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<div class="moz-cite-prefix">I question one part of this conclusion,
but have not set up any tests. The only part I question is "the
string with the highest current AND voltage dictates where the
inverter MPP locks". Rather, it seems to me that the voltage at
which the combined current of all strings is greatest dictates
where the inverter locks. <br>
<br>
A string whose string VMP is higher than the aggregate point will
be pulled to a lower voltage (raising the current of that string
slightly). A string whose string VMP is lower than the aggregate
point will be pulled up to a higher voltage - but as current drops
off sharply above MPP, very soon that string's current will drop
off substantially. Whether the dropoff affects the combined VMP
depends on the relationship of that string to the whole,
especially to the number of parallel strings in the array. That
is, the effects of a difference will have a different effect
whether there are two strings or ten. If there are, say, two
strings, the combined VMP will drop to just slightly above the VMP
of the lower-voltage string, as that's where the combined current
of both strings is greatest. If there are ten strings and one has
a lower VMP than the others (such as a string with one failed
module in a large array), the combined VMP of the other nine
strings will determine the VMP of the array, and the string with
the low VMP will be pulled up past its string VMP and its current
will drop substantially - how much depends on how far it's pulled
above its VMP. <br>
<br>
Could someone please explain to me what I just wrote? ;^) And
could someone please tell me if I'm wrong, and why?<br>
<br>
I think these principles would apply in Kirk's case. I suspect the
combination would work, as Dave suggests. But certainly, the
bastard-stepchild 5.5 Imp module in string three would put out at
most about 4.0 A (3.9 plus a smidge), and the lower voltage of
that string might pull it above Vmp and consequently lower the
entire string's output. I second Ray's encouragement to add the
frame-damaged Sanyo, as all four strings then match pretty
closely.<br>
<br>
My hunch is that it will work for an unmentioned reason - that
degradation on the Sanyos will result in less voltage imbalance. <br>
Back to work...<br>
Allan<br>
<br>
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<font face="Times New Roman, Times, serif"><b>Allan Sindelar</b></font><br>
<small><a href="mailto:Allan@positiveenergysolar.com"><font
face="Times New Roman, Times, serif" color="#000099"><u>Allan@positiveenergysolar.com</u></font></a></small><font
face="Times New Roman, Times, serif"><br>
<span style="font-size: 10pt;">NABCEP Certified Photovoltaic
Installer<br>
NABCEP Certified Technical Sales Professional<br>
New Mexico EE98J Journeyman Electrician<br>
Founder and Chief Technology Officer<br>
<b>Positive Energy, Inc.</b><br>
3209 Richards Lane (note new address)<br>
Santa Fe, New Mexico 87507<br>
<b>505 424-1112</b><br>
<a href="http://www.positiveenergysolar.com/"
target="_blank"><u>www.positiveenergysolar.com</u></a><o:p></o:p></span></font>
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<span style="font-size: 10pt;"><br>
</span> </div>
On 4/29/2013 1:21 PM, Larry Crutcher, Starlight Solar Power
Systems wrote:<br>
</div>
<blockquote
cite="mid:4A22530C-6BC2-4AE4-AD80-4B126D54270D@starlightsolar.com"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
So the string with the highest current AND voltage dictates where
the inverter MPP locks. Other strings are "pulled" up past their
MPP which means lower current from them. Makes sense.
<div><br>
</div>
<div>I was only considering constant current under a load equal to
the potential. In that case a module is constant current. The
chart was helpful to rethink what happens with mis-matched
strings. Thanks!<br>
<div apple-content-edited="true"><br>
Larry Crutcher<br>
<div><br>
PS: I believe you!<br>
<br>
<br>
<br>
</div>
</div>
<br>
<div>
<div>On Apr 29, 2013, at 10:40 AM, Ray Walters <<a
moz-do-not-send="true" href="mailto:ray@solarray.com">ray@solarray.com</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
<div text="#000000" bgcolor="#FFFFFF">
<div class="moz-cite-prefix">Modules aren't exactly constant
current devices. <span><I-V_curve.gif></span><br>
They are for much of their voltage range, up to a voltage
a bit below the MPP, and then the current starts tapering
down to 0 at Voc. The entire string can only pass as much
current as the lowest current module. That means the
higher current module will be forced to operate where it
can, which is at a higher voltage than its MPP point, as
David said. <br>
If you don't believe us, hook up a couple of mismatched
modules and measure voltage across each module, and the
current. Current is always equal in a series connection,
then the modules make up for it by moving up or down along
their IV curve.<br>
<br>
<pre class="moz-signature" cols="72">R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760</pre>
On 4/29/2013 9:44 AM, Larry Crutcher, Starlight Solar
Power Systems wrote:<br>
</div>
<blockquote
cite="mid:E5D21F48-A1AD-451D-AB85-6A5E2ED6C950@starlightsolar.com"
type="cite">
<meta http-equiv="Content-Type" content="text/html;
charset=ISO-8859-1">
David,
<div><br>
</div>
<div>Since a PV module is a constant current device, I
don't believe this will take place. For a given amount
of light (plus all factors affecting power), the current
potential will be there. What force can act upon the
current to reduce it? Consequently, the lowest Vmp the
inverter can track will dictate the voltage for all
strings. This means less power from the highest Vmp
modules. Do you agree?
