*1.1. Motivation*

Building-integrated photovoltaic (BIPV) systems consist of solar photovoltaic (PV) cells and modules that are integrated in the building envelope as part of the building structure, replacing conventional building materials [1,2]. BIPV is now being proposed as an economically viable solution to the increasing demand for renewable electricity generation, since the relatively minor added cost of PV cells to the overall building component's cost results in conceivable payback times [3,4]. Furthermore, developments in thin-film PV technology reduce the costs of adding PV to structural elements even further [5,6].

The use of BIPV is encouraged by the European Strategic Energy Technology (SET) Plan [7] and the European Energy Performance of Buildings Directive (EPBD) [8]. The EPBD requires that all new buildings in the 28 member states are near Zero Energy Buildings (NZEB) from 2020 on. The implementation can be a combined result of reducing the energy demand and increasing the energy generation on site. In high-rise buildings, the amount of roof surface where PV panels can be placed might be unsufficient to cover the demand of the building. Placing PV in the façades offers a solution to this [9,10]. A high amount of research is conducted towards the different aspects of BIPV, such as BIPV Thermal (BIPVT) installations [11–13], a life-cycle analysis of BIPV installations [14,15], ab optimal design to match the electric loads in NZEB [16], the refurbishment and renovation of older buildings using BIPV [17–19], the thermal impact on the building [20–23], novel PV materials for use in BIPV products [24–27], the role of Building Information Management (BIM) in the design of new buildings with BIPV [28–30], and specific case studies [31–35]. This manuscript focuses on the electrical installation aspects of façade BIPV modules where a high degree of modularity is envisioned. At first sight, the electrical installation of BIPV systems does not seem to differ from building-applied photovoltaics (BAPV). This paper will highlight that the expectations and boundary conditions are different and lead to specific requirements for the design of new power electronics converters.

A wide variety of BIPV modules is commercially available on the market [3,36–39]. In general, two classes of BIPV modules are distinguished, namely roofing BIPV modules [40] and façade BIPV modules [3,41]. The first category consists of PV modules that are part of the roof structure of a building, comprising in-roof systems, full roof solutions, and solar tiles. The second category further distinguishes cold and warm façades, depending on whether the BIPV modules contain a ventilated air gap or not. Additionally, accessories exist such as parapets, balconies, and solar shadings not belonging directly to the building skin [42].

More than BAPV systems, BIPV systems have a profound impact on the electrical installation of the building and on the planning of the construction works, as the BIPV modules are installed during the construction phase and not fitted after construction has completed. Hence, the way in which the BIPV modules are interconnected and converter-interfaced is important to consider in order to minimize the additional required installation time and system engineering effort. In that regard, module-level converters (MLCs) that are factory preinstalled in the BIPV modules are promising [43]. Their main advantage of modularity prevails over the use of conventional string-level inverters, which are usually praised for their lower system cost [44]. Furthermore, adopting a DC instead of an AC distribution architecture simplifies the design of the module-level converters, resulting in an increased compactness, efficiency, and reliability [45,46]. Apart from that, as the cost of BIPV modules reduces further over time, the impact of the electrical installation architecture and the converters becomes more prevalent in the overall cost of the system.

Besides modularity, module-level converters reduce the impact of partial shading and are capable of supporting a wide variety of BIPV modules and associated electrical specifications [36,47–49]. Especially partial shading, leading to different I-V characteristics and maximum power points (MPP) of neighbouring modules is more prevalent in BIPV systems as compared to BAPV [50–53].

However, preinstalling module-level converters in BIPV modules leads to higher required levels of fault tolerance of the converters as it is undesirable or practically infeasible to replace the converter after an internal failure. Therefore, this paper discusses the possible failure modes of module-level converters, including its causes, consequences, and detection methods, and introduces techniques to ensure the fail-safe operation of the module-level converter if necessary. Additionally, the need for non-isolated or isolated module-level converters will be addressed.

#### *1.2. Research Questions and Objectives*

The research questions in this paper are (1) Given the difference with BAPV, what are the specific requirements of a BIPV electrical installation? General ideas will be translated to concrete evaluation points that serve as Key Performance Indicators (KPIs); (2) What are the advantages of Low-Voltage DC (LVDC) grids compared to traditional AC grids to electrically interconnect BIPV modules?; (3) Can the electrical requirements be translated to practical converter design recommendations?; and (4) What is the impact of the LVDC grid configuration (low resistance grounded—TN-S or high resistance grounded—IT) on the reliability and fault-tolerance of the system?
