Which inverter for which module?
The question is of course: Which of the above problems apply for which module technology? And which inverter provides the matching solution? An initial orientation is provided by the following overview, as well as in the table in Fig. 3.
Crystalline silicon (also c-Si): The thick, encapsulated cells are quite robust chemically and do not tend to corrode even in case of negative potentials. Grounding is usually not necessary. The considerable thickness of these modules usually also leads to their parasitic capacity being relatively small. Most crystalline modules can therefore easily be operated with all inverters. There are two exceptions to this rule, however:
• Some cell types, especially those with both poles on one side, tend to exhibit polarization effects when operating under positive potential (problem no. 2). Positive grounding of the PV array usually solves the problem – as mentioned earlier, most transformerless devices are not suitable for this.
• Some glass-film modules have a grounded metallic structure integrated into the backside film, so that their parasitic capacity can be surprisingly great (problem 3). In order to prevent capacitive leakage currents, only inverters should be used here that have no significant fluctuations of potential on the DC side (transformer devices or transformerless inverters with quiet rail topology).
Thin-film silicon (a-Si): Cells based on amorphous silicon have a tendency towards corrosion of the TCO, which leads to a permanent loss of output (problem no. 1). The solution is to negatively connect the generator to ground, which is why most transformerless inverters are not a viable option.
Cadmium telluride (CdTe): A similar relationship as for amorphous silicon is also suspected to exist for cadmium telluride-based thin-film modules. Negative grounding is also recommended here, unless the manufacturer explicitly recommends a different solution.
Copper indium selenide (CIS) or copper indium gallium selenide (CIGS): Due to their substrate structure, no TCO corrosion could be observed here so far; grounding is not necessary in most cases. However, it has to be considered, however, that there is a particularly large variety of manufacturing processes for CIGS modules. A manufacturer recommendation should be obtained in the individual case.
Flexible solar cells: Besides CIGS technology, flexible cells currently available base on amorphous silicon; however, it uses a substrate structure and has no glass contact. TCO corrosion has not been observed here, grounding is not required. However, their thin structure can lead to problems: parasitic capacities of flexible laminates can be particularly great when directly attached to a metal surface or in case of moisture and can therefore lead to especially large leakage currents when operated with certain transformerless inverters (problem no. 3). In order to avoid unwanted deactivation, it is recommended to use an inverter that has no notable fluctuations of potential on the DC side (transformer device or transformerless inverter with quiet rail topology).
The inverters available allow realizing any conceivable plant configurations. It is also no problem to use any available module technology as long as the planner takes into consideration its particular characteristics and chooses the right inverter. As the world market leader in the area of PV inverters, SMA is able to offer the appropriate device for any purpose.