*3.4. LVDC Grid*

A low-voltage DC architecture is adopted because of three main arguments. Primarily, an LVDC grid has a higher compatibility with the DC devices in the system. PV systems, energy storage systems, electric vehicles, LED lighting, IT equipment, and drives operate natively on DC or require DC along the power conversion chain [97–101]. Consequently, an LVDC grid can simplify the conversion steps, which is not only limited to DC-DC power optimizers. That results in an increased efficiency, a reduced component part count, and an enhanced component-level reliability. Secondly, an LVDC grid can transmit more power provided the same copper conductor cross section, thereby lowering costs [99,101]. Thirdly, because power converters are predominantly present in LVDC grids, power flows are actively controllable as compared to passive rectifiers used in AC systems.

Compared to the previous arguments that hold for LVDC grids in general, what are the specific advantages of an LVDC grid in a BIPV context? At first, the fewer conversion steps lead to a lower component count, which leads to an inherent higher reliability and an increased power density. This is beneficial for a frame-integration of the modules. Secondly, since both the input and output power are DC quantities, no energy buffering is required to filter out the power pulsation occurring at twice the grid frequency. The required buffer capacitance is given by Equation (1). To increase the power density and decrease costs, this is usually an electrolytic capacitor. The long-term reliability of this component is often questioned and is, therefore, a distinct advantage if electrolytic capacitors can be left out of the design [44,102].

$$\mathcal{C}\_{\rm DC} = \frac{P\_{\rm MPP}}{2 \cdot \omega\_{\rm grid} \cdot \mathcal{U}\_{\rm DC} \cdot u\_{ripple}} \tag{1}$$

The voltage level can be seen as a degree of freedom which allows for a balance of the converter and cabling costs in the system. A higher voltage level leads to smaller cable cross sections and, thus, to cheaper cabling. However, a lower voltage level can reduce the required gain of the converter and allows the employment of components with a lower voltage rating, which might lead to cost reductions for the converter. To date, there are no internationally recognized standards or agreements on the exact voltage level of future LVDC grids. In telecom applications, a unipolar 48 V grid has been used for decades. In datacenters, a transition is going on from AC to bipolar DC with a voltage level of 380 V (+190 V/0/−190 V) [103]. Rodriguez et al. proposed the use of a bipolar 1500 V (+750 V/0/−750 V) grid for LVDC distribution that can be further divided in a bipolar 750 V (+375 V/0/−375 V) for high power loads and a unipolar 48 V bus for low power loads [104].
