Besides ‘classic’ solar cells made of crystalline silicon, novel kinds of crystalline cells have established themselves on the market, such as back junction solar cells (BJC) and various thin film technologies. With this great variety, the question is whether all of these different cell technologies can be combined with any kind of inverter, or whether specific configurations reduce output, or even damage individual system components . To answer this question, possible failure mechanisms and the impact of the inverter topology will be explained.
First, the potentials of the terminals and consequently of each cell of a string, will be defined according to Figure 21.29.
The solar generator voltage VSG can be measured between the two terminals. The solar generator’s voltages with respect to ground at the positive and the negative poles are VPlus and VMinus respectively. These voltages might be virtually pure DC voltages, but, depending on the inverters hardware topology and control strategy, major sinusoidal or rectangular AC voltages may also be present.
After several decades of experience with conventional crystalline Si solar modules it can be stated, that no correlation has been observed between a degradation of the module and the type of inverter used. However, the degradation of some modules based on special cells and also some thin
Figure 21.29 Definition of potentials at the input side of an inverter
film modules show such correlations. In the case of the back junction cells, a reversible degradation in efficiency has been observed due to the so-called polarization effect . This effect is caused by small leakage currents flowing from the cells through the embedding material and through the front glass to the frame or support structure. These ultra-low currents cause an accumulation of charge on the antireflective coating of the cell, which has a strong impact on the cell efficiency. The time constant of the process is several hours or days, and it is fully reversible. The direction and the strength of the current depend on the potential of the cell relative to ground – with positive potential, negative charges congregate on the antireflective coating of the cell, leading to a reduction in efficiency of up to 30%. A negative potential leads to a positive surface charge, which can even increase the cell efficiency. The polarisation effect can therefore be prevented by positive grounding of the PV generator which means, the positive terminal of a string is connected to ground.
A similar effect has been observed with some string ribbon cells – here, a negative potential relative to ground caused a reversible degradation of the efficiency. In this case, negative grounding is appropriate .
Laboratory experiments and field experience shows, that some thin film technologies are also prone to leakage-current-induced degradations. The probable cause is a corrosion of the transparent front contact (TCO) due to sodium ions moving from the front glass to the interface between the glass and the TCO . The migration of sodium ions can be avoided by negative grounding of the generator.
The input terminal potentials of an inverter are determined by the topology, the switching regime of the power switches and by an eventual internal or external grounding of the generator. It is very important to notice that inverters with and without galvanic isolation by a transformer may have exactly the same potentials. Therefore, galvanic isolation by a transformer without additional grounding measures does not solve these problems!
To clarify the behaviour of inverters, three groups, A-C, of typical potentials have been proposed .
Group A consists of inverters where the solar generator’s potentials are ‘quiet’ to ground potential, while none of the poles has a fixed connection to the ground potential. Depending on the topology, the partial voltages VPlus and VMinus can be symmetrical or asymmetrical about the zero line (ground potential). Except for a small 100 Hz ripple in single-phase units, no other alternating voltage interferes with these voltages, as shown in Figure 21.30.
Group B consists of certain transformerless single-phase inverters, in which a sinusoidal alternating voltage interferes with the voltages to ground, as illustrated in Figure 21.31. The
Figure 21.30 Basic potential curve for inverters belonging to group A. In this example, the potentials are symmetric to ground which is typical for inverters with galvanic isolation (transformer), but without additional grounding measures
alternating voltage corresponds to exactly half of the mains voltage, i. e. 115 V and 50 Hz in European grids.
Group C consists of systems, where one pole of the solar generator is connected to the ground potential (neutral conductor) within or outside the inverter; in other words, the solar generator has a fixed potential according to Figure 21.32. Certain transformerless topologies offer an inherent, thin film friendly grounding. In case of transformer-based inverters, special grounding kits are provided by some manufacturers.
The increasing adoption of thin film panels in systems with high system voltages has increased awareness of the problems described above, and panel manufacturers continue to improve their cell technologies, edge seals and frames. Manufacturers also perform highly accelerated lifetime tests (HALT) under exposure to high temperatures and humidity while applying the maximum allowable system voltage as a negative or positive bias voltage. Some manufacturers expect that, for their products, technological advances have eliminated these problems completely and that, considerating the restriction due to capacitive leakage currents, basically any inverter can be used.
If no clear information on inverter selection is provided by the panel manufacturer, the following recommendations should be observed for thin film panels:
• use transformerless inverters with an inherently grounded minus pole;
• use inverters with transformers and an internally grounded minus pole (grounding kit);
• use frameless panels; if necessary, mount with isolating clips or with clamps affixed on the back surface.
Ultimately, the panel manufacturers must specify whether their panels are suitable for operation with all inverter topologies or for which of the three classes A-C they can be used. Panel developments must ensure that future models can be combined with all inverter topologies.
Figure 21.32 Basic potential curve with negative grounding for systems belonging to group C