Patent ID: 8614904

Claim:
A multi-cell converter having at least one phase leg connecting to opposite poles of a direct voltage side of the converter and comprising a series connection of switching cells, each said switching cell having at least two current paths between first and second terminals thereof with a first current path formed by one or more first semiconductor assemblies connected in series and having each a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith, and a second path including a series connection of on one hand at least one second semiconductor assembly having a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith and on the other at least one energy storing capacitor, a mid point of said series connection of switching cells forming a phase output being configured to be connected to an alternating voltage side of the converter, wherein said first semiconductor assemblies of said switching cells are arranged in stacks comprising each at least one semiconductor assembly, the converter comprises an arrangement configured to obtain electric contact through semiconductor assemblies in said stack while ensuring that the semiconductor assemblies of said first path go into a permanently closed circuit state in case of a failure of the respective switching cell, and said second path of each switching cell has a member configured to keep said second path non-conducting upon occurrence of said failure.