*4.1. Fast Fault Propagation*

Although the SSCBs current limiting inductance ensures a maximum fault current magnitude, it does not always prevent the fault from propagating through the system and tripping multiple protection devices. To show this the experimental setup shown in Figure 10 is used.

In this setup, a constant voltage source of 350 V and two constant current loads of 5 A are connected to a low inductive dc bus via three SSCBs. This situation can occur, for example, in a dc household that is disconnected from the main grid, where the photovoltaic (PV) panels are providing the energy for loads in two other groups inside the house.

**Figure 10.** (**a**) Schematic and (**b**) picture of the experimental setup connecting a constant voltage source to two constant current loads through three SSCBs.

To show that, in some cases, the fault propagates through the system and trips all the SSCBs before the SSCB in the faulted group can react, a short-circuit with a very low fault resistance (0.75 Ω) is induced at the load-side terminal of CB3. The experimental results for the voltage over the current limiting inductance of CB2 *UL*2and the currents flowing in each circuit breaker are shown in Figure 11.

Observe that after the short-circuit is induced, the fault current starts flowing from the converters' output capacitances to the fault. Therefore, the current in CB3 is increasing rapidly, while the currents in CB2 and CB1 are decreasing rapidly. Also note that, although CB2 feeds a load, the discharge of the load converter's capacitance contributes to the fault current. Furthermore, even though the fault occurs at the load side of CB3, the voltage over the current limiting inductance and CB2 exceeds its threshold before CB3 can act and selectivity is lost.

It is important to realize this is not a consequence of utilizing di/dt detection. If only overcurrent detection is used, the currents in CB1 and CB2 would exceed their limits by the time CB3 clears the fault, because of the high current rate-of-change. Therefore, a challenge for the selectivity of non-unit protection schemes is the fast propagation of low impedance faults through low inductive sections of grids. In radial grids, directional detection can be used to overcome this challenge, but for meshed grids this does not work.

**Figure 11.** Experimental results for the system shown in Figure 10, showing that fault propagation can cause unnecessary tripping in low inductive systems.
