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There are many different types of shading (and mismatch). Many of these profiles can, but won't necessarily create multiple MPPs.In order for there to be more than one MPP for an array, the IV curve of the array must become nonmonotonic. There are real situations where significant harvest can be lost with a series connected array without creating a nonmonotonic curve.
For those interested in reading more detail on the matter, I'd like to recommend "Impact of PV Module Mismatch on the PV Array Energy Yield and Comparison of Module, String and Central MPPT." This is a peer reviewed paper out of ABB Corporate research which can be found on IEEEXplore. It was published in the Photovoltaic Specialists Conference in 2012.
The product of this paper is three graphs which display the "Energy Yield Variation" as a function of the "Standard deviation of normal distribution of Isc and Voc" for three different locations and for 5 different MPPT and array configurations.
The graphs show that, with consistent modules (2.5% Standard deviation of normal distribution of Isc and Voc) the increase in energy yield from Module level MPPT is 0.25-0.5% depending on location. However, the paper states:
"Another observation that can be made in Fig. 9 - Fig. 11 is
that for low deviations of Isc and Voc the additional annual
energy from applying module MPPT, compared to the
reference case, is lower than 1 %. Nevertheless, it should be
highlighted that no other parameters that may cause
mismatch, such as soiling, non uniform solar irradiation or
partial shading have been taken into account."
Anyways, please read the paper and form your own opinions, but my take on this paper is that it sets the minimum benefit of module level MPPT, and ANY additional variables will increase the benefit. Soiling, variable aging, and variable irradiance (shading) are very real considerations for any site which can have very significant effects on harvest. Additionally, without module level monitoring, there is no way to determine whether or not a modules are even meeting their standard deviation specifications.
Soiling and aging effects are especially hard to model, and the paper references establishes some techniques to quantify the potential benefit of microinverters under these conditions through measurements at an actual test site. Although the study did not use any global MPPT inverters, nonmonotoic conditions which "fooled" the inverter were excluded from the results.