VSC-converter

Claim 1

A VSC-converter for converting direct voltage into alternating voltage and conversely and which comprises at least one phase leg with an NPC-connection, four current valves connected in series, including at least one semiconductor device of turn-off type and a first rectifying member connected in anti-parallel therewith, in which a point on the phase leg between two inner valves of the series connection is for connection to a phase of an alternating voltage network and opposite ends of the phase leg are for connection to a pole conductor each of a direct voltage network or a direct voltage intermediate link, and in which a series connection of two second clamping rectifying members directed in the same direction with respect to said series connection as the first rectifying members are connected between on one hand to a point between one of the outer valves of the series connection and the adjacent inner valve and on the other hand to a point between the other of the outer valves in the series connection and the adjacent inner valve, and a midpoint between the two clamping rectifying members is connected to a zero potential defined by capacitors connected in series between said pole conductors, wherein at least two of the current valves and the clamping rectifying members are arranged in relatively close proximity to each other and are oriented with respect to each other so that commutation currents generate upon commutation of the converter therein will flow in substantially opposite directions.