One of the main causes of power generation losses in photovoltaic systems is partial shading inphotovoltaic modules. Solar panelsare composed of photovoltaic cells connected in series or in parallel, with diodes included in different configurations. The curve of a photovoltaic cell varies as a function of the radiation received and its temperature.  In addition, solar moduleshave diodes that allow current to flow through an alternative path when a sufficient number of cells are shaded or damaged. There are two typical bypass diode configurations: overlapping and non-overlapping. Note that the analysis in PV panelswith overlapping diodes is more complex because there may be different paths for current flow.

 In the following we will see the individual behavior of a module and of a set of modules connected to a shaded inverter in both cases.

Behavior of a partially shaded module photovoltaic cell

To analyze the behavior of a partially shaded module, it is necessary to know:

• The curve of cells in direct and reverse polarization.

• The curve of cells as a function of radiation.

• The bypass diode curve.

Behavior of an array of photovoltaic cells of a partially shaded module.

When shading a PV module, the behavior of a PV cell in its reverse (negative voltage) region must be taken into account, since shaded PV cells may be reverse biased.

Behavior of a partially shaded photovoltaic module

A solar panel consists of a set of photovoltaic cells, with shunt diodes connected in parallel. The connection configuration determines the I-V curve of the photovoltaic module, this configuration can be:

• Superimposed shunt diodes. When the module is shaded, its negative voltage at the operating point may be 1/3 of its open-circuit voltage, and the power consumed may exceed 1/3 of the maximum power of the solar panel.
  

• Non-overlapping shunt diodes. When the module is shaded, it doubles the open-circuit voltage and halves the short-circuit current. In addition, when the module is reverse biased, its voltage and power consumption are lower. However, when the generated power is low, this consumption can be appreciable due to the power consumption of forward biased diodes.
  

Performance of a photovoltaic field with partially shaded modules

In an unshaded PV array, with no damaged panels, all modules have the same MPP. But when any of the modules are fully or partially shaded, the I-V curve of the array changes, so the MPP of each panel may be different. This would result in energy losses. This loss is not proportional to the shaded area, but can be much higher.

The losses in a photovoltaic field depend mainly on:

• The configuration of the bypass diodes.

• Inverter voltage limits on the DC side.

• The layout of the modules.

• Electrical configuration.

Inverter limits with partially shaded modules

A solar inverter connected to the PV array cannot always achieve MPP due to its operating voltage range and MPP tracking algorithm. PV arrays with bypass diodes are more susceptible to power loss due to module shading.

In conclusion, shading losses depend on the configuration of the bypass diodes of the PV modules. Power losses in panels with overlapping diodes can be one third of their maximum power, because in addition to the bypass diodes, one third of their PV cells can also consume power. In modules without overlapping bypass diodes the power losses are only caused by the power consumption of the diodes, although it would only be noticeable if there were too many bypass diodes and shaded modules.

Source: Influence of the shadows in photovoltaic systems with different configurations of bypass diodes