Apparatus and a method for voltage conversion

An apparatus for converting a direct voltage into direct voltage comprising a VSC-converter (8) having a direct voltage intermediate link (9) with a positive (10) and a negative (11) pole and at least one phase leg (12, 13) interconnecting the two poles and having at least two current valves (14-17) connected in series, each current valve having at least one semiconductor device (18) of turn-off type and a rectifying member (42) connected in anti-parallel therewith, the apparatus further comprising a transformer (19) with two opposite ends of a first winding (20) thereof connected to an output (21, 22, 30) each of the VSC-converter and with a second winding (23) thereof connected to an arrangement adapted to form a direct voltage, the apparatus also comprising a unit (7) adapted to control the VSC-converter and said arrangement for obtaining said voltage conversion, characterized in that the VSC-converter comprises at least one snubber capacitor (33-36, 50) connected to said current valves, that said arrangement comprises a direct converter (26) having at least one phase leg (24, 25) connected through the opposite ends thereof to opposite ends of said second winding (23) of the transformer (19) and having at least two current valves (37, 38) 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 that a midpoint (27) of said phase leg of the direct converter is provided with an output for forming a terminal (29) for said direct voltage between this output (28) and a further output of the direct converter.

A method for converting direct voltage into alternating voltage and conversely through an apparatus comprising a VSC-converter (8) having a direct voltage intermediate link (9) 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 an alternating phase voltage, said VSC-converter and said arrangement being controlled for obtaining said voltage conversion, characterized in that the control is carried out for an apparatus in which the. VSC-converter comprises at least one snubber capacitor (33-36, 50) connected to said current valves thereof, said arrangement comprising a direct converter (26) having at least one phase leg connected through the opposite ends thereof to opposite ends of said second winding (23) 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, a midpoint of said phase leg of the direct converter being provided with a phase output for forming a terminal for said alternating phase voltage between the output (27) and a further phase output (28) of the direct converter, and that the control comprises the steps of:

• a) controlling the semiconductor devices (18) 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 (20),
• b) commutate the phase leg or phase legs (24, 25) of the direct converter for changing the end of the second transformer winding (23) 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 doing 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.

A method for converting a direct voltage into direct voltage through an apparatus comprising a VSC-converter (8) having a direct voltage intermediate link (9) 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 a direct voltage, said VSC-converter and said arrangement being controlled for obtaining said voltage conversion, characterized in that the control is carried out for an apparatus in which the VSC-converter comprises at least one snubber capacitor (33-36, 50) connected to said current valves, said arrangement comprising a direct converter (26) having at least one phase leg connected through the opposite ends thereof to opposite ends of said second winding (23) 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, a midpoint of said phase leg of the direct converter being provided with an output for forming a terminal for said direct voltage between the output (27) and a further output (28) of the direct converter, and that the control comprises the steps of:

• a) controlling the semiconductor devices (18) 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 (20),
• b) commutate the phase leg or phase legs (24, 25) of the direct converter for changing the end of the second transformer winding (23) to which the respective phase output is connected in such a sequence and at such delays that a constant desired voltage is obtained on said terminal and doing 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.