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
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 <drake.chamber...@redwoodalliance.org>;
Date: Thu, May 12, 2011 12:27 pm
To: RE-wrenches <re-wrenches@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:Any help with these questions would be appreciated.
- 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.
Thank you,
DrakeDrake Chamberlin
ATHENS ELECTRIC
OH License 44810
CO license 3773
NABCEP Certified PV
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