| Abstract |
In the field of high power electronics for transmission applications there is currently a strong tendency towards using modular converter topologies employing cascaded cells.These multilevel converters are well suited for high power applications, due to good control performance, extensive modularity and excellent harmonic distortion. This paper concerns
fault tolerance of such a cascaded converter on cell level, and a method is proposed to exclude a cell from the chain of cells in case a semiconductor switching device fails. The described method concerns a semiconductor device in presspack encapsulation that normally fails into a permanent short circuit. In particular, it is applicable to converters that use Integrated Gate Commutated Thyristors (IGCT). The application of the converter considered here is STATCOM, where several tens or even hundreds of these cells are involved. Hence, a failure in one of the applied semiconductor switching
devices should not lead to a malfunction of the whole converter. A feasible method for shorting out a failed cell is proposed
and experimentally validated by two consecutive tests. The principle is to initiate a shoot-through in a cell hence
discharging the DC cell capacitance completely where the circuitry for di/dt-reduction during turn-on limits the surge current. Hereby the used semiconductor switching devices are put into a permanently shorted state without using additional hardware. From the observations it can be concluded that the surge leads to a stable SCFM process (Short Circuit Failure Mode) which can conduct the converter current in both directions. The test is not fully conclusive as to how long the SCFM can be sustained. However, experiences from thyristors used in HVDC applications indicate that the SCFM state can be relied on to last for several years. |
