4.5.1. Earth Fault Selectivity

Assume an earth fault occurs at the output terminal of the MLC. The required response will depend on the LVDC grid configuration. In an IT grid, it is possible to organize the protection centrally, for example, as an extra feature of the central AC/DC converter. If all the protection equipment and logic is contained within this central box, communication is necessary between the central converter and the MLCs. When an earth fault is detected, the central converter can force a sequential shutdown of the MLCs in order to localize and isolate the faulty converter. This specific strategy is only possible in the case of an IT grid configuration, as it allows a certain time to react and search the earth fault. It comes, however, at the cost of a more complex protection scheme, based on communication between the MLCs and a central controller. The MLC needs to be equipped with a relay such that it can entirely disconnect from the LVDC grid when required. The central inverter needs to be equipped with an isolation monitoring device (IMD). Due to the extra costs and increased control complexity, this protection scheme is not recommended. The situation is, however, different for a TN-S grid configuration. When an earth fault occurs, the situation is similar to a short-circuit fault. The earth fault protection can, thus, be identical to the local short circuit protection measures. This can be, for example, a fuse. The fault current will be determined by the amount of PV generation of the other converters, the total capacitance of the LVDC grid, and the fault impedance.

#### 4.5.2. Galvanic Isolation: Yes or No?

A transformer is often regarded as a component that reduces the overall efficiency and strongly increases the cost and volume of a DC/DC converter. It has, however, been successfully implemented in several designs [43,111,112] to realize the high step-up from the PV voltage level to the LVDC voltage level. In this work, an extra advantage of galvanic isolation will be analyzed.

As can be seen from the fault tree in Figure 5, the presence of a transformer affects only the situation where an earth fault occurs on the PV side. The three possible situations for this fault (assuming PV+ to PE) are analyzed in Table 2. A similar analysis is possible for a PV − to PE fault, but it is omitted here as the outcome is the same. This table provides evidence that a (high frequency) transformer in between the PV terminals and the LVDC grid improves the fault-tolerance and, thus, the reliability of the system by creating a very local IT grid. As in an IT system, a first fault has no direct consequence but merely places the potentials on the same level. If the converter has a local isolation monitoring device installed, it can sense this fault and shut down, but this is not a strict requirement. A second fault will instantly lead to a short-circuit at the PV terminals, which is measurable and requires a converter shut down and disconnection from the LVDC grid.


**Table 2.** An overview on the converter PV side earth faults for different grid configurations and distinguishing galvanic isolations.
