terminals in full sun. The
product of these measurements was about 50% higher than the expected wattage
rating of the panels, which did not suprise me. To plot IV curves, would I
then perform a series of additional measurements with the panel under
varying (meter external) loads?

Rick