Quick identification technique of fault conditions in cascaded H-Bridge multilevel converters

With reference to multilevel converters in cascaded H-bridge configuration, the paper presents a technique suitable to quickly identify fault conditions during normal operations. First a modulation technique generalized to m-level converters is synthetically illustrated in order to evidence the presence of different modulation patterns and the corresponding criteria of selection in normal operating conditions. Then, all the "single- fault" conditions are classified, evidencing symmetrical situations, and some consequences on the converter voltage- vector star are analysed. On this basis, a procedure is presented for on-line identification of the corrupted switching device and of the converter module where it is located. No additional sensors are requested, because the procedure is based only on the values of phase-voltages and currents already detected for the closed loop converter control. The fault identification is carried out in a few modulation intervals, also thanks to modifications automatically introduced in the modulation algorithm exploiting redundancies of different converter voltage vectors. Numerical results are presented with reference to some significant fault conditions in cascaded H-bridge inverter having an assigned number of voltage levels.