<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">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>
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<br><div><div>On Apr 29, 2013, at 10:40 AM, Ray Walters <<a href="mailto:ray@solarray.com">ray@solarray.com</a>> wrote:</div><br class="Apple-interchange-newline">
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<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">
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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>
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</div>
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<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>
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<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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<div class="WordSection1"><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>
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