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A thin section of tape creates partial shading for testing two arrays—one with distributed MPPT, the other without. |
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Distributed MPPT gains the bottom array an additional 412 watts (40% more output). |
The main article’s simplified explanation of how distributed MPPT devices work left out an important detail: the module’s built-in diodes (usually two or three that each control a section of the module) that help mitigate shading effects. While it’s true that the lowest current will bog down a string of dissimilar modules in series, bypass diodes will help route power around a shaded module (or module section) once the resistance of the module’s shaded section exceeds that of the diode.
Batteryless grid-tied inverters actually vary the load on the array to “activate” bypass diodes once the array’s power loss due to a dissimilar module in series is greater than that module’s (or module section’s) loss. This is good news for systems without distributed MPPT—otherwise, a shaded cell could shut down an entire array.
Distributed MPPT equipment can recover the power from shaded modules (or module sections) and keep bypass diodes from activating, simply by finding the available peak current and voltage from the partially shaded module, then calculating and placing the required load on the module so that the current will still flow through the shaded portion of the module, instead of the bypass diode. This keeps module sections and entire modules from going “offline” so they still contribute what they can. This can be accomplished either via a parallel configuration where the module voltage is boosted to match the other module voltages and the available current is added to the total, or through a series configuration where the current of the shaded module is increased to match the other modules’ currents.
A test compared two rows of 11 modules: the bottom row has Tigo Module Maximizers-ES (series) units installed on each module, and the top row does not (see photo at left and screen shot, above). To mimic a shading scenario on both arrays, the two modules on the right have approximately 50% of the bottom row of cells covered. The screen shot shows the output of all modules under these conditions. The bypass diodes on both shaded modules in the top row (the non-Tigo string) have been activated—showing zero output. The remaining modules on the string have reduced output as well (about 116 W each). When the inverter varies the load on the array to activate the bypass diodes, the string is no longer operating at its maximum power point, reducing each module’s output. In contrast, with the assistance of the Tigo devices, the bottom string is getting about two-thirds of the output from the shaded modules, and the other modules are operating at their maximum power (about 141 W each).
The length of time the unshaded modules on the non-Tigo string will operate at the reduced output depends on the inverter’s MPPT algorithms. Inverters may scan the string periodically to find a new MPP. Another irradiance change (such as passing clouds) also can cause the inverter to search for the optimal peak power point for the unshaded modules and restore the output of those modules to their MPP. Even with distributed MPPT equipment installed, if entire PV cells have severe shading (opacity at 80% to 100%) and their is high irradiance, bypass diodes may still activate to prevent reverse current in the shaded module sections.
