Xantrex versus Sunny Boy [RE-wrenches]

Bill Brooks billb at endecon.com
Fri Aug 1 14:26:42 PDT 2003


Kent,

To be crystal clear, this is not at all referring to modules in series being
in two or three orientations. I believe both you and Matt are
misunderstanding the original question. Marco was clear that he understood
that multiple orientation in a single series string to be a Cardinal sin
(which it generally is, but the confessional is open).

Both you and Matt L. are overstating the multiple orientation issue.
Although it is best to have all subarrays in the same orientation going into
an inverter, when the strings are in PARALLEL this is really not a major
issue. When in parallel, the module currents are additive (picture a bunch
of I-V curves of differing current magnitudes stacked on top of one
another).

Since the inverter can only pick one voltage to run at, the big question is
what is the voltage of all three orientations. Only an I-V curve tracer can
tell the exact numbers, but think about what is happening. The highest
irradiance orientation will have the highest temperature and therefore the
lowest voltage per irradiance. The higher irradiance on that string will
produce a slightly higher operating voltage providing some compensation for
the high operating temperature. Going around to each string you will find
that the max power voltage is within a few percent of each other.

When it is all said and done, all three arrays will be operated with a few
percent of their own max power voltage. Being a few percent off in max power
voltage has almost no effect on max power because their is very little power
variation +/-3% around max power voltage.

You should be able to prove this yourself by looking at an installation that
has SB2500s, one east, one south, one west at the same location with a 25
degree tilt. Look at the operating point of each inverter. The currents will
be very different, but the operating voltages should be very similar.

Another reason that this is okay is that the inverter will tend toward the
dominant subarray so it will tend to operate the array at a slightly LOWER
voltage than the less illuminated arrays. This is the more forgiving side of
the power-voltage curve. Let's prove it once and for all, write a paper and
be done with it.

Bill.



-----Original Message-----
From: Kent Sheldon [mailto:kentsheldon at sbcglobal.net]
Sent: Monday, September 01, 2003 1:55 PM
To: RE-wrenches at topica.com
Subject: RE: Xantrex versus Sunny Boy [RE-wrenches]


Bill, I'll be very curious to see your data. I was surprised that you would
have recommended a single inverter/MPPT for an installation with three very
different plane of arrays combined into one input. This will keep the whole
system running on the weakest sub-array at all times. Regards, Kent
***snip***

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