<div apple-content-edited="true"><br>
Larry Crutcher<br>
<div><br>
<br>
<br>
<br>
<br>
</div>
</div>
<br>
<div>
<div>On Apr 28, 2013, at 9:33 AM, David Katz <<a
moz-do-not-send="true"
href="mailto:dkatz@aeesolar.com">dkatz@aeesolar.com</a>>
wrote:</div>
<br class="Apple-interchange-newline">
<meta http-equiv="content-type" content="text/html;
charset=ISO-8859-1">
<div dir="auto">
<div>Kirk</div>
<div>The 43volt 5.5 amp panel will operate at 3.6
amps, so it's voltage will move up toward open
circuit voltage, raising the mppt voltage of that
string. I would bet that the voltage of that
entire string will rise to match the higher
voltage string by moving toward open circuit
voltage. It should work fine.</div>
<div>David Katz<br>
<br>
Sent from my iPhone</div>
<div><br>
On Apr 28, 2013, at 9:01 AM, "Kirk Herander" <<a
moz-do-not-send="true"
href="mailto:kirk@vtsolar.com">kirk@vtsolar.com</a>>
wrote:<br>
<br>
</div>
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<p class="MsoNormal"><span
style="font-size:14.0pt">Hello everyone,<o:p></o:p></span></p>
<div><span style="font-size:14.0pt"> </span><br
class="webkit-block-placeholder">
</div>
<p class="MsoNormal"><span
style="font-size:14.0pt">I am at present
adding to a -20- panel array of old Sanyo
HIT “lipped” 200 watt panels. They are
configured in -4- series strings of -5-
panels, with a string MPP voltage of 279 vdc
(55.8 v, 3.6 a each). The customer wants to
add another nominal 2 kw using the (now)
Panasonic 235 watt HIT’s which have an MPP
of 43.0 vdc, 5.5 a. I am locked in to an
existing SMA 6000US inverter. Obviously I
cannot get all the strings to the same
voltage. But I can get relatively close by
rewiring:<o:p></o:p></span></p>
<div><span style="font-size:14.0pt"> </span><br
class="webkit-block-placeholder">
</div>
<p class="MsoNormal"><span
style="font-size:14.0pt">String 1: -7- 200
watt @ 55.8 ea. = 390.6 v MPP<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">String 2: same as
string 1<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">String 3: -6- 200
watt @ 55.8 plus -1- 235 watt @ 43.0 = 377.8
v MPP<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">String 4: -9- 235
watt @ 43.0 = 387 v MPP<o:p></o:p></span></p>
<div><span style="font-size:14.0pt"> </span><br
class="webkit-block-placeholder">
</div>
<p class="MsoNormal"><span
style="font-size:14.0pt">Under this scenario
there is a delta of about 12 vdc between
high and low MPP. I assume the entire array
will operate at the 377.8 V MPP. I know that
the 235 watt panel will be current limited
also in string 3. Am I missing anything
obvious in doing it like this? How will the
inverter arrive at an operational MPP
voltage?<o:p></o:p></span></p>
<div><span style="font-size:14.0pt"> </span><br
class="webkit-block-placeholder">
</div>
<p class="MsoNormal"><span
style="font-size:14.0pt">PS. In a pinch I
could install a leftover 200 watt HIT I have
in the shop. That way I could have -3-
strings of -7- 200’s @ 390.6 v MPP, and one
string of 235’s @ 387v MPP. But I would
prefer not to use this panel since the frame
is damaged.<o:p></o:p></span></p>
<div><span style="font-size:14.0pt"> </span><br
class="webkit-block-placeholder">
</div>
<p class="MsoNormal"><span
style="font-size:14.0pt">Kirk Herander<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">VT Solar, LLC<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">dba Vermont Solar
Engineering<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">NABCEP<sup>TM </sup>Certified
Inaugural Certificant<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">NYSERDA-eligible
Installer<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">VT RE Incentive
Program Partner<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:14.0pt">802.863.1202<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
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