HVDC device for converting between alternating voltages and direct current voltages

A device for converting alternating voltage into direct voltage, and, conversely, direct voltage into alternating voltage comprising:

• a series connection of at least four units, each having a turn-off type semiconductor element and a first diode connected anti-parallel therewith, said series connection extending between a positive and negative pole of a direct voltage source;
• an alternating voltage phase line connected to a first midpoint dividing the series connection into two parts, said first midpoint constituting a phase output for the series connection;
• means providing a midpoint between the two poles of said direct voltage having a voltage difference with each pole which is the same magnitude but of opposite sign;
• a pair of second midpoints formed in the series, connected through second diodes to said midpoint between said two poles, said second diodes having a conduction direction with respect to the phase output opposite to the conducting direction of a first diode in the unit connected between the respective second midpoint and the output phase, each of said second diodes having a turn-off type semiconductor element connected anti-parallel with each of said second diodes;
• and
• an apparatus for controlling the semiconductor elements of the units to generate a train of pulses according to a pulse width modulation pattern on the phase output, said apparatus alternately connecting the alternating voltage phase line to the midpoint, and the two poles of the direct voltage;
• said apparatus controlling the semiconductor elements of the units between the two second midpoints in response to a pulse width modulation frequency of at least one order of magnitude higher than the fundamental frequency of the alternating voltage of the phase line, and controlling the semiconductor elements connected anti-parallel with said second diodes, and in the remaining of said units, with a frequency substantially lower than said pulse width modulation frequency, and substantially the same as a frequency range one or two times said fundamental frequency.