*4.2. Commutation of Inductive Currents*

If the current has an alternative path, the interrupting SSCB does not always dissipate the excess inductive energy after clearing the fault. Consequently, for a transient period, the inductive current will flow through the remainder of the system, which can trip other SSCBs in the system. To show this the experimental setup shown in Figure 12 is used.

**Figure 12.** (**a**) Schematic and (**b**) picture of the experimental setup connecting a constant voltage source and two constant current loads connected through an inductive line.

For this experiment, a constant voltage source of 350 V is connected to two constant current loads, each consuming 10 A, via an inductive line and two SSCBs. This situation can occur, for example, when a dc household is connected to a main grid. The line in this experiment is emulated by the *π*-equivalent emulation circuit presented in Section 3.

To show that, in some cases, commutated inductive currents can trip SSCBs in non-faulted parts of the system, a short-circuit with a short-circuit resistance of 4.0 Ω is induced at the load side of CB2. The current in the line and the currents in the circuit breakers for this experiment are shown in Figure 13.

**Figure 13.** Experimental results for the system shown in Figure 12, showing that the commutation of inductive currents can cause unnecessary tripping (the right figure is a zoom in).

Note that, after the short-circuit occurs, the fault current starts flowing from the output capacitances of the converters. Therefore, the currents in the line and CB2 rise quickly, while the current in CB1 decreases rapidly, until the threshold of CB2 is exceeded. Subsequently, after CB2 opens, the inductive current in the line (that was first shared by CB1 and CB2) is commutated to CB1 almost immediately and its di/dt detection is tripped and selectivity is lost. However, if the di/dt measurement was not tripped, the overcurrent detection would have also been tripped since the current through CB1 also briefly exceeds 21 A. Afterwards, since the inductive line current does not have an alternative path, the inductive energy is dissipated in CB1's MOV's.

When SSCBs are operating near their rated current, the commutation of inductive currents would likely cause a cascade tripping circuit breakers in the system. Moreover, this challenge cannot be solved by directional detection, even in radial systems. Therefore, the commutation of inductive currents poses a challenge for the selectivity of non-unit protection schemes.
