Partial shading does not have as drastic effect in the parallel connection of PV modules as in a series connection. The MPP voltage of a PV module is largely independent of illumination, thus even at different irradiance levels, the MPPs of modules connected in parallel occur at a nearly common voltage. In other words, a small deviation from the MPP voltage causes only a small reduction to the power produced by a
parallel-of three unshaded modules, the parallel connection parallel-of two unshaded modules, and the parallel connection of two unshaded and one shaded modules and the I-U curve of an individual shaded module. The P-U curves are shown in Figure 6.11. In this case, PMPP of the partially shaded parallel connection is about 315.69 W that is over 60 W more than in the above-presented case of the partially shaded series connection of three ules. It is about 79.3 % of the value of the unshaded parallel connection of three mod-ules, which is about 398 W. In this case, the mismatch losses are only about 0.682 W or 0.216 % of the theoretical maximum power output, which is about 316.37 W.
Figure 6.10. The I-U curves of the parallel connection of three unshaded modules, the parallel connection of two unshaded modules, and the parallel connection of two un-shaded and one un-shaded modules and the I-U curve of one un-shaded module. The ambient
temperature is 25 °C.
Figure 6.11. The P-U curves of the parallel connection of three unshaded modules, the parallel connection of two unshaded modules, the parallel connection of two unshaded
and one shaded modules and the P-U curve of one shaded module. The ambient tem-perature is 25 °C.
In the second case, one module of a parallel connection of 12 modules is shaded and the others are unshaded. Figure 6.12 shows the I-U curves and Figure 6.13 the P-U curves of the parallel connections in this case. In this case, PMPP of the partially shaded parallel connection is about 1509.7 W. It amounts to about 95 % of the value of the un-shaded parallel connection, which is about 1592 W. In this case, the mismatch losses are only about 0.755 W or 0.050 % of the theoretical maximum power output, which is about 1510.4 W.
Figure 6.12. The I-U curves of the parallel connection of 12 unshaded modules, the parallel connection of 11 unshaded modules, the parallel connection of 11 unshaded
and one shaded modules and the I-U curve of one shaded module. The ambient tem-perature is 25 °C.
Figure 6.13. The P-U curves of the parallel connection of 12 unshaded modules, the parallel connection of 11 unshaded modules, the parallel connection of 11 unshaded and one shaded modules and the P-U curve of one shaded module. The ambient
tem-perature is 25 °C.
In a parallel connection of PV modules, a voltage is the same for all the mod-ules. Thus, in the case of a partially shaded parallel connection of PV modules, the mismatch losses are due to the fact that modules are not operating in their MPPs. As can be seen from Figures 6.11 and 6.13, UMPP of a shaded module is slightly greater than UMPP of an unshaded module. Thus, partial shading moves the MPP of a parallel con-nection towards the open-circuit condition. However, the movement of the MPP is very low compared to a series connection. In Figures 6.11 and 6.13, UMPP of unshaded mod-ules and unshaded parallel connections is about 22.67 V and UMPP of a shaded module is about 23.94 V. When one module of a parallel connection of three modules is shaded and the others are unshaded, UMPP of the parallel connection is about 22.835 V. When one module of a parallel connection of 12 modules is shaded and the others are un-shaded, UMPP of the parallel connection is about 22.703 V. Hence, the UMPP of a par-tially shaded parallel connection decreases slightly as the length of the parallel connec-tion increases.
The mismatch losses of a parallel connection when one module of the parallel connection is shaded and the others are unshaded are presented as a function of the length of the parallel connection in Figure 6.14. The length of the parallel connection is increased from two to twelve modules. As can be seen from Figure 6.14, the mismatch losses of the parallel connection increase slowly with the increasing length of the paral-lel connection.
Figure 6.14. The mismatch losses of a parallel connection as a function of the length of the parallel connection when one module of the parallel connection is shaded and the
others are unshaded. The ambient temperature is 25 °C.
As the above-presented scenarios demonstrated, the behaviour of a parallel con-nection of PV modules under partially shaded conditions is incomparable compared to a series connection. That can be seen clearly by comparing Figures 6.6 and 6.14. The mismatch losses of the partially shaded parallel connection are about 98.9 % smaller than the mismatch losses of the partially shaded series connection in the case of three modules and about 98.8 % smaller in the case of 12 modules when one module is shaded and the others are unshaded.
Partial shading can have significant effects on the operation of photovoltaic generators, as illustrated in Chapter 6. There is mainly four ways to reduce these effects: different PV array topologies, grid connection configurations, MPPT techniques and converter topologies. The two first mentioned ways are studied in this thesis. The term configura-tion of a PV generator is used to mean the ensemble of a PV array topology and a grid connection configuration. The shadow sensitivity of three different PV generator con-figurations is studied by simulations using the model of a PV generator and parameters for the NAPS NP190GKg PV module presented in Chapter 5. A shading model is used to produce changing shading conditions. The aim of the simulations is to study how the configuration of a PV generator, the movement direction of a shadow and the sharpness of the shadow affect mismatch losses.