August 2010

Which inverter is the right one?

The choice of PV modules is virtually endless

Specialist retailers already offer thousands of module types today, adding new ones regularly. Various new models are released every year, especially in the area of thin-film technology, but there is also an enormous range of inverters. What seems like a double dilemma at first, is actually the solution. There is in fact a right inverter for every available module technology.
Yet, it is not always easy to find the right combination of PV module and inverter. The modules simply have too many different characteristics. There are various sizes and power classes. They are available with and without frames, as flexible or inflexible versions, manufactured using the crystalline or thin-film procedures and also made of various materials. Likewise, there are various models of PV inverters, employing different topologies and concepts.
Therefore, first you find here a short overview of inverter characteristics relevant for module compatibility, as well as of the module technologies available. Afterwards, we focus on the most common problems in the area of PV modules and show a list of suitable combinations of PV modules and inverters that avoid these problems; organized by module technology.

Which inverter characteristics are relevant?

In terms of module compatibility, the distinction between transformerless and galvanically isolated inverters is probably the most important one. Thanks to their galvanic isolation, devices with a transformer allow grounding the PV array, a requirement for some module types. Thus, all modules of the PV system shift to positive potential when grounding the negative pole of the PV system, or to negative potential when grounding the positive pole.
This is not always possible with transformerless inverters – at least not with the devices currently available. Here, the generation potential is determined by the electronics, usually devided more or less symmetrically into positives and negatives. A certain percentage ofalternating current on the DC side is also determined by the electronics: particularly the more efficient topologies cause the potential of the PV array to oscillate at half of the grid amplitude. This potential oscillation may become a problem, however, if the PV modules have high parasitic capacities; in this case capacitive leakage currents may occur. Inverters with so-called "quiet rail" topology, such as the Sunny Tripower by SMA, avoid this oscillation and only have voltage ripples of a few volts, similar to transformer inverters (Fig. 1).

Abb. 1: Generatorpotenzial bei unterschiedlichen Wechselrichter-Topologien und Erdungsvarianten (Quelle: SMA)

Overview of available module technologies

Until today, modules with cells made of mono- and polycrystalline silicon dominate the market with a share of more than 80 percent. They consist of approx. 0.2 mm thick silicon wafers that are either laminated between two panes of glass or between a film layer and a pane of glass. They are usually covered with a grid of contact fingers on the front; some are contacted with both poles through the back side.
Much less semiconductor material is required for silicon thin-film cells. Here, an amorphous silicon layer with a diameter of only a few micrometers is deposited in a high vacuum and divided into individual cells that are connected accordingly. On the front side, electric contact between the cells is established through a transparent conductive oxide (TCO) layer. The manufacturing process usually begins with the outer glass pane; after this the TCO, the amorphous silicon and the metallic contact layer on the back are applied. This so-called superstrate structure is also used in connection with cadmium telluride (CdTe), while copper-indium-selenide (CIS) modules usually use the reversely assembled substrate structure. Here you begin with the backside, onto which the backside contact, the semiconductor material and the TCO are vapor-deposited layer for layer (see Fig. 1). Most important difference: in the substrate structure the laminate film lies between the cover glass and the TCO, thereby preventing direct contact, in contrast to the superstrate structure.
Flexible laminates form their own, rather young market segment. Here the various functional layers are deposited onto a film, which makes it possible to produce flexible, thin and extremely light cells. These can then be attached directly to the surface of conventional building materials (e.g., metal roofs, awnings or aircraft wings).
Of all other module types, only few have reached the mass production stage. Other types of cells on the threshold of serial production include dye-sensitized solar cells, which are produced by applying a sort of organic or anorganic ink, as well as so-called solar concentrator cells, in which an optical system concentrates sunlight amplified at up to 1000 times onto a highly efficient stacking cell.

Abb. 2: Substrat- und Superstrat-Aufbau bei Dünnschichtmodulen (Quelle: SMA)

The four most common challenges in the area of modules

1.) TCO corrosion: It occurs when sodium from the cover glass of the module reacts with moisture, under the influence of negative potential against ground. Another precondition is apparently direct contact between the TCO and the cover glass, as found in superstrate technologies. Corrosion results in the TCO turning milky starting from the edge of the module and losing its electric conductivity. The module will suffer irreversible power losses – slowly at first and completely once a certain degree of severity is reached.
2.) Polarization: The charge carriers set free in the semiconductor during the photovoltaic process may accumulate around internal boundary areas under certain circumstances. This alters the original characteristic curve and reduces the cell's output. This effect is usually still reversible so that permanent damage to the modules will not occur.
3.) Capacitive leakage currents: In essence, a PV module is an electrically chargeable surface connected to a grounded frame – it behaves similarly to a capacitor. If the module is charged with potential fluctuations on the DC side by the inverter, periodic displacement currents will occur, which also depend on the substructure and the weather. So the "module capacitor" is periodically charged and discharged, which leads to flowing of according currents. In the least favorable scenario, these currents can become so large that the legally required residual current monitoring system of the inverter trips and separates the grid connection.
4.) Insulation resistance (Riso): Every PV system should be insulated against ground as well as possible in order to avoid leakage currents. In the worst case, they may lead to injuries or damages. Therefore, modern inverters check the insulation resistance of the PV array every time prior to connecting. For systems with galvanic isolation, Riso monitoring with deactivation is not required, since only double failures can lead to a hard short circuit, so only a warning message will appear.
However, a transformerless device may not connect at values above 1 mA (=1 kOhm/V), a value that a completely functional, large PV array with a peak power of a few kW can easily reach under wet conditions. Such a situation occurs more easily when the total surface of a PV array belonging to an inverter is large. Sometimes, however, damaged plugs may also be the cause of the problem.

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.

Abb.3: Empfohlene Kombinationen von SMA Wechselrichtern und Zelltechnologien