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String Sizing with Smart Modules

Applies to:

JES Series
Smart Module JBox

TS4-L
Long Strings

 

Introduction

Smart Modules utilize a state of the art technology designed to reduce balance of system costs by allowing longer strings in solar arrays. This can reduce the number of strings by 30%, which directly correlates to a 30% reduction in combiner box, wiring, fuses, and hardware overhead costs, as well as reducing the labor requirement for installation. This reduction equates to more than a $0.05 / watt decrease in total system cost. 

Smart-Curve enables longer strings by limiting the Voc output voltage of the module. This reduced Voc is still used to design maximum string lengths, but with the added benefit of a 0% derate factor for temperature. Vmp calculations are not affected by Smart-Curve. An example comparing traditional string design and Smart-Curve enabled string design is below.

 

How To Calculate Maximum String Size?

 

The Smart Modules' string design must satisfy 2 conditions: 

1. Code: 

  

Max. Voc for code: 600V Max. Voc for code: 600V
Inverter MPP voltage range: 250-550V Inverter MPP voltage range: 250-550V
Module Voc: 40V Reduced Voc: 34V
Module Vmp: 32V Module Vmp: 32V
Temp. Corrected Voc: 40 * 1.25 = 50V  Temp. Corrected Voc: 34V * 1 = 34V
Max. string: 600V ÷ 50V =  Max. string: 600V ÷ 34V =
12 modules 17 modules

  

 2. Inverter Voltage:

  


From the example above: 550V ÷ 32V = 17 modules per string. Verify string Vmp limits by dividing inverter max MPP voltage range by the Smart Module's Vmp. 

In Case of discrepancy between the Code requirement and the inverter voltage requirement pick the smaller number of the two.

 

Impact

This example shows us a 33% increase in string power and Vmp, with a corresponding reduction in electrical BOS costs. Interestingly, the Voc of the string is reduced from 600V in the traditional array to 544V in the Smart-Curve design. This is because Smart Modules keep the Voc close to Vmp, and the inverter’s voltage tracking range becomes the new upper limit of system design. 

Note: Only JES series smart modules and modules equipped with TS4-L offer the longer strings functionality. Add-on optimizers, such as the 2ES, are not equipped with this feature.

Always refer to your module datasheet for Voc and Vmp values when sizing your strings.

 

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