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HeatSpring PV Course Blog

MPPTs Stacked Series and Parallel with Shading


Q:

You said, "if the entire string is shaded with MPPT, then the PV source circuit would operate as if the entire string was in the shade.." ...
But then what of the other strings still in the sun?
Assuming for this example that there is some flat horizontal object (like a neighboring building) that causes the bottom landscape series string to experience a lower light level (not zero). So therefore the amperage of that bottom string would be less than the other (parrallel) horizontally strung series strings above the lowest shaded string. I say this because your video said that the AMPERAGE for an array of series 160v strings that are in parallel means their amperage is added.
But at SOME point the shading may be sufficient to drop the voltage.
Which brings me to the next question.

You said to avoid paralleling strings that have different voltages. But due to variation in manufacture etc., no two panels are exactly the same so I've seen strings of 3 that put out 93v or 92v or 94volts ...
But what about when there is greater variance than that between strings because the light level has dropped ENOUGH? At what variance level does it make a significant difference?
I suppose at some point the power per panel will increase to meet the decreasing cost of MPPT charge controllers until every string has its own charge controller. A kind of "Micro String" inverter...

A:

A perfect MPPT inverter that is connected to multiple will operate at the voltage that will make the maximum power.

If you have one string in the sun and another in the shade, the voltages will be similar, because the level of light between direct sun and shaded PV does not effect the voltage much. The string in the shade will contribute less current than the sunny string and the currents will add. This is similar to having an east facing and a west facing string on the same MPPT. The currents will just add.

If you have a situation where there are multiple strings and one of the strings has a shaded module or two, then the array will still do best operating at the voltage of the many unshaded strings. The string that has the shading will be stuck operating at the same voltage as the other modules and will operate as if it were shaded. Kicking in the bypass diodes in this situation will not help. However, when you have an inverter that has multiple arrays on a single MPPT, it is often part of a large ground mount and large ground mounts are typically not put in shaded places.

If we have 2 strings on an MPPT, which is common with commercial rooftop systems that are also commonly 1000V systems with about 20 modules in series, then if a single module is shaded, it would be like having a string of 19 and a string of 20. The question here is what would be the Maximum Power Point where the most power is made. Since the difference between 19 and 20 is about 5%, then meeting in the middle would have the MPP of the 2 strings only 2.5% off each, which is not a big deal, like if there were 6 in series.

For most residential systems these days that are connected to string inverters, each string has its own MPP, so it will just bypass the shaded module and the other modules will work at full power.

Just remember, the MPP should work at the voltage that makes the most power and whatever point makes the most voltage will cause the bypass diodes to kick in or not.

Visualize multiple IV curves stacked up in series and parallel for each solar cell. Some people want to make MPP cells. Probably too expensive. ET solar is selling MPPT modules, which is interesting, but not mainstream.

http://www.etsolar.com/Cell_ptimizer.asp

Interesting stuff.

Expect in future code cycles to see MPPT at every module.

Thanks,
Sean White

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