I'm pondering stringing options for an array using 144-cell modules mounted landscape and ways to minimize susceptibility to lightning. Strings are 15, 16 or 17 modules. Configuration is similar to a ballasted array, with rows of single modules, separated by about 18", but mounted on floats, in a small reservoir. Project is in coastal California area, so lightning is relatively rare.
PV arrays can be susceptible to inductive coupling from nearby lightning pulses which can cause a current because of the rapid change in the electro-magnetic field flux through a wire loop created by an individual string. The current is proportional to the area of the surface surrounded by the loop. One can minimize the area, and thus the coupling by, for example, wiring a string in a single line, starting the positive at one end and returning the wire from the negative end along the panels. One could maximize the area, and the possible bad effects, by putting the modules around the perimeter of as large a square as possible, so obviously a bad thing to do. My questions: 1. In this project, we are wiring in string pairs: i.e. Array strings are paired with adjacent strings in the array field and combined using MC4 branch connectors for home run wiring. There are 12 strings per inverter which are combined into 6 home runs, matching the 6 MPPT inputs for each SMA Core1 inverter. There are no junction boxes. By wiring these pairs so that their circulations are opposite (i.e. one string goes plus to minus around it's wiring loop clockwise, and its pair goes plus to minus counter-clockwise, the induced voltages should cancel, since the changes in flux will be opposite, and the loops are connected at the branch connectors. Q: Am I correct, or would the bypass diodes interfere in some way? 2. In addition to the induction attributable to the string geometry, there is some induction due to the internal wiring of each module. Q: Does the half-cell configuration with its twin half-modules reduce induction? 3. Q: Does rotating half the modules (so half have + at top, and half at bottom in landscape), reduce the induction. We have other reasons to rotate approximately half the modules for wiring purposes. Thanks for your insights. Antony Tersol
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