Apparatus and a method for voltage conversion

A method for converting direct voltage into three-phase alternating voltage and conversely through an apparatus employing a VSC-converter having a direct voltage intermediate link with a positive and a negative pole and at least one phase leg interconnecting the two poles and having at least two current valves connected in series, each current valve having at least one semiconductor device of turn-off type and a rectifying member connected in anti-parallel therewith, the apparatus further comprising a transformer with two opposite ends of a first winding thereof connected to an output each of the VSC-converter and with a second winding thereof connected to an arrangement adapted to form voltage pulses for forming a threephase alternating voltage, said VSC-converter and said arrangement being controlled for obtaining said voltage conversion, wherein the control is carried out for an apparatus in which the VSC-converter has at least one snubber capacitor connected to said current valves thereof, said arrangement comprising a direct converter having at least three phase legs connected through the opposite ends thereof to opposite ends of said second winding of the transformer and having at least two current valves connected in series, each of these current valves being able to conduct current and block voltage in both directions and to turn on by gate control, and the midpoints of said phase legs of the direct converter being provided with phase outputs for forming a terminal for said alternating voltage between these phase outputs, and that the control comprises the steps of:

• a) controlling the semiconductor devices of the VSC-converter for changing the switching state of this converter by changing the connection of at least one of said outputs thereof from one pole of the direct voltage intermediate link to the other for changing the sign of the voltage across said first transformer winding,
• b) commutate the phase legs of the direct converter for changing the end of the second transformer winding to which the respective phase output is connected in such a sequence and at such delays that desired voltage pulses are obtained on said terminal and do this until the current through the second transformer winding has changed direction, and
• c) starting over with controlling the VSC-converter to change switching state again.