[RE-wrenches] SMA High AC Voltage vs MPP

[Drake]

Jamie,

Thanks for the reply.  That does make sense as to how the AC Voltage 
might affect an inverter.  It would certainly seem a likely issue 
with the Sunny Boy line of inverters up to the 7000, which have a 
maximum MPP DC Voltage of 250 V.  The SB 8000 is a 300 V MPP unit.  I 
wonder if the Sunny Design tool is correct about the concern.  I also 
wonder why the SB 8000 went to 300 V DC.

Drake


  At 01:17 PM 5/12/2011, you wrote:
>Drake,
>
>The sunny boy design tool assumes a max grid voltage of 240 (I have 
>version 1.56 that allows you to select 250vAC as the max on screen 
>#2), if your grid voltage exceeds the grid design voltage then the 
>minimum mpp design voltage needs to increase along with it in order 
>to maximize your kWh production.
>
>I forget what the difference is between grid and minimum mpp, IIRC 
>it is 10vDC+ above Grid Voltage so 240vAC would require 250vDC as 
>the minimum mpp.  You would need to check the installation manual 
>for the sunny boy inverter that you are installing.
>
>At high temps, the mpp voltage drops (calculate this as %/deg c 
>above 25 deg c from your module manufacturer supplied temp coef) and 
>may drop below the minimum designed mpp needed for 240vAC or 
>250vAC.  If this happens then either the mppt will adjust below the 
>knee of the power curve in order to increase the voltage (less 
>efficient), or the mpp voltage will drop below the minimum needed 
>and the inverter may shut off.  I have seen both happen.
>
>
>
>Jamie Johnson
>NABCEP Certified PV Technical Sales Professional
>NABCEP Certified Solar PV Installer
>
>General Manager
>SOLAR POWER ELECTRIC
>[]
>
>
>
>-------- Original Message --------
>Subject: [RE-wrenches] SMA High AC Voltage vs MPP
>From: Drake 
><<http://drake.chamberlin@redwoodalliance.org>>drake.chamberlin at redwoodalliance.org>
>Date: Thu, May 12, 2011 12:27 pm
>To: RE-wrenches 
><<mailto:re-wrenches at lists.re-wrenches.org>re-wrenches at lists.re-wrenches.org>
>
>Hello Wrenches,
>
>Using the Sunny Design tool, I got a message saying the design was 
>conditionally compatible.  The message said:
>
>"In case of increased grid voltage, the operating point of the PV 
>generator may slightly deviate from its MPP if the module 
>temperature is high.  This design can therefore only be recommended 
>if the expected average grid voltage at the inverter will not exceed 
>the value used in the calculation."
>
>The SMA tech rep I talked to thought the design was good, but 
>couldn't really answer the questions I had.  These are:
>What is the design AC voltage?
>    * Is it 240 VAC or the maximum voltage the inverter allows, 
> which is 264 VAC?
>    * There is no input for design AC voltage.
>    * Why does the max AC voltage affect the MPP at high temperature?
>        * Since it apparently does, how much does it affect it?
>    * If the grid voltage got to 250 VAC, would that be a problem?
>        * I see that range pretty regularly around here.
>Any help with these questions would be appreciated.
>
>Thank you,
>
>Drake
>
>Drake Chamberlin
>ATHENS ELECTRIC
>OH License 44810
>CO license 3773
>NABCEP Certified PV
>
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Drake Chamberlin
ATHENS ELECTRIC
OH License 44810
CO license 3773
NABCEP Certified PV
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