*3.3. Power Optimizers*

Compared to inverters, power optimizers (POs) benefit from an immunity against AC grid disturbances. They are similar to micro-inverters regarding the distributed MPPT approach but they do a DC/DC and not an AC/DC conversion. Because of this, their internal power electronics circuit topology can be simpler. This is especially true for series power optimizers (SPOs) of which the outputs are connected in series to obtain a high voltage at the end of the string. Their output voltage is currently controlled via an extra current source converter, which can be placed inside the building. The largest benefit of this approach is that their circuit topology can be a standard buck-boost converter with a low amount of required components and low voltage stresses on the components due to their lower output voltage [88–90]. Both factors favour the converter reliability and compactness of SPOs. As indicated in Table 1, SPOs have the same score as string inverters from a modularity and engineering effort point of view. This is because the installation is not necessarily plug-and-play. For small installations this might be relatively straightforward, assuming that, for example, one string per floor level is required and all BIPV modules can be simply put in series. For larger buildings with longer strings, this is not necessarily the case as the amount of SPOs per string is limited by the technology and correct cabling becomes challenging again. Furthermore, one malfunctioning converter can lead to the improper operation of the entire string, as discussed in Section 2.6. In the literature, several detection strategies are proposed to detect and overcome this issue [91,92].

Parallel POs (PPOs) show the best overall performance for use in BIPV electrical installations. As with SPOs, PPOs employ a DC/DC conversion, but all the outputs are placed in parallel to a common DC bus as shown in Figure 1d. The DC bus is controlled by a central voltage source converter which could be the same inverter that controls the LVDC micro grid.

PPOs combine the advantages of a distributed MPPT, a lower amount of internal components compared to inverters, a high degree of flexibility and modularity, no impact of AC grid disturbances, and the highest overall system reliability due to the independence of other converters.

To the author's best knowledge, no papers have been published where the SPO approach is used for BIPV applications. The use of PPOs for BIPV has first been reported by Reference [43], where a 200-V DC bus was used. The same approach was adopted by References [93–95], but a 380-V DC bus was employed. The reliability aspects concerning the embedment of this converter in the frame of a BIPV curtain wall element were treated in Reference [96].


**Table 1.** The qualitative and quantitative analysis of possible BIPV electrical installation architectures. The grey boxes indicate the preferred option.

SI: String Inverter; MI: micro-inverter; SPO: Series Power Optimizer; PPO: Parallel Power Optimizer.
