Faults analysis and remedial strategies in high power neutral point clamped converters

Nowadays, multilevel power converter structures are intensively studied and investigated for several applications, such as railway traction systems, active filtering, reactive power compensation and large induction or synchronous motor drives. However, only a few papers deal with the analysis of faults and the development of adequate remedial strategies. This is one of the most important aspects that must be carefully considered and investigated when transferring converter systems from research laboratories to the applications. Failures of the components in static energy conversion systems must be adequately faced to prevent severe and widespread damages in the converter, or even extended to the load and the grid. In particular, in multilevel converters, system protection is more difficult when compared to simpler two-levels converters, mainly because current fault paths can be much more complex. Problems may be exalted in high power applications, as those considered in this paper, when several kVs and hundreds of kAs characterize the application. In this paper, a study and an analysis of important fault scenarios in a high power (up to 12 MVAs) IGCT based Neutral Point Clamped converter system are presented and possible remedial strategies are discussed. Although the study has been performed mainly by simulation, some interesting experimental results, obtained in the joint laboratory between Ansaldo Sistemi Industriali and the University of Genova, are also presented.