Fast and Distortion-Free Short-Circuit Detection and Protection Scheme With Fault Ride-Through Capability Applied to Four-Leg VSI

The three-phase four-leg (3P4L) voltage source inverter (VSI) with
LC
output filter is a suitable transformer-less topology to feed nonlinear and unbalanced loads in four-wire applications. Both symmetrical and asymmetrical short circuits can occur on the load-side. Moreover, due to the low thermal inertia of semiconductors, fast short-circuit protections are necessary to avoid damage by overheating. This article proposes a fast method to detect and protect a two-level 3P4L-VSI, identifying the faulted phases and switching the control to a distortion-free current mode. In contrast, the healthy phases are not affected and maintain a regular operation using voltage mode control. The proposal is based on the instantaneous trajectory of the output voltage vector in the 3-D
$\alpha \beta$
0 reference frame, which is faster than detection methods based on the rms phase voltage and/or inductor current variation. The proposal is validated through simulation and experimental results using a 1.2 kVA 3P4L-VSI prototype under different short-circuit scenarios.