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Inverter Topology Issues for Grounded PV Modules

The ever-evolving solar panel technology has led to the development of different types of solar cells. As a result, new types of crystalline cells, such as the BSC (Backside Contact Solar Cells), and a number of thin-film technologies have gained a foothold in the market. Some of these technologies require the inverter to operate in special conditions in order to maximize the energy produced or to avoid premature cell degradation.
With the Ingecon Sun family of inverters, it is possible to adapt the conversion stage to the conditions required by the different types of modules, in order to deliver optimum system performance.



Thin-Film Modules


Two construction technologies are available for the thin-film modules: one is based on superstrate technology, typically used for Amorphous Silicon or Cadmium Telluride; while the other is based on substrate technology, generally used for Copper Indium Selenide (CIS) modules.

The manufacturing process used in the superstrate technology means that there is direct contact between the glass and the TCO (Transparent Conductive Oxide) used to obtain the negative pole. With regard to this module type, recent studies have shown that the TCO layer either deteriorates or corrodes after a relatively short period of time in operation. This corrosion is caused by the circulation of leakage currents which appear when the module is subjected to a negative potential with respect to ground. These currents transport the sodium ions (Na+) present in the glass to the TCO, causing this to deteriorate. The circulation of these ions is particularly important in hot and humid conditions. The deterioration of the TCO is irreparable and produces a considerable reduction in the module efficiency.

To mitigate the deterioration of the TCO, the module manufacturers have adopted measures such as improving the module side seal or increasing the distance between the TCO and the grounding structures, such as the frame of the module itself. The first measure prevents the ingress of humidity into the module, while the second measure reduces the circulation of currents that could transport the Na+ ions to the TCO.

In any case, and to prevent the circulation of Na+ ions to the TCO, the Ingecon Sun inverters can be used with a grounding kit, to ground the PV array negative pole.


Backside Contact Modules


The backside contact modules feature solar cells in which the metallization of the positive and negative contacts takes place on the reverse side of the cell. This technology makes it possible to obtain an efficiency of more than 20%, representing a real milestone in the development of solar modules.

Despite the good performance achieved in laboratory conditions, in practice the first high voltage field-installed arrays have not achieved their theoretical efficiency. This drop in performance is the result of the accumulation of charge carriers on the cell surface due to the circulation of small leakage currents through the EVA and the upper glass, a phenomenon known as the polarization effect. Given the fact that the cells have no contact on the upper surface, these charges cannot be released and have an important impact on the cell performance. The direction of the leakage current is a decisive factor in the loss of performance. If the cell is subject to a positive potential with respect to ground, then the negative charges remain on the surface, thereby reducing efficiency. However, if the cell has a negative potential with respect to ground, then the effect is reversed and the cell regains its original efficiency.

To avoid the polarization effect, Ingecon Sun Inverters can be used with a grounding kit, grounding the PV array positive pole. In this way the module cells operate at a negative potential with respect to ground.


The Right Solution for Each Type of Solar Array


PV array grounding can be only done with transformer-based inverter or transformerless inverter connected to a dedicated IT network.

To make it easier to select the most appropriate inverter for each type of module, should there be no recommendations from the PV module manufacturer, you are recommended to use the selection table in Figure 3, which shows the various inverter modules that can be used for the different panel technologies.

The grounding kit for the Ingecon Sun transformer-based inverters offers the possibility of grounding either the PV array positive or negative pole, in a simple operation. This is a kit that is mounted inside the inverter itself, with no external modification to the array.

In order to use the grounding kit for the IngeconSun Power Max family of modular inverters, these must be configured with the master-slave option (IngeconSun Power Maxter). On the other hand, if you are using the grounding kit with Transformerless (TL) inverters, then it is only possible to connect one inverter per transformer winding.

The Figure 4 shows a typical grounding layout for negative pole grounding on a transformer based inverter. The PV array grounding is achieved through a protective device (fuse or thermal magnetic breaker, depending on the version) offering protection against fires in the event of a ground fault at the ungrounded pole.



For more information, please send your e-mails to pved@infothe.com.

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