Selective Harmonic Mitigation for Cascaded Multilevel Inverters in the Event of Unbalanced Phase Condition


 This paper provides the switching angles that mitigate the dominant low-order line-to-line voltage harmonics in a cascaded H-bridge power converter with a single faulty switch. In this paper, first the firing angles for 5-level and 7-level converters are calculated by solving a set of nonlinear equations. Then, in the faulty condition, a set of switching angles for each phase that results in balanced line-to-line voltages is derived. Finally, the provided solutions are used to simulate the faulty operation of a multilevel inverter.



Fig. 1. The seven-level cascaded H-bridge converter.



Fig. 2. Inverter output voltages in the event of a fault at phase ‘a’. (a) phase voltages: VaN is the blue waveform (b) line voltages (c) load voltages and (d) Harmonic spectrum of the Phase, Line and Load voltages.


This paper provided a method to deal with a single fault in a 7-level CHB converter based on SHM method. Using the proposed strategy, the inverter generates balanced line to line voltages in fault condition while the low order harmonics of the load voltages are limited within the acceptable industrial limits (EN 50160 and CIGREWG 36–05). Using the conventional SHM algorithm the maximum achievable line voltage for a seven-level CHB inverter reduces to about 33% in the event of a single fault. While the decrement of output voltage with the proposed fault–tolerant strategy is about 10%. Therefore, the proposed approach improves the maximum achievable fundamental voltage. This strategy can be easily extended and implemented for inverters with more voltage levels.


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