Design of a Fault-tolerant Controller for Three-phase Active front End Converter used for Power Conditioning Applications

Recently, an asynchronous micro-grid PCS, enabled by 10kV SiC MOSFETs, has become an exciting research area. The PCS is lightweight and modular technology due to its high switching frequency, 10 kHz or more. Therefore, it can be manufactured and transported at a relatively lower cost. The integration of the PCS to an MV grid, 13.8kV or more, requires an electrical protection scheme to avoid the converter's fault. Different types of faults might occur in the converter system, such as switch failures or grid side faults. The converter shut-off might lead to power loss to many customers and higher downtime costs for the utilities during a failure. Thus, alternative fault-tolerant techniques are necessary to sustain the converter operation under faulty conditions. This paper investigates short and open circuit switch conditions occurring in 3L-NPC converter topology. Large and small-signal converter models are derived for a given failure. A fault-tolerant controller is designed based on the symmetrical components, and it limits the fault current under the desired limit. A closed-loop simulation of the PCS at 22 kV DC bus and the 10A load current is performed at the RTDS and CHIL setup. Individual switch failures are tested in the RTDS and compared with offline simulation results. The effectiveness of the controller is validated in PLECS.